GC012 Abnormal Lung Shadow On Chest Radiograph CXR, CT
An incidental or symptomatic radiographic finding on chest X-ray or CT scan representing a localized or diffuse opacity that requires systematic evaluation to determine its etiology, which may include infection, malignancy, inflammation, or vascular abnormality.
Abnormal Lung Shadow on Chest Radiograph (CXR, CT)
This GC lecture from HKU's Department of Diagnostic Radiology is fundamentally about what to do when you see something abnormal in the lung on a chest X-ray — and specifically, how to decide whether it's benign or malignant, how to characterize it, how to stage lung cancer, and how to approach diffuse lung disease. It is a radiology-oriented lecture, but it is deeply clinical because every imaging finding drives a management decision.
The lecture is structured around two major categories:
- Focal lung lesions — especially the solitary pulmonary nodule (SPN) and lung cancer
- Diffuse lung lesions — interstitial lung disease, infections (PJP, TB), bronchiectasis, emphysema
You are expected to walk away knowing how to describe a lung shadow, how to use CT (contrast CT, HRCT, PET-CT) appropriately, and when to biopsy. This lecture directly feeds into exam questions about lung nodule workup, benign vs malignant features, CT staging, and the role of ancillary investigations.
Learning Objectives (from the slides):
1. Describe abnormal lung shadow on CXR 2. Understand the clinical applications of various CT techniques 3. Understand the values of other ancillary investigations [1]
Part 1: Focal Lung Lesions
A solitary pulmonary nodule is defined as a focal lung lesion < 3 cm in diameter. [1]
This is the bread-and-butter of lung radiology questions. The clinical scenario in the lecture is a 25-year-old asymptomatic, non-smoking man with an incidental SPN on CXR. The key question is: is this benign or malignant?
Why does the size matter?
- Below 3 cm = "nodule" → could be benign, needs characterization
- Above 3 cm = "mass" → high suspicion for malignancy until proven otherwise
Why does age matter?
- In a 25-year-old non-smoker, the pre-test probability of malignancy is very low. Most SPNs in young patients are benign (granulomas, hamartomas, intrapulmonary lymph nodes).
- In a 65-year-old smoker, the same nodule carries a much higher probability of lung cancer.
SPN Workup Algorithm (Directly from Lecture Slides)
Step 1: Check previous CXR
- No growth over 2 years → Benign
- Interval growth → Suspicious → CT scan
- No previous CXR → CT scan ± Bronchoscopy ± Sputum analysis [1]
Why 2 years? Because most malignant nodules have a doubling time of 30–400 days. A nodule that has been stable for 2 full years is very unlikely to be malignant (the doubling time would be too long to be consistent with cancer).
CT thorax is used to:
- Characterize the lesion
- Determine solitary vs multiple nodules
- Stage a malignant lesion [1]
Hounsfield Units (HU) — First Principles
CT measures density or attenuation value of structures in Hounsfield Units (HU) [1]
| Substance | HU Value |
|---|---|
| Air | -1000 |
| Fat | -50 |
| Water | 0 |
| Soft tissue | 30–50 |
| Calcification | > 150 |
Why does this matter? Because the HU value tells you what the nodule is made of:
- A nodule measuring -55 HU contains fat → this is essentially diagnostic of a hamartoma [1]
- A densely calcified nodule (> 150 HU) with central/uniform calcification → benign (likely granuloma)
- A soft tissue density nodule (30–50 HU) with no calcification → needs further workup
Fat-containing nodule at -55 HU = Hamartoma [1]
High Yield Exam Point
A hamartoma is the most common benign lung tumour. It contains fat, cartilage, and sometimes "popcorn" calcification. On CT, the fat attenuation (negative HU, around -50) is virtually diagnostic. The lecture specifically shows -55 HU.
1.3 Nodule Characteristics on CT
Key characteristics to evaluate:
- Shape: round, lobulated
- Margins: smooth or spiculated
- Calcification: uniform, central, speckled, eccentric
- Fat content
- Cavitation
- Air bronchograms [1]
| Feature | Benign | Malignant |
|---|---|---|
| Margins | Well circumscribed, smooth | Ill-defined, spiculated |
| Calcification | Central, uniform | Eccentric calcification |
| Fat | Fat-containing | No fat |
| Shape | Polygonal/triangular | Irregular, lobulated |
| Position | Subpleural | N/A |
| Enhancement | < 15 HU (minimal/none) | > 20 HU |
| Other | — | Pleural retraction, heterogeneous |
Source: [1]
Why are spiculated margins malignant? Spiculations represent tumour cells infiltrating along interlobular septa, lymphatics, and alveolar walls. This "sunburst" or "corona radiata" appearance is the tumour reaching out into surrounding lung tissue — a classic sign of invasive growth.
Why is eccentric calcification suspicious? In benign lesions (granulomas), calcium deposits centrally or uniformly because the inflammatory process is concentric. In malignant lesions, calcification may be eccentric because the tumour engulfs a pre-existing calcified granuloma, or it represents dystrophic calcification in necrotic tumour tissue — the calcium is off-center.
Why does pleural retraction suggest malignancy? The tumour causes fibrosis and scarring in adjacent lung tissue, which pulls the visceral pleura inward. This tethering effect is a hallmark of desmoplastic reaction around the tumour.
High Yield
An intrapulmonary lymph node is a classic benign nodule: subpleural, polygonal/triangular, well-defined, and small. The lecture specifically shows this as a benign nodule example. [1]
Benign nodule DDx:
- Granulomata (most common — TB, fungal)
- Inflammatory/Infectious lesions
- Arteriovenous malformations
- Hamartoma (fat)
- Adenoma [1]
Cavitating lesion DDx — history and clinical presentation are important:
- Abscess: pyrexia, cough
- Infarction: haemoptysis, chest pain, SOB
- Tumour: weight loss, haemoptysis [1]
Why do lesions cavitate? Central necrosis occurs in:
- Abscess: liquefactive necrosis from infection, contents drain via bronchus, leaving an air-fluid level
- Tumour: central ischaemic necrosis in rapidly growing tumours that outgrow their blood supply (especially squamous cell carcinoma)
- Infarction: coagulative necrosis from vascular occlusion (PE), the infarcted tissue breaks down
Exam Trap
Not all cavities are cancer. The clinical context is essential. A febrile patient with a thick-walled cavity containing an air-fluid level is more likely an abscess. A cachectic smoker with a spiculated cavitating mass and no fever is more likely squamous cell carcinoma. The cavity wall thickness also matters: thin-walled cavities are more likely benign; thick, irregular walls favour malignancy.
When a nodule doesn't clearly fall into benign or malignant categories:
Contrast enhancement on CT:
(Ryan Ho notes specify the thresholds as > 25 HU malignant, 15–25 HU indeterminate, < 15 HU benign [2]. The lecture slide shows "> 20 HU enhancement" as the key discriminator for an indeterminate nodule. Use the lecture framing for exams.)
Management of indeterminate nodule:
- Close follow-up with CXR/CT (3–6 months)
- Biopsy: Percutaneous (CT-guided lung biopsy) or Transbronchial
- Resection [1]
CT-guided lung biopsy: The lecture shows an image of a CT-guided percutaneous needle biopsy. This is used for peripheral lesions that cannot be reached by bronchoscopy. The main complication is pneumothorax.
Part 2: Lung Cancer (Lung Mass)
Further evaluation of suspected lung cancer:
- CT scan: staging ± biopsy
- Bronchoscopy: biopsy + BAL
- Post-bronchoscopy sputum analysis [1]
Contrast CT Thorax for TNM staging:
- T — Primary tumour: size, location, invasion
- N — Nodal involvement: hilar, mediastinal, contralateral
- M — Metastasis: liver, adrenal, bone, brain [1]
Why contrast? IV contrast enhances blood vessels and allows differentiation between:
- Tumour and collapsed lung (collapsed lung enhances more uniformly)
- Tumour and mediastinal structures (to assess invasion)
- Lymph nodes and blood vessels
Modern CT scanners produce 0.5–0.625 mm slices for contrast CT thorax. [1]
Ancillary findings on CT: other nodules, lymph nodes, etc. [1]
The lecture shows a necrotic subcarinal lymph node — this is a classic N staging finding indicating mediastinal nodal involvement [1].
Pancoast tumour: erosion of rib and left transverse process, extension into soft tissues of back [1]
Why is this important? A Pancoast (superior sulcus) tumour is a lung cancer at the apex of the lung that invades:
- Brachial plexus → arm pain, weakness, Horner's syndrome
- Ribs and vertebral bodies → bony destruction
- Sympathetic chain → Horner's syndrome (ptosis, miosis, anhidrosis)
- Subclavian vessels
High Yield
The lecture specifically emphasizes that MRI is superior for evaluating Pancoast (superior sulcus) tumours because it can delineate brachial plexus invasion and chest wall involvement better than CT. [1]
Common sites of lung cancer metastasis shown in lecture: liver metastasis, adrenal metastasis [1]
Why liver and adrenals? These are the most common sites for haematogenous spread of lung cancer, along with bone and brain. The adrenal glands receive extensive arterial blood supply and are a common site for metastatic deposits from many cancers.
CT evaluation purposes:
- Stage disease — assess operability
- Radiation planning — anatomic relationship, disease extent, surrounding tissue thickness
- Assessing treatment response [1]
Pitfalls of CT:
- Mediastinal nodes: size criteria (nodes may be enlarged from inflammation, not metastases); microscopic metastases may be missed
- Indeterminate chest wall or mediastinal invasion [1]
Why is this a pitfall? CT uses a size criterion (typically short-axis > 1 cm) to determine if lymph nodes are pathological. However:
- Reactive nodes from infection/inflammation can be > 1 cm (false positive)
- Micro-metastases in normal-sized nodes are missed (false negative)
- This is why PET-CT and tissue sampling (mediastinoscopy, EBUS) are needed
Bronchoscopy approach depends on lesion location:
| Proximal Lesion | Peripheral Lesion | |
|---|---|---|
| Visualization | Directly visualized | Not visualized directly |
| Sampling | Saline washing and brushing (microbiology & cytology) | Bronchoalveolar lavage — BAL (microbiology & cytology) |
| Biopsy | Biopsy under direct vision (histology) | Transbronchial biopsy (histology) |
Source: [1]
Why the difference? Proximal lesions (endobronchial) can be seen through the bronchoscope, so you can directly biopsy them. Peripheral lesions are beyond the reach of direct visualization — you use fluoroscopic guidance or navigational bronchoscopy to reach them, and BAL to wash the distal airways for cytology.
Other staging techniques for lung cancer:
- Transoesophageal ultrasound + biopsy
- Mediastinoscopy + biopsy
- Thoracotomy + nodal sampling
- PET-CT [1]
MRI:
- Similar limitations to CT
- Expensive
- Superior for superior sulcus (Pancoast) tumours — evaluates chest wall and brachial plexus invasion [1]
Why MRI for Pancoast? MRI has excellent soft tissue contrast and can image in multiple planes (coronal, sagittal). This is critical for delineating the relationship between tumour and the brachial plexus, subclavian vessels, and vertebral bodies — all of which influence surgical planning.
PET/CT uses 18-fluoro-2-deoxy-D-glucose (18-FDG):
- Based on glucose metabolism
- Neoplasms and inflammatory lesions show increased metabolic activity (hot) Advantages:
- Whole body scans — detect occult metastases elsewhere
- High sensitivity (90–95%) Disadvantages:
- Lower specificity — false positives [1]
Why false positives? FDG is taken up by any metabolically active tissue. Active infection, granulomatous disease (sarcoidosis, TB), and even post-surgical inflammation can all be FDG-avid, leading to false positives. Conversely, some well-differentiated tumours (e.g., bronchoalveolar carcinoma / lepidic adenocarcinoma) may be FDG-negative (false negative).
Past Paper Alert – 2025 MCQ Q41
"A 78-year-old male with a 3cm lung mass, bone pain — what is the MOST EFFECTIVE imaging to investigate further?" Answer: Whole body PET-CT scan — because it addresses both staging of the primary tumour AND detection of bone metastases in a single scan. DEXA only measures bone density (for osteoporosis), MRI chest would miss other distant metastases, and ultrasound is not useful for lung masses. [3]
Part 3: Diffuse Lung Lesions
54-year-old lady, increasing SOB, painful joints, non-smoker [1]
The combination of ILD + painful joints should make you think of connective tissue disease-associated ILD (e.g., rheumatoid arthritis, systemic sclerosis, polymyositis/dermatomyositis with anti-Jo-1 antibody) [4].
CXR findings:
CXR of ILD:
- Lower zone predominance
- Reticular shadowing
- Volume loss
- Shaggy heart borders [1]
Why lower zone? Idiopathic pulmonary fibrosis (UIP pattern) and many CTD-associated ILDs have a basilar and peripheral predominance. The "shaggy heart borders" occur because fibrotic changes blur the normally sharp cardiac silhouette (similar to how a shaggy sweater has fuzzy edges). Volume loss occurs because fibrosis contracts and shrinks the lung tissue.
Exam Discriminator
Upper zone predominance ILD = think of silicosis, coal worker's pneumoconiosis, sarcoidosis, and hypersensitivity pneumonitis (chronic). Lower zone predominance ILD = think of IPF, asbestosis, CTD-associated ILD. This zonal distribution is a common differentiator in MCQs.
HRCT:
- Old protocol: 1 mm sections at 10 mm intervals (high resolution with reduced radiation)
- Modern scanners: 0.5–0.625 mm contiguous slices with lower radiation than old HRCT
- Can be performed during inspiration, expiration, and prone (3 sets of images)
- Excellent for studying lung parenchyma, interstitium, and airways [1]
Why prone imaging? When patients lie supine, dependent atelectasis (gravity-dependent collapse of posterior lung bases) can mimic early fibrosis. Prone imaging eliminates this artefact — if the "ground-glass" opacity disappears when prone, it was just atelectasis. If it persists, it's real disease.
Why expiratory imaging? Air trapping (seen in small airway disease, bronchiolitis obliterans, early COPD) is best demonstrated on expiratory scans. Normal lung deflates and increases in density on expiration; trapped air remains dark.
Standard CT vs HRCT
| Feature | Standard Contrast CT | HRCT |
|---|---|---|
| Slice thickness | 2.5–5 mm | 0.5–1 mm |
| Contrast | Yes (IV contrast) | No contrast needed |
| Coverage | Volumetric (whole thorax) | Sampled sections or contiguous thin slices |
| Best for | Mass lesions, staging, vascular structures | Parenchymal, interstitial, and airway disease |
| Limitation | Lower spatial resolution for interstitial detail | May miss small mass lesions (if old sampling protocol) |
HRCT in diffuse lesions:
- Characterize disease: assess for air-trapping; nodular, reticular pattern, honeycombing
- Determine extent of disease: upper/mid/lower; central/peripheral
- Localize site for biopsy
- Post-treatment evaluation [1]
What do these patterns mean?
| HRCT Pattern | What It Represents | Classic Disease |
|---|---|---|
| Reticular | Thickened interlobular septa, fibrosis | IPF, asbestosis |
| Nodular | Small discrete opacities | Sarcoidosis, miliary TB, silicosis |
| Honeycombing | Clustered cystic spaces with thick walls | End-stage IPF (UIP pattern) |
| Ground-glass opacity | Partial filling of alveoli or thickening of interstitium | PJP, NSIP, alveolar haemorrhage |
| Tree-in-bud | Small airway inflammation with plugging | Infectious bronchiolitis, TB |
| Cystic | Thin-walled air-containing spaces | LAM, LCH, PJP |
3.5 Specific Diffuse Diseases Shown in Lecture
- Shown as thin-walled cysts diffusely throughout the lungs [1]
- Occurs almost exclusively in women of reproductive age
- Associated with tuberous sclerosis complex
- Ground-glass opacities, often bilateral and perihilar [1]
- Classic in immunocompromised patients (HIV/AIDS with CD4 < 200)
- The lecture shows both LAM and PJP side by side, likely as differential of diffuse cystic disease
- HRCT image shown in the lecture [1]
- Key findings: dilated airways, "signet ring" sign (bronchus larger than adjacent pulmonary artery), lack of tapering
- Distribution gives clues to aetiology (see table in section 3.2 above)
Past Paper Alert – 2020 MCQ Q75
"55-year-old lady with 2-year history of SOB, sputum production, haemoptysis. CXR shows streaky shadows with tramlines. What radiological investigation to confirm diagnosis?" Answer: HRCT thorax — tramlines on CXR suggest bronchiectasis, and HRCT is the gold standard for confirming bronchiectasis. NOT contrast-enhanced CT (that's for masses/staging), NOT PET-CT (that's for cancer staging), NOT V/Q scan (that's for PE). [5]
| CT Technique | When to Use | What It Shows | Key Points |
|---|---|---|---|
| Non-contrast CT | Nodule characterization, calcification, fat | Attenuation values (HU), density | No IV contrast needed |
| Contrast CT Thorax | Lung cancer staging, vascular assessment | Enhancement of masses, lymph nodes, vessels | 0.5–0.625 mm slices; TNM staging |
| HRCT | Diffuse lung disease, ILD, bronchiectasis | Parenchymal and interstitial detail | Inspiration + expiration + prone; no contrast |
| CT-guided biopsy | Indeterminate or peripheral nodules | Tissue for histology | Risk of pneumothorax |
| PET-CT | Staging lung cancer, detecting occult metastases | Metabolic activity | 90–95% sensitivity; false positives with infection/inflammation |
| CT Aortogram | Suspected aortic dissection (widened mediastinum) | True and false lumen, intimal flap | Pre and post-contrast |
| CTPA | Suspected pulmonary embolism | Pulmonary artery filling defects | eGFR cutoff > 30 for contrast |
Take Home Points:
- Focal or diffuse disease — the first discriminating question
- Choose the optimal imaging modality for the clinical scenario
- Know the suspicious imaging features for lung malignancy
- Understand the indications for lung biopsy
- Know other ancillary investigations and staging techniques [1]
Integration with Related Material
- Pleural shadows on CXR — asbestosis causes pleural plaques (calcified), pleural thickening, and mesothelioma. These are pleural (not parenchymal) shadows. ILD from asbestosis causes basilar fibrosis similar to IPF. [8]
- Haemoptysis + lung mass → bronchoscopy for tissue diagnosis
- Haemoptysis + tramlines → bronchiectasis → HRCT
- Bilateral fine basal crackles with JVP not raised → think ILD/IPF (not heart failure because JVP normal) [9]
- Tramlines on CXR → bronchiectasis
- Stony dull percussion → pleural effusion (not related to parenchymal disease)
Likely Exam Questions
-
A 60-year-old smoker has a 2 cm solitary pulmonary nodule on CXR with spiculated margins. What is the single best next investigation?
- Answer: CT thorax (to characterize and stage) [1]
-
Which CT finding is most suggestive of a benign pulmonary nodule?
- Fat attenuation (-50 HU) suggesting hamartoma [1]
-
A patient with suspected lung cancer has a 4 cm mass. CT shows a necrotic subcarinal lymph node. What does this represent in TNM staging?
- N2 disease (ipsilateral mediastinal lymph node involvement) [1]
-
Which imaging modality is best for evaluating brachial plexus invasion in a Pancoast tumour?
- MRI [1]
-
What is the main disadvantage of PET-CT in lung cancer staging?
- Lower specificity / false positives from inflammatory conditions [1]
-
HRCT is the investigation of choice for which category of lung disease?
- Diffuse parenchymal / interstitial lung disease [1]
-
Why is prone HRCT performed?
- To differentiate dependent atelectasis from true fibrosis (atelectasis resolves when prone)
-
A 55-year-old presents with SOB, tramlines on CXR. Name the investigation to confirm diagnosis and two HRCT findings.
-
List 4 features of a malignant pulmonary nodule on CT.
- Spiculated margins, eccentric calcification, pleural retraction, heterogeneous enhancement (> 20 HU) [1]
-
A CXR shows bilateral lower zone reticular shadowing with volume loss. What is the most likely pattern of disease and what imaging would you request?
- Interstitial lung disease (likely IPF or CTD-ILD); request HRCT thorax [1]
High Yield Summary
Focal vs Diffuse is the first question for any abnormal lung shadow. For focal lesions (SPN < 3 cm), compare with old CXR → CT characterization using HU, margins (smooth = benign, spiculated = malignant), calcification pattern (central = benign, eccentric = malignant), fat content (-50 HU = hamartoma). Indeterminate nodules need follow-up CT at 3–6 months or biopsy. For lung masses (≥ 3 cm), proceed to contrast CT for TNM staging + bronchoscopy for tissue. PET-CT is highly sensitive (90–95%) but has false positives. MRI is specifically superior for Pancoast tumours. For diffuse disease, HRCT is the investigation of choice — it characterizes pattern (reticular, nodular, honeycombing, ground-glass), determines distribution (upper vs lower, central vs peripheral), and guides biopsy. Key past paper themes: tramlines = bronchiectasis → HRCT; lung mass in smoker → CT staging → PET-CT; nodule characterization with HU.
Active Recall - Abnormal Lung Shadow on CXR/CT
[1] Lecture slides: GC 012. Abnormal lung shadow on chest radiograph CXR, CT.pdf [2] Senior notes: Ryan Ho Respiratory.pdf (Section 2.6 Abnormal Lung Shadows) [3] Past papers: 2025 Fourth Summative MCQ.pdf (Q41) [4] Senior notes: Block A - Rheumatology Interactive Tutorial.pdf (Case P2) [5] Past papers: 2020 Fourth Summative Assessment MCQ paper.pdf (Q75) [6] Senior notes: Block A - Chest Pain - Department of Radiology.pdf [7] Senior notes: Block A - Sudden severe chest pain_ acute myocardial infarction; aortic dissection.pdf [8] Lecture slides: GC 083. Shortness of breath in a construction site worker.pdf [9] Past papers: 2024 Fourth Summative MCQ.pdf (EMQ Q1–4)
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