Oligodendroglioma is a slow-growing, diffusely infiltrating CNS glioma arising from oligodendrocytes, typically occurring in the cerebral hemispheres and characteristically associated with IDH mutation and 1p/19q codeletion.

Oligodendroglioma

Oligodendroglioma is a diffuse, infiltrative primary brain tumour arising from oligodendrocytes — the glial cells responsible for producing and maintaining the myelin sheath around axons in the central nervous system (CNS). The name tells you the origin:

"Oligo" = few (Greek oligos) — oligodendrocytes have fewer processes than astrocytes
"dendro" = tree/branch (Greek dendron) — referring to the cell's branching processes
"glioma" = tumour of glial cells (Greek glia = glue)

So an oligodendroglioma is literally a "tumour of the few-branched glial cells."

WHO 2021 Definition – Must Know

Under the WHO CNS5 (2021) Classification, oligodendroglioma is defined not just by histology but by molecular markers: it requires both IDH-mutation AND 1p/19q co-deletion. Without these molecular features, the tumour is classified differently — even if it looks like an oligodendroglioma under the microscope. This is a paradigm shift from purely morphological classification. ^[1] ^[2]

Per the WHO 2021 system:

Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, CNS WHO Grade 2 (previously called "low-grade" or "well-differentiated")
Anaplastic oligodendroglioma, IDH-mutant and 1p/19q-codeleted, CNS WHO Grade 3 (previously called "anaplastic")

This molecular-integrated approach means histological look-alikes that lack IDH mutation or 1p/19q codeletion are reclassified as astrocytomas or other entities.

2. Epidemiology

Primary intracranial tumours are rare (~6/100,000 per year) ^[3]. Among these, gliomas account for ~30% of all primary brain tumours and ~80% of malignant primary brain tumours.
Oligodendrogliomas account for approximately 5–18% of all gliomas (depending on series and era of classification), making them the third most common glioma after glioblastoma (GBM) and diffuse astrocytoma.
Annual incidence: approximately 0.2–0.5 per 100,000 population.

Age: Peak incidence in 40–60 years (median ~44 years). Rare in children and the elderly.
Sex: Slight male predominance (M:F ≈ 1.2–1.5:1).
Geography: More common in Caucasian populations. In Hong Kong and East Asia, gliomas overall are less common than in Western populations, but oligodendrogliomas represent a similar proportion of gliomas when they do occur.

Feature	Oligodendroglioma	Astrocytoma (diffuse)	Glioblastoma (GBM)	Meningioma
Origin	Oligodendrocytes	Astrocytes	Astrocytes (high-grade)	Arachnoid cap cells
WHO Grade	2 or 3	2, 3 or 4	4	Mostly 1
Peak age	40–60	30–50	55–70	50–70
Location	Supratentorial (frontal)	Supratentorial	Supratentorial	Surface-based (convexity, parasagittal)
Molecular hallmark	IDH-mut + 1p/19q codel	IDH-mut, ATRX loss	IDH-wt, TERT, +7/-10	NF2 loss
Prognosis	Best among diffuse gliomas	Intermediate	Worst	Generally excellent

In adults, 80–85% of brain tumours are supratentorial, commonly metastases, glioma, meningioma. ^[3]

3. Risk Factors

Unlike many cancers, brain tumours have few well-established modifiable risk factors. For oligodendroglioma specifically:

Ionizing radiation exposure: The only consistently proven environmental risk factor for gliomas. Prior cranial radiotherapy (e.g., for childhood leukaemia, tinea capitis) increases risk 5–10 years later.
Age and sex: Middle-aged males as above.

Family history / genetic syndromes: Very rare familial clustering. No single Mendelian syndrome consistently predisposes to oligodendroglioma, though:
- Li-Fraumeni syndrome (TP53 germline mutations) → predispose to astrocytomas more than oligodendrogliomas
- NF1/NF2 → optic pathway gliomas, vestibular schwannomas respectively, not typically oligodendrogliomas
IDH mutation as a biological precursor: IDH mutations are thought to be early, possibly initiating events. The mechanism by which individuals acquire somatic IDH mutations is unknown.

Mobile phone use: Large studies (INTERPHONE, Danish cohort, Million Women Study) have NOT shown consistent association.
Head trauma: No proven association.
Diet, smoking, alcohol: No consistent association with glioma risk.

4. Anatomy and Function of Oligodendrocytes

Understanding the normal cell of origin helps you understand the tumour's behaviour.

Function: Oligodendrocytes produce the myelin sheath in the CNS (analogous to Schwann cells in the PNS). A single oligodendrocyte can myelinate segments of up to 50 axons simultaneously.
Location: Found throughout the white matter of the brain and spinal cord, with cell bodies also present in grey matter.
Development: Arise from oligodendrocyte precursor cells (OPCs) in the subventricular zone → migrate widely → differentiate into mature myelinating oligodendrocytes.

Because OPCs are migratory and widely distributed, oligodendrogliomas tend to be diffusely infiltrative — they spread along white matter tracts.
The predilection for the frontal lobe may relate to the high density of OPCs and white matter connections there.
Despite infiltrating brain parenchyma, oligodendrogliomas tend to grow relatively slowly compared to astrocytic gliomas — consistent with a cell-of-origin that is normally quiescent or slowly dividing.

Supratentorial location is overwhelmingly most common — consistent with the general principle that adult brain tumours are mainly supratentorial ^[3].
Frontal lobe is the single most common site (~50–65%), followed by temporal, parietal, and occipital lobes.
Often involves the cortex and subcortical white matter — the tumour characteristically sits superficially in the cerebral hemisphere.
Can cross the midline via the corpus callosum (though less commonly than GBM's "butterfly" pattern).
Rarely infratentorial (cerebellum, brainstem) or spinal — these locations should make you question the diagnosis.

5. Etiology and Pathophysiology

5.1 Molecular Pathogenesis (The "Why" of Oligodendroglioma)

The molecular biology of oligodendroglioma is one of the best-understood stories in neuro-oncology. Let me walk you through it step by step:

IDH1 R132H mutation (or less commonly IDH2 R172 mutation) is present in >95% of oligodendrogliomas.
IDH = Isocitrate Dehydrogenase, a Krebs cycle enzyme.
Normal IDH converts isocitrate → α-ketoglutarate (α-KG).
Mutant IDH gains a neomorphic function: it converts α-KG → 2-hydroxyglutarate (2-HG), an "oncometabolite."

Why does 2-HG cause cancer?

2-HG is structurally similar to α-KG and competitively inhibits α-KG-dependent enzymes, including:
- TET2 (a DNA demethylase) → leads to DNA hypermethylation (the "CpG island methylator phenotype" or G-CIMP) → silences tumour suppressor genes
- Histone demethylases → epigenetic dysregulation → altered gene expression
- Prolyl hydroxylases → stabilises HIF-1α → promotes angiogenesis
The net effect is a global epigenetic reprogramming that blocks normal differentiation and promotes tumourigenesis.

IDH Mutation — The Gatekeeper

IDH mutation is the earliest and defining event in oligodendroglioma pathogenesis. It is shared with IDH-mutant astrocytomas. What separates an oligodendroglioma from an astrocytoma is the second hit: 1p/19q codeletion (oligodendroglioma) vs. ATRX loss/TP53 mutation (astrocytoma). These are mutually exclusive pathways.

1p/19q codeletion = whole-arm loss of the short arm of chromosome 1 (1p) and the long arm of chromosome 19 (19q).
This is the result of an unbalanced translocation t(1;19)(q10;p10) — the derivative chromosomes der(1;19) are lost, leaving only one copy of 1p and 19q.
This is the molecular hallmark of oligodendroglioma and is required for the WHO 2021 diagnosis.

Why does 1p/19q codeletion matter?

It results in loss of several candidate tumour suppressor genes:
- CIC (Capicua transcriptional repressor) on 19q13 — mutated in ~60–80% of oligodendrogliomas (the "second hit" on the remaining allele)
- FUBP1 (Far upstream element binding protein 1) on 1p31 — mutated in ~30% of oligodendrogliomas
CIC and FUBP1 mutations cooperate with IDH mutation to drive tumourigenesis.
1p/19q codeletion is also a strong predictor of chemosensitivity (especially to PCV: procarbazine, CCNU/lomustine, vincristine) and better prognosis.

TERT (Telomerase Reverse Transcriptase) promoter mutations are present in ~95% of oligodendrogliomas.
These mutations (C228T or C250T) create new binding sites for ETS transcription factors → upregulated TERT expression → telomerase reactivation → unlimited replicative potential (one of the hallmarks of cancer).
TERT promoter mutation + IDH mutation + 1p/19q codeletion = the molecular triad of oligodendroglioma.

In anaplastic (Grade 3) oligodendroglioma, additional mutations accumulate:

CDKN2A/B homozygous deletion (9p21) — encodes p16INK4a and p14ARF, key cell cycle regulators. This is associated with worse prognosis even within Grade 3.
PI3K pathway activation (PIK3CA mutations)
Increased mitotic activity, microvascular proliferation, necrosis — histological features of anaplasia

Oligodendrogliomas are diffusely infiltrative — tumour cells migrate along white matter tracts, perineuronal spaces, subpial zones, and perivascular (Virchow-Robin) spaces ("secondary structures of Scherer").
Despite infiltration, they grow slowly — doubling time is months to years.
They have a strong tendency to calcify (~70–90% on imaging) — this is a hallmark feature.
Cortical involvement is common — they are among the gliomas most likely to involve the cortex, which explains why seizures are the most common presenting symptom.
Transformation to higher grade (Grade 2 → Grade 3) occurs over years to decades, unlike GBM which is aggressive from the outset.

6. Classification

Entity	Molecular Requirements	WHO Grade	Key Features
Oligodendroglioma, IDH-mutant and 1p/19q-codeleted	IDH1/2 mutation + 1p/19q codeletion	Grade 2	Well-differentiated, low mitotic rate
Anaplastic oligodendroglioma, IDH-mutant and 1p/19q-codeleted	IDH1/2 mutation + 1p/19q codeletion	Grade 3	Increased mitotic activity, ± microvascular proliferation, ± necrosis

Key principle: Under WHO 2021, you cannot diagnose oligodendroglioma without confirming IDH mutation and 1p/19q codeletion. A tumour that looks like an oligodendroglioma histologically but lacks these markers is classified as something else (e.g., diffuse astrocytoma, IDH-mutant if it has IDH mutation but not 1p/19q codeletion).

Grade 2 (Low-grade):

Moderate cellularity
Low mitotic rate (typically <6 mitoses/10 HPF)
No microvascular proliferation
No necrosis
Characteristic "fried egg" appearance on histology (see below)

Grade 3 (Anaplastic):

Increased cellularity and mitotic activity
Microvascular proliferation (endothelial proliferation creating "glomeruloid" tufts)
Necrosis (including palisading necrosis)
Nuclear atypia, pleomorphism
The presence of CDKN2A homozygous deletion may be used to upgrade a Grade 2-appearing tumour

Previously, oligodendrogliomas were classified purely by histology:

WHO Grade II: Oligodendroglioma
WHO Grade III: Anaplastic oligodendroglioma
The old "oligoastrocytoma" (mixed glioma) category has been largely eliminated under WHO 2021 — molecular testing almost always resolves these into either oligodendroglioma or astrocytoma.

7. Histopathology

Understanding the microscopic appearance is important for both exams and understanding why the tumour behaves the way it does.

"Fried egg" appearance: Tumour cells have round, uniform nuclei surrounded by a clear perinuclear halo (an artefact of formalin fixation, but highly characteristic). This gives a monotonous, honeycomb-like pattern.
"Chicken wire" capillary pattern: A delicate, branching network of thin-walled capillaries that partition the tumour cells into lobules. This vascular pattern is characteristic and contrasts with the more haphazard vasculature of astrocytomas.
Calcification: Very common (~70–90%), sometimes extensive. Can be dystrophic (within tumour) or involve blood vessel walls.
Cortical infiltration: Tumour cells often infiltrate the overlying cortex, creating perineuronal satellitosis (tumour cells clustering around neurons) and subpial aggregation.
Mucin-rich microcysts: Small cystic spaces containing mucinous material.

Marker	Expected Result	Significance
IDH1 R132H	Positive (IHC)	Confirms IDH mutation (covers ~90% of IDH1 mutations)
ATRX	Retained (positive nuclear staining)	Distinguishes from astrocytoma (which shows ATRX loss)
p53	Wild-type pattern (not overexpressed)	Distinguishes from astrocytoma (which often shows TP53 mutation → p53 overexpression)
1p/19q FISH	Co-deleted	Diagnostic requirement
GFAP	Variable positive	Glial marker (not specific)
Olig2	Positive	Oligodendroglial lineage marker
Ki-67 (MIB-1)	Low in Grade 2, elevated in Grade 3	Proliferation index

ATRX Retained = Think Oligodendroglioma

A quick screening approach in practice: if a diffuse glioma shows IDH mutation + retained ATRX nuclear staining, think oligodendroglioma and proceed to 1p/19q testing. If IDH mutation + ATRX loss, it is almost certainly an astrocytoma (no need for 1p/19q testing, as these are mutually exclusive).

8. Clinical Features

Cardinal presentation of brain tumours: ^[3]

High-grade diseases tend to present with subacute onset of S/S of ↑ICP

Low-grade diseases tend to present with slowly progressive focal neurological S/S

Focal neurological deficits (negative symptoms), epilepsy (positive symptoms)

Oligodendrogliomas, being mostly Grade 2 at initial diagnosis, tend to present with seizures as the most common symptom — a positive symptom from cortical irritation. The slowly progressive course aligns with the neoplastic clinical pattern: insidious onset, progressive course, constitutional symptoms ^[4].

8.2 Symptoms

Why are seizures so common in oligodendroglioma?

Oligodendrogliomas have a strong predilection for the cerebral cortex — the grey matter where neuronal cell bodies reside and where seizure activity is generated.
The slow growth of the tumour causes chronic cortical irritation rather than acute destruction.
Frontal lobe location (the most common site) is particularly epileptogenic.
Calcifications may act as additional epileptogenic foci.
Perineuronal satellitosis (tumour cells wrapping around neurons) directly alters neuronal excitability through changes in the extracellular microenvironment (e.g., altered glutamate/GABA signalling).

Seizure types depend on location:

Focal (partial) seizures: Most common — motor, sensory, or complex partial depending on cortical location
Focal to bilateral tonic-clonic (secondary generalisation)
Status epilepticus: Can be the initial presentation

Seizures may precede diagnosis by months to years in low-grade oligodendrogliomas. A young-to-middle-aged adult presenting with new-onset seizures should always prompt neuroimaging to exclude a structural lesion.

Pathophysiological basis:

Mass effect: As the tumour grows, it occupies space within the rigid cranial vault → increases intracranial pressure (ICP).
Vasogenic oedema: Tumour disrupts the blood-brain barrier → plasma leaks into extracellular space → perilesional oedema → further mass effect.
CSF obstruction: If the tumour is large or strategically located (e.g., near the foramen of Monro or third ventricle), it can obstruct CSF flow → obstructive hydrocephalus → raised ICP.

Character of headache from raised ICP ^[3]:

Generalised, dull, constant headache (worse in morning) — morning worsening because recumbent position increases venous return to the head, worsening ICP.
May be exacerbated by Valsalva manoeuvre (coughing, straining, bending forward).
Typically progressive over weeks to months.

Pathophysiological basis:

Frontal lobe involvement (the most common site) → executive dysfunction, personality changes, apathy, disinhibition, poor judgement.
The changes may be subtle initially — often noticed by family members rather than the patient ("My husband isn't himself lately").
Frontal lobe tumours can mimic psychiatric disorders (depression, personality disorder) or frontotemporal dementia ^[5].
In the dominant hemisphere (usually left), language deficits can occur → expressive aphasia (Broca's area involvement) or anomia.

These depend on tumour location and typically develop gradually as the tumour infiltrates and destroys functional brain tissue:

Tumour Location	Expected Deficit	Explanation
Frontal lobe (precentral gyrus)	Contralateral motor weakness (hemiparesis)	Primary motor cortex involvement
Frontal lobe (prefrontal)	Executive dysfunction, personality change, abulia	Prefrontal cortex mediates higher cognitive functions
Temporal lobe (dominant)	Receptive aphasia (Wernicke's), memory impairment	Language and hippocampal memory circuits
Temporal lobe (non-dominant)	Difficulty with emotional prosody, visuospatial processing	Non-dominant hemisphere functions
Parietal lobe	Contralateral sensory loss, neglect (non-dominant), apraxia	Primary sensory cortex and association areas
Occipital lobe	Contralateral homonymous hemianopia	Primary visual cortex
Insular cortex	Visceral sensory changes, speech arrest (dominant side)	Insula is deep and eloquent

More prominent in Grade 3 (anaplastic) oligodendroglioma or large Grade 2 tumours:

Headache (as above)
Vomiting — often without preceding nausea ("projectile vomiting"), due to direct pressure on the vomiting centre in the area postrema of the medulla
Decreased level of consciousness — from diffuse cerebral compression or herniation
Visual disturbance: Transient visual obscurations or blurred vision from papilloedema (optic disc swelling due to raised ICP transmitted along the optic nerve sheath) ^[6]
Diplopia: CN VI (abducens) palsy — the abducens nerve has the longest intracranial course and is a "false localising sign" of raised ICP, not indicating the site of the tumour itself

Nausea and vomiting: Due to raised ICP (area postrema stimulation)
Gait disturbance: If tumour involves motor areas or causes hydrocephalus
Incontinence: Large bifrontal tumours can affect the parasagittal cortex (leg motor and sensory representation) and micturition centres

8.3 Signs

Papilloedema ^[6]: Bilateral optic disc swelling. On fundoscopy: blurred disc margins, dilated superficial capillaries, loss of spontaneous venous pulsation.
- Pathophysiology: ↑ICP → transmitted along subarachnoid space of optic nerve sheath → "tourniquet" effect → ↓ axoplasmic outflow → axonal swelling of disc
Cushing's triad (late, ominous sign): Hypertension + bradycardia + irregular respirations — indicates brainstem compression, imminent herniation
CN VI palsy: Lateral rectus weakness → inability to abduct the eye → convergent squint (esotropia). This is a false localising sign — it indicates raised ICP, not a lesion at the VI nerve nucleus or pons.
Altered consciousness: GCS assessment is critical — deterioration suggests worsening mass effect or herniation

These correspond to the tumour's anatomical location:

Upper motor neuron (UMN) signs (contralateral): Spasticity, hyperreflexia, upgoing plantar (Babinski sign), clonus — from involvement of the corticospinal tract or primary motor cortex
Sensory deficits: Contralateral cortical sensory loss (astereognosis, agraphaesthesia, impaired two-point discrimination) — from parietal cortex involvement
Visual field defects: Contralateral homonymous hemianopia or quadrantanopia — from involvement of optic radiation or visual cortex
- "I keep on bumping into people on my side" — suggests a visual field defect from a mass lesion affecting the visual pathway ^[7]
Aphasia: Motor (Broca's — non-fluent) or sensory (Wernicke's — fluent with impaired comprehension) if dominant hemisphere is involved
Frontal release signs ("primitive reflexes"): Grasp reflex, palmomental reflex — suggest extensive frontal lobe involvement

Witnessed seizure activity during examination (uncommon but possible)
Post-ictal Todd's paralysis (transient focal weakness after a seizure — localises the seizure focus)
Tongue bite marks, incontinence (signs of recent generalised tonic-clonic seizure)

Herniation Type	Location	Key Signs
Uncal (transtentorial)	Temporal lobe → through tentorial notch	Ipsilateral CN III palsy (fixed dilated pupil, ptosis), contralateral hemiparesis → progresses to bilateral signs
Subfalcine (cingulate)	Cingulate gyrus → under falx cerebri	Contralateral leg weakness (ACA compression)
Tonsillar	Cerebellar tonsils → through foramen magnum	Neck stiffness, respiratory arrest, death
Central (downward)	Bilateral downward	Progressive drowsiness → coma, bilateral small pupils → fixed dilated pupils

Clinical Pearl — New-Onset Seizures in a Young/Middle-Aged Adult

Any adult presenting with new-onset seizures (especially focal seizures) must have brain imaging (MRI preferred) to exclude structural lesions. Oligodendroglioma is one of the key differentials because: (1) it commonly presents with seizures, (2) it affects the 40–60 age group, and (3) it is a treatable condition. Do not dismiss new-onset seizures as "epilepsy" without imaging.

Feature	Mechanism	Notes
Seizures	Cortical irritation by infiltrating tumour cells; altered neuronal excitability; calcification as epileptogenic focus	Most common presenting symptom (50–80%)
Headache	↑ICP from mass effect, oedema, ± CSF obstruction	Worse in morning, progressive
Cognitive/personality change	Frontal lobe infiltration disrupting executive and behavioural circuits	Can mimic psychiatric illness or FTD
Hemiparesis	Motor cortex or corticospinal tract involvement	Contralateral to tumour
Aphasia	Dominant hemisphere (Broca's or Wernicke's area) involvement	Think dominant frontal/temporal lobe
Visual field defect	Optic radiation or visual cortex involvement	Homonymous hemianopia/quadrantanopia
Papilloedema	↑ICP → transmitted to optic nerve sheath → axonal swelling	Bilateral; implies significant ↑ICP
CN VI palsy	↑ICP → stretching of CN VI over petrous apex	False localising sign
Vomiting	Direct pressure on area postrema or ↑ICP	May be projectile, without nausea
Personality/behavioural changes	Frontal lobe dysfunction (orbitofrontal, prefrontal circuits)	Disinhibition, apathy, poor judgement

9. Imaging Characteristics (Pre-Diagnostic, For Clinical Approach)

While formal diagnosis will be discussed in the next section, the imaging appearance is integral to the clinical approach and recognition of oligodendroglioma:

Hypodense or isodense cortical/subcortical mass
Calcification is the hallmark — present in ~70–90%, often coarse, chunite, or ribbon-like. This is the single most characteristic CT feature.
Minimal or no contrast enhancement in Grade 2; heterogeneous enhancement in Grade 3
May see cortical expansion ("cortical thickening") with scalloping of the inner table of the skull (indicating very slow growth)

T1-weighted: Hypointense to isointense, heterogeneous
T2-weighted/FLAIR: Hyperintense (bright), often with blurred margins reflecting infiltrative nature
Gadolinium enhancement: Absent or minimal in Grade 2; patchy or ring enhancement in Grade 3
Location: Cortical/subcortical, most commonly frontal lobe
Features suggesting oligodendroglioma (not pathognomonic):
- Cortical-based location
- Calcification (blooming on susceptibility-weighted imaging/SWI)
- Relative lack of perilesional oedema (in low-grade)
- +/- Cystic component

Emergency radiology: On CT, a calcified cortical mass in a young-to-middle-aged adult presenting with seizures should raise strong suspicion for oligodendroglioma ^[8].

High Yield Summary

Oligodendroglioma — Key Points for Exams

Definition: Diffuse glioma arising from oligodendrocytes; WHO 2021 requires IDH mutation + 1p/19q codeletion for diagnosis.
Epidemiology: 5–18% of gliomas; peak age 40–60; slight male predominance; supratentorial (frontal lobe most common).
Molecular triad: IDH mutation + 1p/19q codeletion + TERT promoter mutation.
Pathogenesis: IDH mutation → 2-HG oncometabolite → epigenetic reprogramming → 1p/19q codeletion (unbalanced translocation) → loss of CIC/FUBP1 tumour suppressors.
Histology: "Fried egg" cells + "chicken wire" capillaries + calcification. ATRX retained (cf. lost in astrocytoma).
Grading: Grade 2 (low-grade) vs. Grade 3 (anaplastic — increased mitoses, ± microvascular proliferation, ± necrosis).
Most common presentation: Seizures (50–80%) — due to cortical involvement and slow growth causing chronic irritation.
Other presentations: Headache (↑ICP), cognitive/personality changes (frontal lobe), focal neurological deficits.
Imaging hallmark: Calcified cortical/subcortical mass on CT, usually frontal.
Prognosis: Best among diffuse gliomas — 1p/19q codeletion predicts chemosensitivity and better survival.
Key distinction from astrocytoma: Oligodendroglioma = IDH-mut + 1p/19q codel + ATRX retained; Astrocytoma = IDH-mut + ATRX loss + TP53 mutation (mutually exclusive pathways).

Active Recall - Oligodendroglioma (Definition to Clinical Features)

^[1] WHO Classification of Tumours Editorial Board. Central Nervous System Tumours (WHO Classification of Tumours, 5th Edition, Volume 6), 2021. ^[2] Senior notes: Maksim Medicine Notes.pdf (Neurology section — CNS tumour classification context) ^[3] Senior notes: Ryan Ho Neurology.pdf (p.161 — Intracranial Tumours, general principles and classification) ^[4] Senior notes: Ryan Ho Neurology.pdf (p.55 — Clinical Approach, nature of lesion table) ^[5] Senior notes: Ryan Ho Psychiatry.pdf (p.94 — Frontotemporal Dementia, differential diagnosis including slow-growing mass lesions) ^[6] Senior notes: Ryan Ho Opthalmology.pdf (p.90 — Papilloedema) ^[7] Lecture slides: GC 067. I keep on bumping into people on my side.pdf ^[8] Lecture slides: GC 013. Emergency radiology.pdf ^[9] Lecture slides: GC 108. A mass in the brain brain tumours.pdf

Differential Diagnosis of Oligodendroglioma

When you encounter a patient — typically a 40–60-year-old adult presenting with new-onset seizures, progressive headache, or slowly evolving focal neurological deficits — and imaging reveals a supratentorial, cortical/subcortical mass (especially with calcification), oligodendroglioma enters the differential. But many other conditions can look similar clinically and/or radiologically.

The differential diagnosis must be constructed at two levels:

Clinical-radiological level: "What could this brain lesion be?" — before you have tissue
Histopathological-molecular level: "This looks like an oligodendroglioma under the microscope — could it be something else?" — after you have tissue but before molecular results

The concept of malignancy is different in primary intracranial tumours: often NOT clear cut. A histologically benign-looking tumour can display aggressive clinical behaviour. A histologically malignant-looking tumour rarely shows extracranial metastasis. ^[3]

Gliomas are classified based on genetics rather than histology (e.g., IDH-1/2, 1p/19q codeletion, ATRX mutation). ^[3] ^[10]

The standard neurological approach to "What is the lesion?" considers the nature of the pathology based on clinical features ^[4]:

Nature	Suggestive Features
Neoplastic / paraneoplastic	Insidious onset, progressive course, constitutional symptoms (loss of weight and appetite)
Vascular	Sudden onset, transient nature, risk factors of vascular diseases
Demyelinating	Exacerbating-remitting course
Inflammatory	Fever, generalised malaise, signs of infection or inflammation
Degenerative	Older age group, gradual onset, progressive course
Metabolic / toxic	Precipitating cause (e.g., drugs), encephalopathy
Congenital / developmental	Younger age group, positive family Hx, consanguineous birth
Trauma / physical	Clear cause-effect relationship

Oligodendroglioma falls squarely into the neoplastic category: insidious onset, progressive course. The key differential is therefore against other space-occupying lesions (SOLs) that can present similarly.

3. Differential Diagnosis by Category

A. Other Primary Brain Tumours (Most Important Differentials)

This is the single most important differential because:

Same age group (30–50 years), same presentation (seizures, focal deficits)
Same anatomical location (supratentorial, cerebral hemispheres)
Shares IDH mutation — the two diverge at the molecular level

How to distinguish:

Feature	Oligodendroglioma	Diffuse Astrocytoma
Molecular	IDH-mut + 1p/19q codel + ATRX retained	IDH-mut + ATRX loss + TP53 mutation
Histology	"Fried egg" cells, "chicken wire" vessels	Fibrillary background, irregular nuclei
Calcification	Very common (~70–90%)	Less common (~20%)
Imaging	Cortical, calcified	More often white matter, less calcification
Prognosis	Better (chemosensitive)	Intermediate
Enhancement	Uncommon in Grade 2	Uncommon in Grade 2

Why are these mutually exclusive? Because 1p/19q codeletion and ATRX loss/TP53 mutation arise from different chromosomal mechanisms in oligodendrocyte precursor cells vs. astrocyte precursor cells, respectively. A tumour commits to one lineage early in tumourigenesis — you virtually never see both 1p/19q codeletion and ATRX loss in the same tumour.

Exam Pearl — The Old 'Oligoastrocytoma' Is Dead

Under WHO 2021, the category of "oligoastrocytoma" (mixed glioma) has been largely eliminated. Molecular testing almost always resolves these into either oligodendroglioma (1p/19q codel) or astrocytoma (ATRX loss). If you see this term on an exam, know it is an outdated diagnosis. ^[3] ^[10]

The most common and most aggressive primary brain tumour in adults
Presents with subacute onset (weeks to months), more rapid progression than oligodendroglioma
Typically older patients (55–70 years) — though there is age overlap
Imaging: ring-enhancing lesion with central necrosis, extensive perilesional oedema, may cross midline ("butterfly glioma")

How to distinguish from oligodendroglioma:

Feature	Oligodendroglioma	GBM, IDH-wildtype
IDH status	IDH-mutant	IDH-wildtype
Age	40–60	55–70
Course	Slowly progressive (years)	Rapidly progressive (weeks–months)
Imaging	Calcified, cortical, minimal enhancement (Gr2)	Ring-enhancing, necrotic core, extensive oedema
Enhancement	Absent/minimal (Gr2), patchy (Gr3)	Prominent ring enhancement
Prognosis	Median survival 10–15+ years (Gr2)	Median survival ~15 months

High-grade glioma: ↓T1W, heterogeneous enhancement with surrounding vasogenic oedema (↑T2W) ± central necrosis ^[10]

Low-grade glioma: ↑T2W, non-enhancing expansile lesion without surrounding vasogenic oedema ^[10]

Arises from ependymal cells lining the ventricles
In adults: most commonly supratentorial (often in the frontal or parietal lobes) or intraventricular
In children: most commonly infratentorial (4th ventricle)
Can present with seizures or raised ICP (especially if intraventricular causing CSF obstruction)
Key distinguishing feature: perivascular pseudorosettes on histology, intraventricular location on imaging

Arises from arachnoid cap cells → extra-axial (outside the brain parenchyma, dura-based)
Most common benign intracranial tumour
Typically older patients (50–70 years), F > M
Presents with progressive focal deficits, seizures (if parasagittal/convexity), or raised ICP

How to distinguish from oligodendroglioma:

Feature	Oligodendroglioma	Meningioma
Origin	Intra-axial (within brain)	Extra-axial, dura-based
Imaging	Cortical/subcortical mass	Homogeneously enhancing, dural tail sign
Enhancement	Variable	Strong homogeneous enhancement
Calcification	Common, coarse	Can calcify, but psammomatous (fine, sand-like)
CSF cleft	No (intra-axial)	Yes (CSF between tumour and brain = extra-axial)

Meningioma: extra-axial, dura-based T1W-hypo/isointense, T2W-iso/hyperintense lesion with strong homogeneous enhancement ^[10]

A diffuse large B-cell lymphoma confined to the CNS (no systemic lymphoma)
Associated with immunosuppression (HIV/AIDS, post-transplant) but also occurs in immunocompetent elderly
Typically presents with rapidly progressive cognitive/personality changes or focal deficits
Characteristically involves deep structures (periventricular white matter, basal ganglia, corpus callosum)

How to distinguish from oligodendroglioma:

Feature	Oligodendroglioma	PCNSL
Location	Cortical/subcortical	Deep periventricular/subcortical
Enhancement	Minimal (Gr2)	Homogeneously enhancing
Calcification	Very common	Rare
DWI (diffusion)	Variable	Prominent diffusion restriction
Response to steroids	None	"Ghost tumour" — dramatic shrinkage
Immunosuppression	Not associated	Strongly associated

Primary CNS lymphoma: solitary/multifocal T2W-iso/hypointense, contrast-enhancing lesions in subcortical regions, classically associated with prominent diffusion restriction on DWI ^[10]

Clinical Pearl — Never Biopsy a Suspected PCNSL Without Considering Steroids

If PCNSL is suspected, avoid giving steroids before biopsy if possible — steroids are lympholytic and can cause the tumour to shrink or disappear ("ghost tumour"), making histological diagnosis impossible. This is a common exam pitfall.

Both are low-grade (WHO Grade 1–2), cortically-based tumours that commonly present with seizures — just like oligodendroglioma
Typically present in younger patients (children and young adults) — younger than typical oligodendroglioma
Key differences:
- DNET: Cortical, "bubbly" multicystic appearance on MRI, temporal lobe predilection, associated with drug-resistant epilepsy. No IDH mutation.
- Ganglioglioma: Contains both neuronal and glial elements, often cystic with mural nodule, temporal lobe. BRAF V600E mutation is common.
These are important differentials in younger patients with cortical seizure-producing lesions that may show calcification

These enter the differential when the lesion is midline or near the sella:

Craniopharyngioma: presence of calcification in 50%, symptoms appearing in childhood (50%), classical presentation with hypopituitarism, chiasmal compression (bitemporal hemianopia), obstruction of third ventricle causing hydrocephalus ^[11]

Craniopharyngioma can show calcification like oligodendroglioma, but is sellar/suprasellar (not hemispheric cortical)
Pituitary adenoma presents with endocrine dysfunction and/or visual field defect (bitemporal hemianopia from optic chiasm compression)

Brain metastases are the commonest intracranial tumour in adults, complicating 25% of extracranial malignancies ^[3]

Common origins: lung (16–20%), breast (5%), RCC (7–10%), colorectal (1–2%), melanoma (7%) ^[3]

Why consider metastases?

Any adult with a brain lesion should have metastases considered, especially if there is a known cancer history
However, 10–15% of solitary cerebral mass lesions in patients with pre-existing cancer are NOT metastasis ^[3]

How to distinguish from oligodendroglioma:

Feature	Oligodendroglioma	Brain Metastasis
Number	Usually solitary	Often multiple
Location	Cortical/subcortical, frontal	Grey-white junction, watershed areas
Margin	Infiltrative, blurred	Circumscribed, well-defined
Oedema	Relatively less (Gr2)	Large volume of vasogenic oedema relative to lesion size
Enhancement	Variable	Strong ring or homogeneous enhancement
Calcification	Very common	Rare (except mucinous carcinoma mets)
History	No systemic cancer	Usually known primary cancer

Brain metastasis: round, well-circumscribed contrast-enhancing lesion with variable signal on T1/2W images ± surrounding oedema (if large), involve multiple regions ^[10]

C. Non-Neoplastic Mimics

Presents with subacute headache, focal deficits, fever, raised inflammatory markers
Ring-enhancing lesion on imaging — can mimic high-grade glioma or Grade 3 oligodendroglioma
Key distinguishing features: fever, leucocytosis, elevated CRP/ESR, peripheral ring enhancement with restricted diffusion centrally (pus restricts diffusion → bright on DWI, dark on ADC), and a smooth thin wall on contrast MRI
Clinical context: recent infection (sinusitis, otitis, dental infection), immunosuppression, endocarditis

Sudden onset (seconds to minutes) — oligodendroglioma is insidious
But subacute infarcts can show enhancement and oedema, potentially mimicking tumour on imaging
Vascular territory distribution (not random cortical location) helps distinguish
Sudden onset, transient nature, risk factors of vascular diseases → vascular pattern ^[4]

Large demyelinating plaques ( > 2 cm) can present as a mass lesion with oedema and enhancement, mimicking a brain tumour ("tumefactive demyelination")
Key distinguishing features: younger patient, relapsing-remitting history, incomplete ring enhancement (open ring sign), periventricular location, other typical MS lesions on MRI
Exacerbating-remitting course → demyelinating pattern ^[4]
Important to consider because unnecessary biopsy or surgery on a tumefactive MS plaque is harmful

Benign vascular malformation that can cause seizures and is often calcified
On MRI: characteristic "popcorn" or "mulberry" appearance with hemosiderin rim (dark ring on T2/GRE)
No mass effect, no enhancement, no oedema (unlike tumour)
Can be an incidental finding or present with seizures/haemorrhage

Particularly relevant in endemic areas (less common in Hong Kong but seen in immigrants from Southeast Asia, Latin America)
Parasitic infection (Taenia solium) → cysts in brain parenchyma → calcified lesions
Can present with seizures and show calcification on CT — mimicking oligodendroglioma
Key distinguishing features: multiple small cystic lesions, scolex (dot within the cyst), travel/dietary history, serological testing (EITB assay)

A developmental malformation of the cortex that is a common cause of drug-resistant epilepsy
Can appear as cortical thickening with blurred grey-white junction on MRI
Distinguished from oligodendroglioma by: no mass effect, no enhancement, stable over years, often identified in childhood/adolescence

Differential	Key Distinguishing Feature from Oligodendroglioma	How to Differentiate
Diffuse astrocytoma, IDH-mut	ATRX loss + TP53 mut (no 1p/19q codel)	Molecular testing: ATRX IHC, 1p/19q FISH
GBM, IDH-wt	IDH-wildtype, ring enhancement, necrosis, rapid course	IDH testing; clinical tempo; imaging pattern
Meningioma	Extra-axial, dura-based, homogeneous enhancement	MRI: dural tail, CSF cleft
Brain metastasis	Multiple, grey-white junction, known primary cancer	History, multiplicity, systemic staging
PCNSL	Deep location, diffusion restriction, steroid-responsive	DWI, steroid response, biopsy
Brain abscess	Fever, restricted diffusion centrally, smooth thin wall	Clinical context, DWI, inflammatory markers
Ganglioglioma / DNET	Younger age, temporal, BRAF mutation, no IDH mut	Age, location, molecular
Tumefactive MS	Relapsing-remitting Hx, open ring sign, periventricular	Clinical history, MRI pattern, CSF oligoclonal bands
Cavernoma	"Popcorn" appearance, hemosiderin rim, no enhancement	MRI (GRE/SWI sequences)
Craniopharyngioma	Sellar/suprasellar, calcified, endocrine dysfunction	Location, endocrine workup
Neurocysticercosis	Multiple small cysts, scolex, endemic exposure	Serology, travel history, imaging pattern

High Yield Summary — DDx of Oligodendroglioma

Most important differential: Diffuse astrocytoma, IDH-mutant — distinguished by 1p/19q codeletion (oligodendroglioma) vs. ATRX loss/TP53 mutation (astrocytoma). These are mutually exclusive molecular pathways.
Commonest intracranial tumour in adults overall: Brain metastasis — always consider, especially if > 1 lesion or known cancer history.
Key clinical-radiological clues pointing to oligodendroglioma: Cortical/subcortical location, calcification (70–90%), frontal lobe, slow growth, minimal enhancement (Grade 2), seizures as presenting symptom.
Non-neoplastic mimics to consider: Brain abscess (fever + ring enhancement + restricted diffusion), tumefactive MS (young, relapsing-remitting), cavernoma (popcorn, hemosiderin rim), neurocysticercosis (endemic, multiple cysts).
Never give steroids before biopsy if PCNSL is suspected — steroids cause tumour lysis and can eliminate tissue for diagnosis.
WHO 2021 mandates molecular classification: Histology alone is insufficient. A tumour looking like an oligodendroglioma without IDH mutation + 1p/19q codeletion is NOT an oligodendroglioma.

Active Recall - Differential Diagnosis of Oligodendroglioma

References

^[3] Senior notes: Ryan Ho Neurology.pdf (p.161–165 — Intracranial Tumours, brain metastases, glioma classification and management) ^[4] Senior notes: Ryan Ho Neurology.pdf (p.55 — Clinical Approach, "What is the lesion?" nature table) ^[10] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.431 — MRI appearance of common tumours, glioma classification) ^[11] Senior notes: Block A - I keep on bumping into people on my side_ pituitary tumours; hypopituitarism.pdf (p.6 — Craniopharyngioma features) ^[12] Senior notes: Maksim Medicine Notes.pdf (p.278 — Increased ICP aetiology and hydrocephalus differential) ^[13] Senior notes: Ryan Ho Psychiatry.pdf (p.82 — Dementia differential diagnosis including structural lesions)

Diagnostic Criteria, Algorithm, and Investigations for Oligodendroglioma

1. Diagnostic Criteria

Unlike many medical conditions, oligodendroglioma does not have a single "diagnostic criteria checklist" like, say, the McDonald criteria for MS or the Jones criteria for rheumatic fever. Instead, diagnosis is an integrated process combining clinical suspicion, imaging, histopathology, and — critically — molecular testing. The WHO CNS5 (2021) classification has made molecular markers mandatory for the diagnosis.

To diagnose "Oligodendroglioma, IDH-mutant and 1p/19q-codeleted", you need ALL of the following:

Requirement	Details
1. Histological confirmation of a diffuse glioma	Tissue showing diffusely infiltrating glial tumour (not a circumscribed or non-glial tumour)
2. IDH mutation	IDH1 (most commonly R132H) or IDH2 mutation, demonstrated by immunohistochemistry and/or sequencing
3. 1p/19q whole-arm codeletion	Demonstrated by FISH, array CGH, SNP array, or methylation profiling
4. Grading (Grade 2 or 3)	Based on histological features (mitotic activity, microvascular proliferation, necrosis) and/or CDKN2A/B homozygous deletion

Gliomas are now classified based on genetics rather than histology (e.g., IDH-1/2, 1p/19q codeletion, ATRX mutation) ^[3] ^[10]

The Molecular Requirement Is Non-Negotiable

A tumour that looks like an oligodendroglioma on H&E (fried egg cells, chicken wire vessels) but lacks either IDH mutation or 1p/19q codeletion is NOT classified as oligodendroglioma under WHO 2021. It would be reclassified — e.g., as diffuse astrocytoma (if IDH-mutant but no codeletion), or glioblastoma IDH-wildtype (if neither), or "diffuse glioma, NOS" if molecular testing cannot be performed.

Once you have confirmed the tumour IS an oligodendroglioma (IDH-mut + 1p/19q-codel), grading distinguishes behaviour:

Feature	Grade 2	Grade 3 (Anaplastic)
Cellularity	Moderate	Increased
Mitotic activity	Low (< 6 mitoses/10 HPF)	Significantly increased
Microvascular proliferation	Absent	May be present (glomeruloid tufts)
Necrosis	Absent	May be present (including palisading)
CDKN2A/B homozygous deletion	Absent	If present → indicates worse prognosis within Grade 3
Ki-67 index	Typically < 5%	Often > 5–10%

Why does grading matter? Because Grade 3 tumours warrant earlier and more aggressive adjuvant treatment (radiotherapy + chemotherapy), whereas some Grade 2 tumours may be observed or treated with surgery alone initially.

The choice of investigation depends on the clinical impression: Where is the lesion? What is the lesion? ^[14]

Neurological investigations confirm the clinical suspicion. Caveat of PAN-investigation — beware false positives (white matter changes, silent lacunar infarct) and false negatives (early disease where structural changes are not obvious) ^[14]

The diagnostic pathway follows a logical sequence: Clinical suspicion → Imaging → Tissue acquisition → Histopathology → Molecular characterisation → Integrated diagnosis.

3. Investigation Modalities

3.1 Neuroimaging

Role: First-line, rapid assessment; detects structural lesions, haemorrhage, calcification, mass effect.

CT brain ± contrast: site and multiplicity, mass effect ^[3]

Normal brain tissue does not enhance (due to BBB). Enhancement indicates: outside BBB (e.g., meningioma — homogeneously enhancing), or disruption of BBB (e.g., high-grade tumours, stroke, inflammation) ^[3]

Key findings in oligodendroglioma:

CT Feature	Description	Pathophysiological Basis
Calcification	Coarse, chunite, or ribbon-like calcification — present in 70–90%	Dystrophic calcification within slow-growing tumour; calcium deposits in tumour tissue and blood vessel walls
Density	Hypodense or isodense to brain (mixed density if calcification + cystic areas)	Tumour cells have similar water content to brain; calcification is hyperdense
Mass effect	Mild or absent in Grade 2; more prominent in Grade 3	Slow growth allows brain to accommodate
Cortical location	Cortical/subcortical mass, may show cortical expansion	Tumour arises from and infiltrates the cortex
Inner table scalloping	Erosion/remodelling of inner table of skull	Very slow-growing tumour chronically pressing against skull — bone remodels over years (implies indolent biology)
Contrast enhancement	Minimal or absent in Grade 2; patchy/heterogeneous in Grade 3	Grade 2: BBB relatively intact; Grade 3: BBB disrupted by angiogenesis and necrosis

Note characteristics of lesions on CT: location/distribution, number, size, shape, outline/contour, attenuation or density, contrast enhancement pattern ^[15]

Calcification = Think Oligodendroglioma

Among primary brain tumours, oligodendroglioma is the most commonly calcified glioma. If you see a cortical/subcortical mass with prominent calcification in a middle-aged adult, oligodendroglioma should be at the top of your differential. Other calcified brain lesions include craniopharyngioma (sellar/suprasellar), meningioma (psammomatous type), cavernoma, and neurocysticercosis — but the clinical and radiological context usually distinguishes them.

Role: Gold standard for brain tumour evaluation — superior soft tissue resolution, better anatomical detail, and functional/advanced sequences.

MRI with contrast: generally preferred over CT ^[3]

Standard MRI sequences and findings:

Sequence	Findings in Oligodendroglioma	What It Tells You
T1-weighted	Hypointense to isointense, heterogeneous	Tumour tissue has longer T1 relaxation than normal brain
T1 + Gadolinium	Grade 2: minimal/no enhancement; Grade 3: patchy, heterogeneous enhancement	Enhancement reflects BBB breakdown → correlates with higher grade
T2-weighted	Hyperintense (bright)	Increased water content in tumour cells and oedema
FLAIR	Hyperintense with better delineation of tumour margins than T2 (suppresses CSF signal)	FLAIR = Fluid Attenuated Inversion Recovery — nullifies free water signal (CSF) to highlight parenchymal lesions
SWI/GRE	Blooming artefact from calcification and/or haemorrhage	SWI = Susceptibility Weighted Imaging — exquisitely sensitive to calcium and blood products
DWI/ADC	Variable diffusion; Grade 3 may show restricted diffusion	Restricted diffusion reflects high cellularity (tightly packed cells impede water movement)

Low-grade glioma: ↑T2W, non-enhancing expansile lesion without surrounding vasogenic oedema ^[10]

High-grade glioma: ↓T1W, heterogeneous enhancement with surrounding vasogenic oedema ± central necrosis ^[10]

Key MRI features suggesting oligodendroglioma specifically:

Cortical/subcortical location (often frontal) — the tumour "sits in" the cortex, unlike astrocytomas which tend to be more deep white matter-based
Calcification (blooming on SWI) — though CT is actually better for detecting calcification
Relatively sharp margins on FLAIR despite being infiltrative
Heterogeneous signal due to mix of solid tumour, calcification, cystic components, and sometimes haemorrhage
Minimal perilesional oedema in Grade 2 (cf. GBM which has extensive oedema)

Functional imaging: for pre-operative planning ^[10]

Technique	Purpose	How It Works
Functional MRI (fMRI)	Identify important cortical areas to guide surgical resection	Detects blood-oxygen-level-dependent (BOLD) signal changes during task performance (e.g., motor, language tasks) → maps eloquent cortex
MRI Diffusion Tensor Imaging (DTI / tractography)	Identify important tracts to guide surgical resection	Tracks directionality of water diffusion along white matter tracts → visualises corticospinal tract, arcuate fasciculus, optic radiation relative to tumour
MR Spectroscopy (MRS)	Distinguish tumour from non-neoplastic tissue; assess tumour grade	Measures metabolite concentrations: ↑choline (cell membrane turnover), ↓NAA (neuronal loss), ↑choline/NAA ratio suggests neoplasm; ↑lactate/lipid in high-grade tumours
MR Perfusion (DSC/ASL)	Assess tumour vascularity and grade	Measures relative cerebral blood volume (rCBV) — ↑rCBV suggests higher grade (more neovascularisation)

MR Spectroscopy Pearl

In oligodendroglioma, MRS characteristically shows elevated choline (reflecting high cell membrane turnover) and presence of 2-hydroxyglutarate (2-HG). Advanced MRS protocols can now directly detect the 2-HG oncometabolite produced by mutant IDH — this can non-invasively predict IDH mutation status before surgery. This is cutting-edge but increasingly used in specialised centres.

Modality	Role	Findings
18F-FDG PET	Limited role in brain tumours (high background cortical metabolism)	Low-grade gliomas may be hypometabolic; high-grade may be hypermetabolic — but poor sensitivity due to high normal brain FDG uptake
Amino acid PET (11C-MET, 18F-FET)	Better for brain tumour grading, delineation, and post-treatment monitoring	Increased amino acid uptake in tumour tissue (lower background than FDG). Grade 3 oligodendrogliomas show higher uptake than Grade 2

PET-CT is not good for brain as the background activity is too hypermetabolic ^[16]

Modality	Role
CT/MR Angiography	Assess relationship to major vessels for surgical planning
SPECT	Historical; distinguishes benign from malignant brain lesions, predicts histological grade, selects areas for biopsy ^[17] — largely superseded by advanced MRI/PET

Histological examination of biopsy specimen remains the most important component of diagnostic evaluation of brain tumours ^[17]

Histological confirmation is obtained at the time of surgical exploration for resection ^[17]

Why do we need tissue? Because imaging alone cannot definitively diagnose oligodendroglioma — you need histological confirmation of a diffuse glioma AND molecular testing for IDH and 1p/19q. No amount of imaging substitutes for tissue.

Methods of Tissue Acquisition

Method	When Used	Advantages	Disadvantages
Maximal safe surgical resection	Preferred whenever feasible	Therapeutic (debulking) + diagnostic; large tissue sample for molecular testing	Risk of neurological deficit if near eloquent cortex
Stereotactic needle biopsy	Inoperable tumours (deep, eloquent location, diffuse, poor performance status)	Minimally invasive; can reach any location	Small tissue sample (may miss heterogeneous areas); sampling error; cannot reduce mass effect
Open biopsy	When stereotactic is insufficient or anatomy prevents stereotactic approach	Larger sample	More invasive than stereotactic

Maximal Safe Resection — The Principle

Maximal safe resection without causing serious neurological sequelae is the key component of treatment ^[17]. The concept is that you remove as much tumour as safely possible — every additional percentage of resection improves survival in oligodendroglioma — but you never sacrifice function. This is why preoperative fMRI and DTI tractography are performed: to map eloquent cortex and white matter tracts before the surgeon goes in.

3.3 Histopathological Examination

Once tissue is obtained, the neuropathologist performs:

Identifies the characteristic morphological features:

Feature	Description	Significance
"Fried egg" cells	Round, uniform nuclei with perinuclear halo (formalin fixation artefact)	Classic oligodendroglioma morphology (though not pathognomonic)
"Chicken wire" vasculature	Delicate, branching capillary network dividing cells into lobules	Characteristic vascular pattern
Calcification	Dystrophic calcification within tumour or vessel walls	Very common; supports diagnosis
Cortical infiltration	Perineuronal satellitosis, subpial aggregation	Typical infiltration pattern of oligodendroglioma
Mitotic figures	Counts per 10 HPF	Low in Grade 2; elevated in Grade 3
Microvascular proliferation	Endothelial proliferation forming glomeruloid tufts	Feature of Grade 3 (anaplasia)
Necrosis	Geographic or palisading necrosis	Feature of Grade 3 (anaplasia)

IHC is performed on the tissue sections using antibodies against specific proteins:

IHC Marker	Expected Result in Oligodendroglioma	Interpretation
IDH1 R132H	Positive (cytoplasmic staining)	Detects the most common IDH1 mutation (~90% of IDH1-mutant cases). If negative, proceed to IDH sequencing — the mutation may be a non-R132H variant or IDH2
ATRX	Retained (positive nuclear staining)	Crucial distinction: ATRX is retained in oligodendroglioma but LOST in astrocytoma. Retained ATRX + IDH-mutant → strongly suggests oligodendroglioma → proceed to 1p/19q testing
p53	Wild-type pattern (not overexpressed)	TP53 mutations (leading to p53 overexpression on IHC) are characteristic of astrocytoma, not oligodendroglioma
Olig2	Positive (nuclear)	Confirms oligodendroglial lineage — but also positive in some astrocytomas, so not specific
GFAP	Variable positive	General glial marker
Ki-67 (MIB-1)	Low (< 5%) in Grade 2; higher in Grade 3	Proliferation index — helps grading

The IHC screening approach in practice:

3.4 Molecular and Cytogenetic Testing

This is where the diagnosis is confirmed or refuted. These tests are mandatory under WHO 2021.

Method	How It Works	Pros	Cons
FISH (Fluorescence In Situ Hybridisation)	Fluorescent probes bind to specific chromosomal regions (1p36 and 19q13) on tissue sections. Count ratio of test to reference signals. If < threshold → deletion	Widely available; can be done on FFPE tissue	Only tests specific loci — cannot confirm whole-arm deletion (partial deletions are NOT equivalent); cannot detect unbalanced translocation directly
Array CGH (Comparative Genomic Hybridisation)	Hybridises tumour vs. normal DNA to arrays → detects copy number changes across entire genome	Confirms whole-arm codeletion; detects CDKN2A homozygous deletion simultaneously	Requires sufficient tumour DNA; more expensive
SNP Array	Similar principle to array CGH but uses single-nucleotide polymorphism markers	Detects copy number changes + copy-neutral LOH	Cost; requires bioinformatics
DNA Methylation Profiling	Classifies tumours based on genome-wide DNA methylation patterns (e.g., Heidelberg classifier)	Most comprehensive — can classify tumour type AND detect 1p/19q, IDH status, CDKN2A, MGMT, TERT simultaneously	Expensive; turnaround time; requires specialised lab

FISH vs. Whole-Arm Codeletion — An Important Nuance

FISH at specific loci (1p36 and 19q13) can sometimes give false positive results if there is a partial deletion at those loci rather than a true whole-arm codeletion. Partial deletions are NOT the same as the unbalanced translocation t(1;19) that defines oligodendroglioma — they can occur in astrocytomas and GBM. Whenever possible, use array-based methods or methylation profiling to confirm whole-arm codeletion. This is a known pitfall.

Method	Details
IHC for IDH1 R132H	First-line screen — detects the most common mutation (~90% of IDH-mutant gliomas). Fast, cheap, widely available
IDH sequencing (Sanger or NGS)	Required if IHC is negative in suspected low-grade glioma — to detect non-R132H IDH1 variants and IDH2 mutations
DNA methylation profiling	Can infer IDH status from the G-CIMP methylation pattern

Test	Target	Clinical Significance
TERT promoter mutation	C228T or C250T	Present in ~95% of oligodendrogliomas; helps confirm diagnosis; also present in GBM (but GBM is IDH-wt)
CDKN2A/B homozygous deletion	9p21 locus	If present in a 1p/19q-codeleted glioma → indicates more aggressive biology and worse prognosis even within Grade 3
MGMT promoter methylation	MGMT gene	Predicts response to alkylating chemotherapy (temozolomide, PCV). Most oligodendrogliomas are MGMT-methylated (due to global hypermethylation from IDH mutation/G-CIMP) → this contributes to their chemosensitivity
CIC mutation	19q13	Mutated in 60–80% of oligodendrogliomas (second hit on remaining 19q allele). Supports diagnosis
FUBP1 mutation	1p31	Mutated in ~30% of oligodendrogliomas. Supports diagnosis

3.5 Additional Investigations

These are not for diagnosing oligodendroglioma per se, but are part of the comprehensive workup:

EEG: epilepsy — distinguish subtype, localisation of epileptic focus ^[18]

Role: Characterise seizure type and localise epileptic focus — important for pre-surgical planning if epilepsy surgery is being considered
Findings: Focal slowing or epileptiform discharges (spikes, sharp waves) in the region corresponding to the tumour
Why it matters: Many oligodendroglioma patients present with seizures; EEG helps guide anticonvulsant therapy and surgical approach

Test	Purpose
CBC, RFT, LFT, coagulation	Pre-operative baseline; assess fitness for surgery/anaesthesia
Electrolytes (Na, K, Ca)	Hyponatremia may indicate SIADH (can occur with any intracranial pathology raising ICP)
Tumour markers (if metastasis suspected)	AFP, β-hCG (germ cell tumours); LDH (lymphoma); CEA, CA 19-9 (if metastatic disease considered)
HIV, hepatitis B/C serology	If PCNSL is a differential (immunosuppression association)

Generally NOT performed if there is a mass lesion with mass effect or raised ICP → risk of tonsillar herniation
May be considered after imaging excludes significant mass effect to analyse CSF cytology (leptomeningeal disease) or exclude infection (if abscess is in the differential)
In oligodendroglioma: CSF is usually unremarkable (cf. MS where oligoclonal bands are present; PCNSL where malignant cells may be found)

Never LP Before Imaging in Suspected SOL

If a space-occupying lesion (SOL) with raised ICP is suspected, NEVER perform lumbar puncture before neuroimaging. Removing CSF from the lumbar space when there is a supratentorial mass creates a pressure gradient → downward tonsillar herniation → brainstem compression → death. Always CT or MRI first.

Important when the tumour is near the visual pathway (optic radiation, occipital lobe, or causing general raised ICP with papilloedema)
Automated perimetry (e.g., Humphrey VF) documents any homonymous hemianopia, quadrantanopia, or enlarged blind spot
Also serves as a baseline before any surgical intervention — surgeons need to know pre-existing visual deficits

Formal cognitive assessment before treatment
Documents baseline cognitive function — especially important for frontal lobe tumours where executive dysfunction may be subtle
Helps guide rehabilitation and monitor post-treatment cognitive changes

Step	Investigation	Key Decision Point
1. Suspect	Clinical assessment	New-onset seizures in young/middle-aged adult → image the brain
2. Detect	CT Brain	Calcified cortical mass? → strongly suspect oligodendroglioma
3. Characterise	MRI Brain + Gd	Location, enhancement, oedema, advanced sequences → plan surgery
4. Obtain tissue	Surgery/biopsy	Histological examination remains the most important component
5. Classify histologically	H&E + IHC	Diffuse glioma? IDH1 R132H IHC? ATRX retained?
6. Confirm molecularly	1p/19q, IDH seq	1p/19q codeleted + IDH-mutant → Oligodendroglioma confirmed
7. Grade	Histology + molecular	Mitoses, MVP, necrosis, CDKN2A/B → Grade 2 vs. Grade 3
8. Plan treatment	Integrated diagnosis	Grade + extent of resection + molecular profile → guide adjuvant therapy

High Yield Summary — Diagnosis of Oligodendroglioma

Diagnosis requires tissue + molecular confirmation: IDH mutation AND 1p/19q codeletion are both mandatory under WHO 2021. Histology alone is insufficient.
CT hallmark: Calcified cortical/subcortical mass (calcification in 70–90% — most commonly calcified glioma).
MRI is the preferred imaging modality: T2/FLAIR hyperintense, minimal enhancement in Grade 2, cortical location, frontal lobe predilection.
IHC screening: IDH1 R132H positive + ATRX retained → strongly suggests oligodendroglioma → confirm with 1p/19q FISH/array.
ATRX retained vs. lost is a rapid IHC screen to distinguish oligodendroglioma (retained) from astrocytoma (lost) — these are mutually exclusive.
FISH alone can give false positives — partial deletions at 1p36/19q13 loci do not equal whole-arm codeletion. Array-based or methylation methods are superior.
MGMT methylation is typically present in oligodendroglioma (due to G-CIMP from IDH mutation) → explains chemosensitivity.
Never LP before imaging if SOL/raised ICP suspected → risk of herniation.
Advanced MRI (fMRI, DTI tractography) is for preoperative planning to map eloquent cortex and white matter tracts.
EEG helps characterise seizures and localise the epileptic focus, not diagnose the tumour itself.

Active Recall - Diagnosis and Investigations of Oligodendroglioma

References

^[3] Senior notes: Ryan Ho Neurology.pdf (p.161–165 — Intracranial Tumours, CT/MRI findings, glioma classification) ^[10] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.431 — MRI appearance of common tumours, glioma classification by genetics) ^[14] Lecture slides: Introduction to Neurological Investigations and Emergencies.pdf (p.3, p.9 — Choice of investigation, caveats) ^[15] Senior notes: Ryan Ho Diagnostic Radiology.pdf (p.39 — Interpretation of CT images, lesion characteristics) ^[16] Senior notes: Ryan Ho Respiratory.pdf (p.144 — PET-CT not good for brain due to high background activity) ^[17] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1262 — SPECT, surgical biopsy as most important diagnostic component) ^[18] Senior notes: Maksim Medicine Notes.pdf (p.235 — Neurological investigations, EEG role)

Management of Oligodendroglioma

The management of oligodendroglioma follows a multimodal, integrated approach that is guided by the molecular-integrated diagnosis (IDH-mutant, 1p/19q-codeleted), tumour grade, extent of resection, and patient factors (age, performance status, symptoms, patient preference).

Key overarching principles:

Surgery first: Maximal safe removal as 1st line in all operable gliomas ^[3] — surgery serves both diagnostic (tissue) and therapeutic (cytoreduction) purposes
Adjuvant therapy is grade-dependent: Grade 2 may be observed after surgery; Grade 3 requires adjuvant radiotherapy and/or chemotherapy
Oligodendrogliomas are uniquely chemosensitive: The 1p/19q codeletion predicts excellent response to alkylating agents (PCV, temozolomide) — this is the key biological advantage
Treatment intent is to prolong progression-free and overall survival while preserving quality of life — oligodendrogliomas are NOT curable but patients can live 10–20+ years with appropriate management
Seizure control is a critical aspect of management throughout the disease course

Histology and staging guide management: surgical resection, radiotherapy, systemic treatment with chemotherapy, immunotherapy, radiotherapy, palliative care including pain control ^[19]

3. Treatment Modalities

3.1 Medical Therapy — Symptom Management

These are given concurrently throughout all phases of disease management.

Anticonvulsants: treatment or secondary prophylaxis of seizures, usually levetiracetam (Keppra) ^[3]

NOT for infratentorial lesions, usually not for primary prophylaxis ^[3]

Why levetiracetam?

Non-enzyme-inducing: Does not interact with chemotherapy agents or dexamethasone via CYP450 induction
Older AEDs (phenytoin, carbamazepine, phenobarbital) are enzyme inducers → they accelerate metabolism of chemotherapy drugs (e.g., temozolomide), corticosteroids, and many other medications → reduced efficacy
Levetiracetam has a favourable side-effect profile, is well-tolerated, and achieves good seizure control

AED	CYP450 Interaction	Use in Brain Tumour Patients
Levetiracetam	Non-inducing	Preferred first-line
Lacosamide	Non-inducing	Alternative first-line
Valproic acid	Inhibitor (mild)	Alternative; may have anti-tumour synergy (preclinical data)
Phenytoin	Strong inducer	Avoid — ↓ chemo efficacy
Carbamazepine	Strong inducer	Avoid — ↓ chemo efficacy
Phenobarbital	Strong inducer	Avoid — ↓ chemo efficacy

Key points about seizures in oligodendroglioma:

Seizure control often improves after surgery (tumour debulking removes the epileptogenic focus)
Seizure control may also improve with chemotherapy — tumour shrinkage reduces cortical irritation (a unique feature of chemosensitive tumours like oligodendroglioma)
Primary prophylaxis (giving AEDs to a patient with a brain tumour who has NOT had a seizure) is NOT recommended by current guidelines (AAN, EANO) — there is no evidence it prevents seizures and it exposes patients to unnecessary side effects

Exam Pearl — Non-Enzyme-Inducing AEDs in Neuro-Oncology

In any brain tumour patient receiving chemotherapy, always choose a non-enzyme-inducing AED (levetiracetam, lacosamide, valproate, lamotrigine). Enzyme-inducing AEDs (phenytoin, carbamazepine, phenobarbital) reduce serum levels of temozolomide, irinotecan, and many targeted agents through CYP3A4 induction. This is a commonly examined principle.

High-dose glucocorticoids: reduction of cerebral oedema. Use: PO (or IV if severe) dexamethasone 10mg then 4mg Q4h or 8mg BD ^[3]

Why dexamethasone specifically?

Dexamethasone has minimal mineralocorticoid activity (unlike prednisolone or hydrocortisone) → less salt and water retention → less risk of worsening cerebral oedema
It is a potent glucocorticoid → stabilises the blood-brain barrier → reduces vasogenic oedema around the tumour → rapid symptomatic relief (headache, focal deficits often improve within hours)
Long half-life allows twice-daily dosing

Mechanism of action:

Vasogenic oedema occurs because the tumour disrupts the BBB → plasma proteins and fluid leak into the extracellular space → mass effect → worsened neurological function
Glucocorticoids tighten BBB endothelial tight junctions and reduce endothelial permeability → less fluid leakage → reduced oedema

Important caveats:

C/I: suspect CNS lymphoma → steroid causes acute lysis of lymphocytes → ↓ diagnostic yield in biopsy ^[3]

Side Effect	Mechanism	Clinical Relevance
Peptic ulcer	↓ prostaglandin-mediated gastric protection	Prescribe PPI prophylaxis
Immunosuppression	↓ T-cell and macrophage function	↑ infection risk, especially with concurrent chemo
Hyperglycaemia / DM	↑ gluconeogenesis, ↓ insulin sensitivity	Monitor blood glucose; may need insulin
Myopathy	Proximal muscle wasting	Prolonged use → steroid myopathy
Osteoporosis	↓ osteoblast activity, ↑ osteoclast activity	Consider Ca/VitD ± bisphosphonate if long-term
Cushingoid features	Exogenous hypercortisolism	Fat redistribution, moon face, striae

Principle: Use the lowest effective dose for the shortest duration. Taper as soon as clinically possible — especially if definitive treatment (surgery) is imminent.

3.2 Surgery — The Cornerstone

Surgical: maximal safe removal as 1st line in all operable gliomas ^[3]

Principle: maximal removal within safety limit. Aim: cytoreduction (↓ tumour mass) + functional preservation. Concept of 'resection margin' does not apply in neurosurgery ^[3]

Tissue diagnosis: You cannot confirm oligodendroglioma without tissue — molecular testing requires physical tumour samples
Cytoreduction: Greater extent of resection (EOR) correlates with longer progression-free survival (PFS) and overall survival (OS). Studies consistently show that gross total resection (GTR) or near-total resection provides the best outcomes
Symptom relief: Debulking reduces mass effect and intracranial pressure → improves headache, neurological deficits, and seizure control
Delays malignant transformation: Reducing tumour burden reduces the pool of cells that can acquire additional mutations → delays progression from Grade 2 to Grade 3

Surgical approach: craniotomy (flap of bone cut and reflected), burr hole (often for biopsy), craniectomy, others (transphenoidal, transoral) ^[3]

Approach	When Used	Details
Craniotomy with tumour resection	Most oligodendrogliomas (cortical/subcortical, accessible)	Open the skull → resect tumour under direct visualisation with intraoperative adjuncts
Stereotactic biopsy	Inoperable tumours (deep, eloquent, diffuse, poor performance status)	Frame-based or frameless needle biopsy — small sample for diagnosis only
Awake craniotomy	Tumour near eloquent cortex (speech, motor areas)	Patient is awake during cortical mapping → surgeon stimulates cortex while patient performs tasks → identifies functional areas to preserve

Image-guided surgery: frameless stereotaxy — small skull markers allowing intraoperative localisation of handheld probe in relation to skull, tumour (localised in pre-op imaging) ± important structures (by fMRI or tractography) → guidance of resection ^[3]

Technique	Purpose	How It Works
Frameless neuronavigation	Guide resection in real-time	Pre-operative MRI data co-registered with patient's head position → surgeon sees probe position on a screen overlaid on MRI images
5-ALA-guided resection	Principle: 5-aminolevulinic acid (5-ALA) accumulated in tumour cells → intraoperative UV light application → tumour shows different colour ^[3]	Oral 5-ALA pre-op → selectively metabolised to protoporphyrin IX in tumour cells → fluoresces pink/red under UV light. Helps distinguish tumour from normal brain. More useful in high-grade gliomas; less reliable in low-grade
Evoked potentials	Detection of cortical electrical response to sensory stimuli or peripheral motor response to cortical electrical stimuli ^[3]	Monitors motor and somatosensory pathways during surgery → alerts surgeon if resection threatens functional tracts
Awake craniotomy	Intraop electrical stimulation of cortex → observe speech response to map speech cortex → guide resection to avoid affecting speech ^[3]	Patient performs language tasks while cortex is stimulated → positive responses indicate eloquent cortex → surgeon avoids these areas
Intraoperative MRI	Real-time assessment of extent of resection during surgery	MRI scanner in operating theatre → surgeon can check if residual tumour remains before closing (not widely available — noted as not available in HK) ^[3]
Intraoperative ultrasound	Real-time localisation of tumour during surgery	Cheaper alternative to iMRI; helps identify tumour borders

Extent of Resection	Definition	Impact
Gross total resection (GTR)	No visible residual tumour on post-op MRI (within 24–72 hours)	Best outcomes — longer PFS and OS
Near-total resection (NTR)	> 90% resection, small residual	Good outcomes; adjuvant Tx may compensate
Subtotal resection (STR)	< 90% resection	Worse outcomes; adjuvant Tx always indicated
Biopsy only	Diagnostic only, no cytoreduction	Worst surgical outcome; adjuvant Tx essential

Greater extent of resection is consistently associated with longer survival in oligodendroglioma, even after adjusting for grade and molecular markers. Aim for GTR whenever safely achievable.

Factor	Consideration
Tumour in eloquent cortex	Motor strip, Broca's area, Wernicke's area → GTR may cause unacceptable deficit → consider awake craniotomy, accept STR, or biopsy only
Deep/midline location	Corpus callosum, basal ganglia, thalamus, brainstem → higher surgical risk → stereotactic biopsy preferred
Bilateral/diffuse infiltration	Extensive bilateral disease → surgery cannot achieve meaningful debulking
Poor performance status	KPS < 60 or significant comorbidities → may not tolerate GA/craniotomy
Patient refusal	Informed consent is mandatory

Post-op complications ^[3] include:

Complication	Mechanism
Haemorrhage (epidural, subdural, intracerebral)	Surgical trauma to vessels
Cerebral oedema	Post-surgical inflammation and BBB disruption
New or worsened neurological deficit	Injury to eloquent cortex or white matter tracts during resection
Infection (wound, meningitis, abscess)	Breach of sterile barriers
CSF leak	Breach of dura/arachnoid → CSF tracks to wound or nasal cavity
Seizures (new or worsened)	Cortical irritation from surgery
DVT/PE	Immobilisation, hypercoagulable state from malignancy
Hydrocephalus	If CSF pathways are obstructed by blood or oedema post-operatively

3.3 Radiotherapy (RT)

Why does RT work?

Ionising radiation damages DNA in tumour cells → causes double-strand breaks → if the cell cannot repair the damage, it undergoes apoptosis or mitotic catastrophe
Normal brain tissue has superior DNA repair mechanisms compared to tumour cells → this "therapeutic ratio" allows preferential tumour kill while sparing normal tissue (to a degree)
RT is NOT curative in diffuse gliomas (because infiltrating cells extend beyond the visible tumour), but it significantly delays progression

RT Modality	Description	Use in Oligodendroglioma
External beam RT (EBRT) — conformal	Focused radiation beams shaped to the tumour volume (3D-CRT or IMRT)	Standard modality for adjuvant RT
IMRT (Intensity-Modulated RT)	Advanced conformal technique with variable beam intensity → better dose distribution → less dose to surrounding normal brain	Preferred technique when available
Proton beam therapy	Uses protons instead of photons → superior dose conformity (Bragg peak) → less exit dose to normal tissue	Available in few centres; theoretical advantage in reducing long-term neurotoxicity; not widely available in HK
Stereotactic radiosurgery (SRS)	Single high-dose fraction to a small, well-defined target	Not standard for diffuse gliomas (infiltrative margins make focal SRS less suitable); used mainly for brain metastases ^[3]

Indication	Dose	Schedule
Grade 2 oligodendroglioma (if RT indicated)	50.4–54 Gy	28–30 fractions over 5.5–6 weeks
Grade 3 anaplastic oligodendroglioma	59.4–60 Gy	30–33 fractions over 6–6.5 weeks

Why fractionation?

Dividing the total dose into small daily fractions exploits the 4 Rs of radiobiology: Repair (normal tissue repairs sublethal damage between fractions better than tumour), Redistribution (cells in resistant cell-cycle phases enter sensitive phases), Reoxygenation (hypoxic tumour cells become oxygenated between fractions → more radiosensitive), Repopulation (normal tissue repopulates faster than tumour)

Grade	When RT Is Indicated
Grade 2	High-risk features: age ≥ 40, subtotal resection, large tumour, progressive neurological deficits, high symptom burden. If low-risk (age < 40 + GTR), may defer RT and observe with serial MRI
Grade 3	Always indicated after maximal safe resection

Factor	Consideration
Prior RT to the same field	Re-irradiation is possible but limited by cumulative dose to normal brain → higher risk of radiation necrosis
Very young children	RT to the developing brain causes severe long-term neurocognitive toxicity → defer or avoid if possible
Very poor performance status	If life expectancy is < 3–6 months, RT may not provide meaningful benefit
Patient preference	Some patients (especially young, asymptomatic Grade 2) opt to defer RT to avoid immediate side effects

Timing	Side Effect	Mechanism
Acute (during RT)	Fatigue, scalp erythema, alopecia (within field), headache, nausea	Direct tissue irritation and inflammation
Early-delayed (1–6 months)	"Pseudoprogression" — transient worsening on MRI that mimics tumour progression	Inflammatory response, BBB disruption, demyelination → oedema and enhancement that can be mistaken for tumour growth
Late ( > 6 months)	Radiation necrosis, neurocognitive decline, leukoencephalopathy, endocrinopathy (if pituitary in field), radiation-induced tumours (decades later), cerebrovascular disease	Vascular endothelial damage → ischaemia → necrosis; white matter demyelination; direct pituitary damage

Pseudoprogression vs. True Progression

After chemoradiation, MRI may show increased enhancement and oedema within 3–6 months. This can be pseudoprogression (treatment-related inflammation) rather than true tumour growth. Pseudoprogression is more common in MGMT-methylated tumours (which most oligodendrogliomas are). Advanced imaging (MR perfusion, MR spectroscopy, amino acid PET) can help distinguish the two. Do not reflexively escalate treatment — observe and repeat imaging in 4–8 weeks.

3.4 Chemotherapy — The Advantage of 1p/19q Codeletion

This is where oligodendroglioma's molecular biology translates directly into clinical benefit. The 1p/19q codeletion predicts exceptional chemosensitivity, particularly to alkylating agents.

MGMT methylation: Most oligodendrogliomas have MGMT promoter methylation (as part of the global G-CIMP phenotype caused by IDH mutation). MGMT (O6-methylguanine-DNA methyltransferase) is a DNA repair enzyme that removes alkyl groups from DNA. When MGMT is silenced by methylation → the tumour cannot repair alkylating agent-induced DNA damage → more cell death
1p/19q codeletion itself: Associated with enhanced sensitivity through incompletely understood mechanisms — possibly loss of DNA repair genes on these chromosomal arms
Intrinsically lower proliferative rate: Gives more time for drug-induced DNA damage to accumulate

Chemotherapy Regimens

The landmark regimens proven in large randomised trials (RTOG 9402, EORTC 26951):

Drug	Mechanism	Route	Notable Side Effects
Procarbazine	Alkylating agent — methylates DNA → cross-links → cell death	Oral	Myelosuppression, nausea, MAO inhibitor effect (tyramine-rich food interaction), neurotoxicity
CCNU (Lomustine)	Nitrosourea alkylating agent — crosses BBB well → DNA cross-links	Oral	Cumulative myelosuppression (especially thrombocytopenia), pulmonary fibrosis (cumulative)
Vincristine	Vinca alkaloid — binds tubulin → inhibits mitotic spindle assembly → mitotic arrest	IV	Peripheral neuropathy (dose-limiting), constipation (autonomic), minimal myelosuppression

PCV regimen schedule (standard):

Lomustine 110 mg/m² PO Day 1
Procarbazine 60 mg/m² PO Days 8–21
Vincristine 1.4 mg/m² IV Days 8 and 29
Cycle length: 6 weeks; typically 6 cycles adjuvant

Evidence: The RTOG 9402 and EORTC 26951 trials demonstrated that RT + PCV significantly improved both PFS and OS in 1p/19q-codeleted oligodendrogliomas compared to RT alone. Median OS with RT + PCV exceeded 14 years in the codeleted subgroup — a truly remarkable result for a brain tumour.

Temozolomide-based chemoradiation: standard therapy following maximal safe resection. Concomitant use with external RT then adjuvant chemo for 6 cycles (5 days each, Q1 month) ^[3]

Feature	Details
Mechanism	Oral alkylating agent → methylates DNA at O6-guanine, N7-guanine → DNA mismatch repair attempts to excise → futile cycling → double-strand breaks → apoptosis
Route	Oral (excellent oral bioavailability ~100%)
BBB penetration	Good — one of few chemo agents that crosses the BBB effectively
Schedule (Stupp protocol)	Concomitant: 75 mg/m²/day during RT (42 consecutive days) → then adjuvant: 150–200 mg/m² Days 1–5 of each 28-day cycle × 6–12 cycles
Key side effects	Myelosuppression (lymphopenia → ↑ PJP risk → prescribe co-trimoxazole prophylaxis), nausea (give ondansetron), fatigue, hepatotoxicity (rare)

TMZ vs. PCV in oligodendroglioma:

Feature	PCV	TMZ
Level of evidence	Strongest (Level I RCT data from RTOG 9402, EORTC 26951)	Less direct evidence for 1p/19q-codeleted tumours specifically (CATNON trial was for astrocytomas); CODEL trial ongoing
Tolerability	Worse — cumulative haematological toxicity, neurotoxicity, nausea	Better tolerated — main issue is lymphopenia
Route	Oral + IV	Fully oral
Convenience	Complex 6-week cycle	Simple 5/28-day cycle
Clinical practice	Preferred by many neuro-oncologists based on RCT data	Widely used in practice due to better tolerability; often substituted for PCV

PCV vs. TMZ — Current Practice 2025/2026

The CODEL trial (cooperative group RCT specifically for 1p/19q-codeleted oligodendrogliomas) is addressing whether TMZ alone, RT + TMZ, or RT + PCV is optimal. Interim results suggest RT + chemotherapy (PCV or TMZ) is superior to TMZ alone. Until final results, both RT + PCV and RT + TMZ are considered acceptable standard-of-care options. Many centres use TMZ due to tolerability, but PCV has the strongest long-term survival data.

Grade	Setting	Regimen
Grade 2, high-risk	Adjuvant after surgery	RT followed by PCV (6 cycles) — strongest evidence; OR RT + concurrent/adjuvant TMZ
Grade 2, low-risk	Observation	Chemotherapy deferred until progression
Grade 3	Adjuvant after surgery (always)	RT + PCV (strongest evidence) OR RT + TMZ (Stupp-like protocol)
Recurrence	Salvage	TMZ (if PCV-naive), PCV (if TMZ-naive), re-challenge with prior agent, clinical trial, bevacizumab

Anti-angiogenesis agent, e.g., bevacizumab (2nd line) ^[3]

Feature	Details
Drug	Bevacizumab (Avastin)
Mechanism	Monoclonal antibody against VEGF-A (vascular endothelial growth factor) → inhibits tumour angiogenesis → "starves" the tumour of blood supply; also reduces vascular permeability → ↓ oedema
Role in oligodendroglioma	Salvage / 2nd line — used at recurrence when chemotherapy options are exhausted
Benefits	Rapid reduction in contrast enhancement and oedema → symptomatic improvement; allows steroid dose reduction ("steroid-sparing effect")
Limitations	Does NOT improve overall survival in gliomas (only PFS); "pseudo-response" — enhancement decreases but tumour may continue to infiltrate on FLAIR
Side effects	Hypertension, proteinuria, poor wound healing (hold before/after surgery), thromboembolic events, GI perforation (rare), intracranial haemorrhage (rare)

CSF shunting for hydrocephalus (e.g., in posterior fossa lesions, leptomeningeal meningitis, CSF mets) ^[3]

If the tumour causes obstructive hydrocephalus (e.g., large tumour near the foramen of Monro or third ventricle):

Procedure	Indication	Details
External Ventricular Drain (EVD)	Emergency temporising	Burr hole → catheter into lateral ventricle → external collection bag. Temporising measure.
Endoscopic Third Ventriculostomy (ETV)	Obstructive hydrocephalus	Creates a hole in the floor of the third ventricle → CSF bypasses the obstruction → flows directly into the prepontine cistern
Ventriculoperitoneal (VP) shunt	Persistent hydrocephalus	Catheter from lateral ventricle → subcutaneous tunnel → peritoneal cavity. Permanent diversion. Risk: infection, over-drainage, peritoneal seeding (very rare in gliomas)

3.7 Grade-Specific Management Summary

Risk Category	Criteria	Management
Low risk	Age < 40 AND GTR achieved AND minimal symptoms	Watchful waiting may be adopted ^[3] — serial MRI Q3–6 months for first 2 years, then Q6–12 months. Treat at progression.
High risk	Age ≥ 40 OR subtotal resection OR significant residual tumour OR neurological deficits OR uncontrolled seizures	Adjuvant RT + PCV (RTOG 9802 model) OR RT + TMZ. Start 4–6 weeks after surgery (allow wound healing).

Why observe some Grade 2 patients?

Young patients with GTR have very long natural history (median OS > 15 years even without immediate adjuvant treatment)
Early RT does not improve OS compared to deferred RT in low-risk Grade 2 gliomas (EORTC 22845 trial — though this preceded molecular classification)
Deferring RT avoids cumulative neurotoxicity — especially important for young patients who may live decades
At progression, treatment can be initiated with equal efficacy

Phase	Management
Surgery	Maximal safe resection
Adjuvant	RT + PCV (strongest evidence: RTOG 9402, EORTC 26951) OR RT + TMZ
Duration	RT: 59.4–60 Gy in 30–33 fractions; PCV: 6 cycles; TMZ: 6–12 cycles
Monitoring	Post-treatment MRI Q3 months for 2 years → Q6 months

Oligodendrogliomas inevitably recur — the goal at recurrence is to maximise quality of life and extend survival:

Option	When Considered
Re-resection	If tumour is accessible, causing significant mass effect, and patient has good performance status
Re-irradiation	If sufficient time since prior RT ( > 1–2 years) and recurrence is focal; limited by cumulative dose toxicity
Alternative chemotherapy	Switch regimen (TMZ if PCV-naive, or vice versa); rechallenge with same agent if long treatment-free interval
Bevacizumab	Salvage setting — symptom improvement, steroid-sparing, but no OS benefit
Clinical trials	Targeted therapies (IDH inhibitors — vorasidenib), immunotherapy, novel agents
Best supportive care	If performance status is poor, all options exhausted

Vorasidenib — A Paradigm Shift for IDH-Mutant Gliomas

Vorasidenib is a dual IDH1/IDH2 inhibitor that crosses the BBB. The INDIGO trial (published 2023) showed that vorasidenib significantly prolonged PFS compared to placebo in patients with Grade 2 IDH-mutant gliomas (including oligodendrogliomas) after surgery alone. This represents the first targeted therapy for IDH-mutant gliomas and may change the management paradigm for Grade 2 oligodendroglioma — potentially offering an alternative to upfront RT/chemo for residual disease. FDA-approved in 2024. This is cutting-edge and likely to be examined.

Applicable throughout all phases, especially at recurrence:

Domain	Interventions
Rehabilitation	Physiotherapy (motor deficits), occupational therapy (ADLs), speech therapy (aphasia), cognitive rehabilitation
Psychosocial support	Counselling, social work, support groups — brain tumour patients face unique challenges (personality change, cognitive decline, loss of driving licence, employment difficulties)
End-of-life care	Pain management, seizure prophylaxis (rectal/buccal midazolam for breakthrough seizures at home), advance care planning, hospice referral
Driving restrictions	Most jurisdictions require seizure-free interval (often 6–12 months) before resuming driving after a brain tumour diagnosis/surgery/seizure

Prognosis ^[3]:

Factor	Favourable	Unfavourable
Molecular	IDH-mutant + 1p/19q-codeleted (= oligodendroglioma)	IDH-wildtype; CDKN2A/B homozygous deletion
Grade	Grade 2	Grade 3
Age	Younger (< 40)	Older (≥ 40–50)
Extent of resection	GTR	STR or biopsy only
Performance status	KPS ≥ 70	KPS < 70
Neurological function	Preserved	Significant deficits
MGMT methylation	Methylated (chemosensitive)	Unmethylated (less responsive to alkylating agents)

Survival estimates:

Subgroup	Median Overall Survival
Grade 2 oligodendroglioma, GTR, adjuvant Tx	> 15–20 years
Grade 2 oligodendroglioma, observation	12–15 years
Grade 3 oligodendroglioma, RT + PCV	~14 years (RTOG 9402)
Grade 3 oligodendroglioma, RT alone	~7 years

Oligodendroglioma has the best prognosis among all diffuse gliomas — dramatically better than GBM (15 months) or IDH-mutant astrocytoma (5–8 years for Grade 3).

High Yield Summary — Management of Oligodendroglioma

Surgery first: Maximal safe resection is the cornerstone — greater extent of resection = better survival. Concept of resection margin does not apply in neurosurgery (brain tumours infiltrate diffusely).
Grade 2, low-risk: Observation with serial MRI after GTR. Treat at progression.
Grade 2, high-risk: Adjuvant RT + PCV (or RT + TMZ) after surgery.
Grade 3: Always RT + PCV (or RT + TMZ) after surgery.
PCV has strongest RCT evidence (RTOG 9402, EORTC 26951) showing median OS > 14 years in 1p/19q-codeleted tumours. TMZ is widely used for tolerability but CODEL trial results are pending.
Chemosensitivity of oligodendroglioma is driven by: 1p/19q codeletion + MGMT methylation (from IDH-driven G-CIMP).
Non-enzyme-inducing AEDs (levetiracetam) for seizures — avoid phenytoin/carbamazepine with chemotherapy.
Dexamethasone for cerebral oedema — but NEVER before biopsy if PCNSL is suspected.
Bevacizumab is salvage/2nd-line — reduces oedema and enhancement but does NOT improve OS.
Vorasidenib (IDH1/2 inhibitor) — first targeted therapy for IDH-mutant gliomas; INDIGO trial showed PFS benefit in Grade 2; FDA-approved 2024.
Intraoperative adjuncts: neuronavigation, fMRI, DTI tractography, awake craniotomy, evoked potentials, 5-ALA fluorescence.

Active Recall - Management of Oligodendroglioma

References

^[3] Senior notes: Ryan Ho Neurology.pdf (p.163–165 — Management of brain tumours, surgery, medical therapy, glioma management, prognosis) ^[10] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.431 — Glioma classification, MRI appearances) ^[19] Lecture slides: Cancer Block - Diagnostic work up and investigations for lung cancer.pdf (p.26 — Summary: histology and staging guide management principles)

Complications of Oligodendroglioma

Complications in oligodendroglioma arise from three distinct sources: the tumour itself (disease-related), the treatment (iatrogenic), and the long-term consequences of living with a chronic brain tumour. Understanding these is critical because oligodendroglioma patients survive for decades — far longer than most brain tumour patients — meaning they accumulate more long-term complications than, say, a GBM patient who survives only 15 months.

Grade II/III: treatable but incurable → almost ALL will recur in ~10y with ↑ grading ^[3] ^[10]

This single fact — that oligodendroglioma is treatable but incurable — shapes the entire approach to complications. Every treatment decision involves balancing tumour control against the cumulative toxicity burden over a potentially 15–20+ year disease course.

Why does this happen? As the tumour grows (or recurs at higher grade), it occupies increasing intracranial volume. The Monro-Kellie doctrine states that the cranial vault has a fixed volume containing brain (~80%), blood (~10%), and CSF (~10%). Any increase in one component must be compensated by a decrease in the others — once compensatory mechanisms are exhausted, ICP rises exponentially.

Complication	Mechanism	Clinical Features
Raised ICP	Mass effect from tumour + perilesional vasogenic oedema + ± obstructive hydrocephalus	Headache (worse morning, with Valsalva), vomiting, papilloedema, CN VI palsy, ↓consciousness
Obstructive hydrocephalus	Large tumour compressing foramen of Monro or third ventricle → blocks CSF outflow from lateral ventricles	Acute decompensation with rapid ↓GCS, vomiting, upgaze palsy (Parinaud syndrome from tectal compression)
Uncal (transtentorial) herniation	Expanding temporal lobe tumour pushes medial temporal lobe (uncus) through tentorial notch → compresses CN III → then cerebral peduncle → then brainstem	Ipsilateral fixed dilated pupil (CN III palsy), contralateral hemiplegia, then bilateral signs, reduced consciousness ^[12]
Subfalcine herniation	Expanding frontal tumour pushes cingulate gyrus under falx cerebri	Contralateral leg weakness (ACA compression), may kink anterior cerebral artery → ACA territory infarct
Tonsillar herniation	Terminal event — cerebellar tonsils herniate through foramen magnum → compress medulla	Cushing's triad (systemic HT, bradycardia, respiratory depression) ^[12] → cardiorespiratory arrest

Herniation Is a Neurosurgical Emergency

Brain herniation is the immediate cause of death in most brain tumour patients who die from disease progression. Recognising the early signs — particularly a new CN III palsy (dilated pupil, ptosis) in a patient with a known brain tumour — requires immediate action: IV dexamethasone, consider mannitol/hypertonic saline, emergency neurosurgical consultation, and possible emergency decompression or EVD.

Seizures are the most common presenting symptom, but they also represent an ongoing complication throughout the disease course:

Drug-resistant epilepsy: ~30–40% of oligodendroglioma patients develop seizures that are difficult to control despite appropriate AEDs. The epileptogenic zone may be larger than the visible tumour because infiltrating tumour cells alter neuronal networks beyond the MRI-visible margin.
Status epilepticus: Prolonged seizure activity (> 5 minutes for convulsive SE) is a medical emergency. In the context of oligodendroglioma, SE can be triggered by tumour progression, medication non-compliance, electrolyte disturbance, or intercurrent illness. It causes neuronal death from excitotoxicity and can be fatal if untreated.
Sudden unexpected death in epilepsy (SUDEP): A rare but recognised risk in any patient with epilepsy, including tumour-related epilepsy.

Why is seizure control particularly important? Beyond the immediate danger, uncontrolled seizures prevent patients from driving, working, and living independently — profoundly impacting quality of life over a disease course lasting decades.

As the tumour infiltrates and destroys brain tissue, deficits worsen:

Location	Progressive Deficit	Impact on Patient
Frontal lobe	Executive dysfunction → personality change → abulia → global cognitive decline	Loss of independence, caregiver burden, may mimic dementia
Motor cortex	Progressive hemiparesis → hemiplegia	Wheelchair dependence, ↑fall risk
Dominant hemisphere	Progressive aphasia (expressive and/or receptive)	Communication breakdown, social isolation
Parietal lobe	Hemisensory loss, hemispatial neglect	↑fall risk, inability to self-care
Visual pathway	Progressive homonymous hemianopia	Loss of driving licence, ↑fall risk

This is one of the most important long-term complications:

Almost ALL will recur in ~10y with ↑ grading ^[3] ^[10]

Grade 2 → Grade 3: Over years to decades, Grade 2 oligodendrogliomas acquire additional mutations (CDKN2A/B deletion, PI3K pathway activation, ↑ proliferative fraction) → transform into anaplastic (Grade 3) oligodendroglioma
The transformed tumour is more aggressive, faster-growing, and less treatment-responsive — though still better than GBM
Clinical manifestation: Worsening symptoms, new enhancement on MRI (previously non-enhancing), more rapid tumour growth
Unlike the astrocytoma lineage, oligodendrogliomas do NOT transform into IDH-wildtype GBM — they maintain their IDH-mutant, 1p/19q-codeleted identity but become more aggressive within that lineage

Intratumoral haemorrhage is uncommon but can occur, particularly in higher-grade or vascular tumours
Presents with acute deterioration: sudden severe headache, rapid neurological worsening, ↓GCS
May mimic a primary intracerebral haemorrhage on initial CT — always consider underlying tumour in a non-hypertensive patient with lobar haemorrhage
Management: emergency neurosurgical evaluation, possible craniotomy for clot evacuation

Brain tumour patients are in a hypercoagulable state (Trousseau's phenomenon) — the tumour releases tissue factor and other procoagulants
DVT and pulmonary embolism are common — incidence up to 20–30% over the disease course
Compounded by immobility (post-surgical, neurological deficits), and corticosteroid use
Management: therapeutic anticoagulation with LMWH (preferred) or DOACs — the risk of intracranial haemorrhage is generally low and acceptable in most brain tumour patients (a common misconception is that brain tumour is an absolute contraindication to anticoagulation — it is not)

Any intracranial pathology causing raised ICP can trigger SIADH (Syndrome of Inappropriate ADH secretion) → dilutional hyponatraemia
Presents with malaise, lethargy, confusion, seizures (worsening the seizure burden from the tumour itself)
Also precipitated by certain AEDs (carbamazepine) and chemotherapy (vincristine in PCV, cyclophosphamide)

Present from diagnosis in many patients (especially with frontal lobe tumours) and worsens progressively
Domains affected: attention, executive function, processing speed, memory
May mimic or be misdiagnosed as frontotemporal dementia or depression ^[5]
Caused by: direct tumour infiltration, perilesional oedema, seizure-related injury, and disruption of white matter networks even at a distance from the tumour

Complication	Mechanism	Timing
Post-operative haemorrhage	Bleeding from tumour bed or bridging veins	Hours to days
New neurological deficit	Injury to eloquent cortex or white matter tracts during resection	Immediate post-op
Wound infection / meningitis	Breach of sterile barriers, CSF leak	Days to weeks
CSF leak / CSF fistula	Dural closure failure → CSF tracks to wound or nasal cavity → ascending infection risk	Days to weeks
Post-operative seizures	Cortical irritation from surgical manipulation, gliosis	Days to weeks (acute); long-term
Cerebral oedema	Post-surgical inflammation, BBB disruption	Days
DVT/PE	Immobilisation + hypercoagulable state	Weeks
Pneumocephalus	Air entry into cranial cavity after craniotomy	Hours to days; usually self-limiting

These are critically important because oligodendroglioma patients live long enough to develop late radiation effects:

Timing	Complication	Mechanism	Clinical Significance
Acute (during RT)	Fatigue, scalp erythema, focal alopecia, headache, nausea	Direct tissue inflammation	Self-limiting; manage symptomatically
Early-delayed (1–6 months)	Pseudoprogression — MRI shows ↑enhancement and oedema mimicking recurrence	Inflammatory response, transient BBB disruption, demyelination	Must distinguish from true progression — repeat MRI in 4–8 weeks; use advanced imaging
Early-delayed (1–6 months)	Somnolence syndrome	Transient demyelination	Excessive sleepiness, resolves spontaneously
Late ( > 6 months)	Radiation necrosis	Vascular endothelial damage → ischaemia → coagulative necrosis	Focal deficits, mass effect; can mimic recurrence; may require bevacizumab or surgical excision
Late (years)	Neurocognitive decline	White matter demyelination, microangiopathy, cortical thinning	Progressive memory loss, executive dysfunction, ↓processing speed; major long-term concern
Late (years)	Endocrine abnormalities: ↓GH (94%), ↓ACTH (43%), ↓TSH (10%)	Pituitary/hypothalamic damage from radiation scatter ^[17]	Fatigue, weight gain, adrenal crisis — monitor endocrine function annually
Late (years)	Leukoencephalopathy	Diffuse white matter injury from radiation	Gait disturbance, incontinence, dementia — may mimic normal pressure hydrocephalus
Late (decades)	Radiation-induced secondary malignancy	Radiation causes DNA damage in normal cells → carcinogenesis	Meningiomas, sarcomas, GBM in the irradiated field — typically > 10 years after RT
Late (years)	Cerebrovascular abnormalities (stroke, intracranial haemorrhage) ^[17]	Radiation accelerates atherosclerosis, causes vasculopathy (Moyamoya-like) in irradiated vessels	↑ risk of ischaemic stroke and haemorrhage in the irradiated field
Late	Optic neuropathy	Radiation damage to optic nerves/chiasm if in field	Progressive visual loss — may be irreversible
Late	Hearing loss	Cochlear damage (especially if temporal lobe irradiated)	Sensorineural hearing loss

Late Radiation Effects — The Long-Term Price of Cure

Because oligodendroglioma patients survive 10–20+ years, they are uniquely vulnerable to late radiation effects that shorter-surviving brain tumour patients never live long enough to develop. Neurocognitive decline is the most common and most devastating — it is cumulative, progressive, and largely irreversible. This is precisely why RT is deferred in young, low-risk Grade 2 patients: to delay the onset of cognitive toxicity as long as possible. Every year of RT deferral is a year of preserved cognitive function.

2.3 Chemotherapy Complications

Drug	Complication	Mechanism
Procarbazine	Nausea/vomiting (severe)	Direct GI mucosal irritation and central emetic zone stimulation
Procarbazine	MAO inhibitor effect → hypertensive crisis	Procarbazine inhibits monoamine oxidase → tyramine in cheese, wine cannot be metabolised → sympathomimetic crisis. Patients must follow a tyramine-restricted diet.
Procarbazine	Allergic rash, peripheral neuropathy	Hypersensitivity; direct neurotoxicity
CCNU (Lomustine)	Cumulative myelosuppression (especially thrombocytopenia)	Cumulative damage to haematopoietic stem cells; dose-limiting toxicity — worse with each cycle
CCNU	Pulmonary fibrosis (cumulative dose-dependent)	Direct pneumocyte toxicity
CCNU	Hepatotoxicity	Direct hepatocellular damage
Vincristine	Peripheral neuropathy (dose-limiting)	Disrupts axonal microtubules → impaired axonal transport → sensorimotor neuropathy (glove-and-stocking pattern)
Vincristine	Constipation / paralytic ileus	Autonomic neuropathy affecting gut motility
All agents	Bone marrow suppression → pancytopenia → infection, bleeding	Direct cytotoxic effect on rapidly dividing haematopoietic precursors

Complication	Mechanism	Management
Myelosuppression (lymphopenia > thrombocytopenia > neutropenia)	Alkylating agent damages haematopoietic precursors	Monitor CBC regularly; dose-reduce or hold if Grade 3–4 cytopenias
Lymphopenia → Pneumocystis jirovecii pneumonia (PJP)	Profound CD4+ T-cell depletion (combination of TMZ + dexamethasone + tumour immunosuppression)	Prophylactic co-trimoxazole (TMP-SMX) throughout concurrent chemoradiation — this is mandatory
Nausea/vomiting	Central emetogenic effect	Pre-medicate with ondansetron (5-HT3 antagonist)
Hepatotoxicity	Drug-induced liver injury	Monitor LFTs periodically
Fatigue	Multifactorial — anaemia, direct CNS effect, cytokine release	Supportive; exclude treatable causes
Teratogenicity	Alkylating agent → DNA damage to germ cells	Contraception mandatory during and 6 months after treatment; fertility counselling pre-treatment
Secondary myelodysplasia / AML	Alkylating agent damage to haematopoietic stem cells → clonal haematopoiesis → MDS/AML	Rare; occurs years later; monitor CBC long-term

Complication	Mechanism
Hypertension	VEGF inhibition → endothelial dysfunction → ↑ vascular tone
Proteinuria / nephrotic syndrome	VEGF is required for glomerular endothelial integrity → loss of filtration barrier
Poor wound healing	VEGF is essential for angiogenesis in wound repair — hold bevacizumab ≥ 28 days before and after surgery
Thromboembolic events (arterial > venous)	Endothelial dysfunction → platelet activation
GI perforation (rare)	Impaired mucosal healing
Intracranial haemorrhage (rare but feared)	Fragile tumour vasculature + antiangiogenic effect

3. Long-Term Survivorship Complications

These are unique to oligodendroglioma because patients survive long enough to experience them:

The most devastating long-term complication — caused by a combination of:

Tumour infiltration damaging white matter networks
Surgery disrupting cortical and subcortical connections
Radiation-induced leukoencephalopathy (progressive white matter damage)
Chemotherapy-related cognitive toxicity ("chemo brain")
Chronic AED use (some agents impair cognition — especially older drugs)
Cumulative seizure-related hippocampal injury

Pattern: Progressive decline in attention, processing speed, executive function, and memory. Personality may change (apathy, disinhibition). Eventually may reach criteria for dementia.

Management: Neuropsychological monitoring, cognitive rehabilitation, optimise AED regimen (avoid sedating agents), treat depression, consider methylphenidate or donepezil (limited evidence but sometimes tried).

Domain	Issue
Employment	Cognitive decline + seizures + focal deficits → difficulty maintaining employment → financial hardship
Driving	Seizures and visual field defects → loss of driving licence in most jurisdictions
Relationships	Personality change, mood disorders, caregiver burden → relationship strain
Mental health	Depression (30–50%), anxiety, adjustment disorders; frontal lobe tumours → apathy or disinhibition
Fertility	Alkylating chemotherapy and cranial RT → gonadotoxicity → infertility. Fertility preservation should be discussed before treatment

Endocrine abnormalities after RT: ↓GH (94%), ↓ACTH (43%), ↓TSH (10%) ^[17]

If the radiation field includes or scatters to the hypothalamic-pituitary axis:

GH deficiency (most common — hypothalamus is most radiosensitive) → fatigue, ↓muscle mass, ↑ adiposity
ACTH deficiency → secondary adrenal insufficiency → fatigue, hypotension, may precipitate adrenal crisis during illness
TSH deficiency → secondary hypothyroidism → fatigue, weight gain, constipation
Gonadotropin deficiency → hypogonadism → amenorrhoea (women), ↓libido, osteoporosis

Management: Annual endocrine screening (fasting GH, IGF-1, cortisol, TSH, fT4, LH, FSH, oestradiol/testosterone) and hormone replacement as needed.

Meningiomas, sarcomas, and even GBM can develop within the irradiated field, typically > 10–20 years after RT
Incidence: ~2–5% at 20 years post-RT
Present as new neurological symptoms or new lesion on surveillance MRI
Must be distinguished from recurrence of the original oligodendroglioma

Category	Key Complications	Timing
Tumour-related	Raised ICP, herniation, status epilepticus, progressive deficits, malignant transformation, VTE, SIADH, haemorrhage	Throughout disease course
Surgery-related	Haemorrhage, new deficits, infection, CSF leak, seizures	Perioperative
RT-related	Pseudoprogression (early), radiation necrosis, neurocognitive decline, endocrinopathy, cerebrovascular disease, secondary malignancy (late)	Weeks to decades
Chemo-related	Myelosuppression, PJP (TMZ), peripheral neuropathy (vincristine), cumulative thrombocytopenia (CCNU), nausea, infertility, secondary MDS/AML	During and after treatment
Long-term survivorship	Neurocognitive decline, psychosocial dysfunction, endocrinopathy, infertility, secondary malignancy	Years to decades

High Yield Summary — Complications of Oligodendroglioma

Malignant transformation: Grade 2 → Grade 3 over ~10 years is virtually inevitable. Treatable but incurable.
Cerebral herniation (uncal, subfalcine, tonsillar) is the final common pathway of death from progressive disease — recognise early signs (CN III palsy, ↓GCS, Cushing's triad).
Drug-resistant epilepsy occurs in 30–40% — impacts quality of life, driving, employment.
Late radiation effects are the most important long-term concern: neurocognitive decline (progressive, irreversible), radiation necrosis, endocrinopathy, cerebrovascular disease, secondary malignancy. This is why RT is deferred in low-risk Grade 2 patients.
TMZ causes lymphopenia → mandatory PJP prophylaxis with co-trimoxazole during concurrent chemoradiation.
PCV complications: Procarbazine has MAO inhibitor activity (tyramine restriction); vincristine causes dose-limiting peripheral neuropathy; CCNU causes cumulative thrombocytopenia.
VTE is common (20–30% over disease course) — brain tumour is NOT an absolute contraindication to anticoagulation.
Pseudoprogression vs. true progression: Must be distinguished after chemoradiation — more common in MGMT-methylated tumours (i.e., most oligodendrogliomas). Do not reflexively escalate treatment.
Fertility: Alkylating agents and cranial RT are gonadotoxic — counsel and offer fertility preservation before treatment.
Psychosocial: Depression, loss of driving, employment difficulties, caregiver burden — oligodendroglioma is a chronic neurological illness requiring holistic multidisciplinary care.

Active Recall - Complications of Oligodendroglioma

References

^[3] Senior notes: Ryan Ho Neurology.pdf (p.163–165 — Management complications, prognosis, post-op complications, medical therapy side effects) ^[5] Senior notes: Ryan Ho Psychiatry.pdf (p.94 — FTD differential including slow-growing mass lesions) ^[10] Senior notes: Adrian Lui Pediatrics Notes.pdf (p.431 — Glioma prognosis: treatable but incurable, recurrence with increased grading) ^[12] Senior notes: Maksim Medicine Notes.pdf (p.278 — Raised ICP clinical features, herniation signs, Cushing's triad) ^[17] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (p.1262–1264 — RT complications including endocrine abnormalities, cerebrovascular abnormalities, surgical complications)

High Yield Summary

Oligodendroglioma — Key Points for Exams

Definition: Diffuse glioma arising from oligodendrocytes; WHO 2021 requires IDH mutation + 1p/19q codeletion for diagnosis.
Epidemiology: 5–18% of gliomas; peak age 40–60; slight male predominance; supratentorial (frontal lobe most common).
Molecular triad: IDH mutation + 1p/19q codeletion + TERT promoter mutation.
Pathogenesis: IDH mutation → 2-HG oncometabolite → epigenetic reprogramming → 1p/19q codeletion (unbalanced translocation) → loss of CIC/FUBP1 tumour suppressors.
Histology: "Fried egg" cells + "chicken wire" capillaries + calcification. ATRX retained (cf. lost in astrocytoma).
Grading: Grade 2 (low-grade) vs. Grade 3 (anaplastic — increased mitoses, ± microvascular proliferation, ± necrosis).
Most common presentation: Seizures (50–80%) — due to cortical involvement and slow growth causing chronic irritation.
Other presentations: Headache (↑ICP), cognitive/personality changes (frontal lobe), focal neurological deficits.
Imaging hallmark: Calcified cortical/subcortical mass on CT, usually frontal.
Prognosis: Best among diffuse gliomas — 1p/19q codeletion predicts chemosensitivity and better survival.
Key distinction from astrocytoma: Oligodendroglioma = IDH-mut + 1p/19q codel + ATRX retained; Astrocytoma = IDH-mut + ATRX loss + TP53 mutation (mutually exclusive pathways).

High Yield Summary — DDx of Oligodendroglioma

Most important differential: Diffuse astrocytoma, IDH-mutant — distinguished by 1p/19q codeletion (oligodendroglioma) vs. ATRX loss/TP53 mutation (astrocytoma). These are mutually exclusive molecular pathways.
Commonest intracranial tumour in adults overall: Brain metastasis — always consider, especially if > 1 lesion or known cancer history.
Key clinical-radiological clues pointing to oligodendroglioma: Cortical/subcortical location, calcification (70–90%), frontal lobe, slow growth, minimal enhancement (Grade 2), seizures as presenting symptom.
Non-neoplastic mimics to consider: Brain abscess (fever + ring enhancement + restricted diffusion), tumefactive MS (young, relapsing-remitting), cavernoma (popcorn, hemosiderin rim), neurocysticercosis (endemic, multiple cysts).
Never give steroids before biopsy if PCNSL is suspected — steroids cause tumour lysis and can eliminate tissue for diagnosis.
WHO 2021 mandates molecular classification: Histology alone is insufficient. A tumour looking like an oligodendroglioma without IDH mutation + 1p/19q codeletion is NOT an oligodendroglioma.

High Yield Summary — Diagnosis of Oligodendroglioma

Diagnosis requires tissue + molecular confirmation: IDH mutation AND 1p/19q codeletion are both mandatory under WHO 2021. Histology alone is insufficient.
CT hallmark: Calcified cortical/subcortical mass (calcification in 70–90% — most commonly calcified glioma).
MRI is the preferred imaging modality: T2/FLAIR hyperintense, minimal enhancement in Grade 2, cortical location, frontal lobe predilection.
IHC screening: IDH1 R132H positive + ATRX retained → strongly suggests oligodendroglioma → confirm with 1p/19q FISH/array.
ATRX retained vs. lost is a rapid IHC screen to distinguish oligodendroglioma (retained) from astrocytoma (lost) — these are mutually exclusive.
FISH alone can give false positives — partial deletions at 1p36/19q13 loci do not equal whole-arm codeletion. Array-based or methylation methods are superior.
MGMT methylation is typically present in oligodendroglioma (due to G-CIMP from IDH mutation) → explains chemosensitivity.
Never LP before imaging if SOL/raised ICP suspected → risk of herniation.
Advanced MRI (fMRI, DTI tractography) is for preoperative planning to map eloquent cortex and white matter tracts.
EEG helps characterise seizures and localise the epileptic focus, not diagnose the tumour itself.

High Yield Summary — Management of Oligodendroglioma

Surgery first: Maximal safe resection is the cornerstone — greater extent of resection = better survival. Concept of resection margin does not apply in neurosurgery (brain tumours infiltrate diffusely).
Grade 2, low-risk: Observation with serial MRI after GTR. Treat at progression.
Grade 2, high-risk: Adjuvant RT + PCV (or RT + TMZ) after surgery.
Grade 3: Always RT + PCV (or RT + TMZ) after surgery.
PCV has strongest RCT evidence (RTOG 9402, EORTC 26951) showing median OS > 14 years in 1p/19q-codeleted tumours. TMZ is widely used for tolerability but CODEL trial results are pending.
Chemosensitivity of oligodendroglioma is driven by: 1p/19q codeletion + MGMT methylation (from IDH-driven G-CIMP).
Non-enzyme-inducing AEDs (levetiracetam) for seizures — avoid phenytoin/carbamazepine with chemotherapy.
Dexamethasone for cerebral oedema — but NEVER before biopsy if PCNSL is suspected.
Bevacizumab is salvage/2nd-line — reduces oedema and enhancement but does NOT improve OS.
Vorasidenib (IDH1/2 inhibitor) — first targeted therapy for IDH-mutant gliomas; INDIGO trial showed PFS benefit in Grade 2; FDA-approved 2024.
Intraoperative adjuncts: neuronavigation, fMRI, DTI tractography, awake craniotomy, evoked potentials, 5-ALA fluorescence.

High Yield Summary — Complications of Oligodendroglioma

Malignant transformation: Grade 2 → Grade 3 over ~10 years is virtually inevitable. Treatable but incurable.
Cerebral herniation (uncal, subfalcine, tonsillar) is the final common pathway of death from progressive disease — recognise early signs (CN III palsy, ↓GCS, Cushing's triad).
Drug-resistant epilepsy occurs in 30–40% — impacts quality of life, driving, employment.
Late radiation effects are the most important long-term concern: neurocognitive decline (progressive, irreversible), radiation necrosis, endocrinopathy, cerebrovascular disease, secondary malignancy. This is why RT is deferred in low-risk Grade 2 patients.
TMZ causes lymphopenia → mandatory PJP prophylaxis with co-trimoxazole during concurrent chemoradiation.
PCV complications: Procarbazine has MAO inhibitor activity (tyramine restriction); vincristine causes dose-limiting peripheral neuropathy; CCNU causes cumulative thrombocytopenia.
VTE is common (20–30% over disease course) — brain tumour is NOT an absolute contraindication to anticoagulation.
Pseudoprogression vs. true progression: Must be distinguished after chemoradiation — more common in MGMT-methylated tumours (i.e., most oligodendrogliomas). Do not reflexively escalate treatment.
Fertility: Alkylating agents and cranial RT are gonadotoxic — counsel and offer fertility preservation before treatment.
Psychosocial: Depression, loss of driving, employment difficulties, caregiver burden — oligodendroglioma is a chronic neurological illness requiring holistic multidisciplinary care.

Oligodendroglioma

1. Definition

2. Epidemiology

Incidence and Burden

Demographics

Comparison with Other Intracranial Tumours

3. Risk Factors

Established

Possible/Under Investigation

NOT Established (Common Misconceptions)

4. Anatomy and Function of Oligodendrocytes

Normal Oligodendrocyte Biology

Relevance to Tumour Behaviour

Preferred Tumour Location

5. Etiology and Pathophysiology

5.1 Molecular Pathogenesis (The "Why" of Oligodendroglioma)

Step 1: IDH Mutation — The Initiating Event

Step 2: 1p/19q Co-deletion — The Defining Event

Step 3: TERT Promoter Mutation

Step 4: Additional/Later Molecular Events

5.2 Molecular Classification Summary

5.3 Tumour Growth and Behaviour

6. Classification

6.1 WHO CNS5 (2021) Classification

6.2 Grading Criteria

6.3 Historical Classification (For Context)

7. Histopathology

Classic Histological Features

Immunohistochemistry and Molecular Markers

8. Clinical Features

8.1 Overview of Presentation

8.2 Symptoms

A. Seizures (Most Common Presenting Symptom — ~50–80%)

B. Headache (~30–50%)

C. Cognitive and Personality Changes

D. Focal Neurological Deficits (Negative Symptoms)

E. Symptoms of Raised Intracranial Pressure

F. Other Symptoms

8.3 Signs

A. Signs of Raised ICP

B. Focal Neurological Signs

C. Seizure-Related Signs

D. Signs Suggesting Herniation (Emergency)

8.4 Summary Table: Symptoms and Signs with Pathophysiological Basis

9. Imaging Characteristics (Pre-Diagnostic, For Clinical Approach)

CT Brain

MRI Brain

Active Recall - Oligodendroglioma (Definition to Clinical Features)

References

1. Clinical Context for Differential Diagnosis

2. Framework for Approaching the Differential

3. Differential Diagnosis by Category

A. Other Primary Brain Tumours (Most Important Differentials)

i. Diffuse Astrocytoma, IDH-mutant (WHO Grade 2, 3 or 4)

ii. Glioblastoma (GBM), IDH-wildtype (WHO Grade 4)

iii. Ependymoma (WHO Grade 2 or 3)

iv. Meningioma (WHO Grade 1, 2 or 3)

v. Primary CNS Lymphoma (PCNSL)

vi. Ganglioglioma / DNET (Dysembryoplastic Neuroepithelial Tumour)

vii. Pituitary Tumours / Craniopharyngioma

B. Brain Metastases

C. Non-Neoplastic Mimics

i. Brain Abscess

ii. Cerebral Infarct (Stroke)

iii. Demyelinating Disease (Tumefactive MS)

iv. Cerebral Cavernous Malformation (Cavernoma)

v. Neurocysticercosis

vi. Cortical Dysplasia (Focal)

4. Differential Diagnosis Algorithm

5. Summary: Key Differentials at a Glance

Active Recall - Differential Diagnosis of Oligodendroglioma

1. Diagnostic Criteria

1.1 WHO 2021 Diagnostic Requirements

1.2 Grading Criteria Within Oligodendroglioma

2. Diagnostic Algorithm

3. Investigation Modalities

3.1 Neuroimaging

A. CT Brain (Without and With Contrast)

B. MRI Brain (With Gadolinium Contrast) — The Preferred Modality

C. Advanced MRI Techniques (For Preoperative Planning)