Metastatic lung disease radiology and for 10 mark precise and for radiology pg exam

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Metastatic Lung Disease - Radiology (10 Marks)

Source: Grainger & Allison's Diagnostic Radiology; Schwartz's Principles of Surgery, 11e

Definition & Routes of Spread

Secondary (metastatic) lung tumours reach the lung via four pathways:

Haematogenous - most common; tumour cells lodge in pulmonary capillaries and grow as discrete nodules
Lymphatic - gives lymphangitic carcinomatosis
Direct extension - from hilar nodes, pleura, or a primary bronchial carcinoma into adjacent tissue
Endobronchial - rare; submucosal deposits cause airway obstruction

Common primary sites: Breast, gastrointestinal tract, kidney, testis, head and neck, and bone/soft-tissue sarcomas.

Radiological Patterns

1. Discrete Pulmonary Nodules (Haematogenous Metastases)

The hallmark pattern. Key features:

Feature	Detail
Number	Multiple (occasionally solitary, seen in 2-3% of all series)
Distribution	Peripheral and basal - most evident on CT
Shape	Usually spherical and well-defined
Edge	Irregular in adenocarcinoma ("spiculated" or corona radiata sign)
Calcification	Rare, except in osteosarcoma and chondrosarcoma
Cavitation	Seen in squamous cell carcinoma metastases (e.g. from head and neck primary)
Growth rate	Explosive doubling in choriocarcinoma/osteosarcoma (<30 days); very slow in thyroid carcinoma

CXR: Detects most nodules >1 cm. High-kV technique used to reveal lesions behind heart, diaphragm, and mediastinum.

CT: MDCT shows nodules >3 mm. Below 6 mm, differentiation from granulomas (TB, histoplasmosis) is unreliable.

With multiple non-calcified nodules on plain CXR: probability of metastases >90% (near 100% in non-endemic areas).
For CT-only visible nodules <1 cm: probability may fall to 50%, depending on community prevalence of infectious granulomas.

Fig. 8.33 - Irregular pulmonary metastases (metastatic adenocarcinoma from unknown primary) - note irregular nodule outlines and large left pleural effusion

Fig. 8.33 from Grainger & Allison - Irregular pulmonary metastases, metastatic adenocarcinoma; note irregular outlines and large left pleural effusion.

2. Cannon-ball Metastases

Term used for large, round, well-defined bilateral pulmonary nodules
Classically associated with: renal cell carcinoma, testicular germ cell tumours, choriocarcinoma
Easily visible on plain CXR

3. Lymphangitic Carcinomatosis

Permeation of pulmonary lymphatics and adjacent interstitial tissue by neoplastic cells.

Primary tumours: Carcinoma of bronchus, breast, stomach, and prostate.

Mechanism:

Bilateral symmetric pattern - from haematogenous emboli spreading via vessel walls into perivascular interstitium
Unilateral/localised pattern - from direct hilar node extension or pleural invasion

Radiological findings:

Modality	Findings
CXR	Fine reticulonodular shadowing; thickened septal (Kerley B) lines; fissure thickening (subpleural oedema); pleural effusion (~30%)
HRCT	Non-uniform, nodular thickening of interlobular septa; irregular thickening of bronchovascular bundles centrally; scattered parenchymal nodules; patchy airspace shadowing; aerated acini subtended by thickened septa (key difference from pulmonary oedema)

Key differentiating point from pulmonary oedema on HRCT: Many acini subtended by the thickened septa remain normally aerated in lymphangitic carcinomatosis.

Fig. 8.34 - Unilateral lymphangitic carcinomatosis (bronchial carcinoma) - thickened septal lines and nodules right lung

Fig. 8.34 - Unilateral lymphangitic carcinomatosis, bronchial carcinoma. Thickened septal lines and nodules confined to right lung.

Fig. 8.35 - Bilateral lymphangitic carcinomatosis. Bilateral thickened septal lines and widespread nodulation. Primary: bronchial carcinoma on sputum cytology.

Fig. 8.36 - HRCT of lymphangitic carcinomatosis - variable thickening of interlobular septa and enlargement of bronchovascular bundle

Fig. 8.36 - HRCT: variable thickening of interlobular septa and bronchovascular bundle enlargement in lymphangitic carcinomatosis.

4. Miliary Metastases

Innumerable tiny nodules mimicking miliary tuberculosis throughout both lungs
No large masses, no lymphatic obstruction
Rarest pattern of metastatic spread
Primary tumours: thyroid carcinoma, renal carcinoma, bone sarcomas, choriocarcinoma

5. Endobronchial Metastases

Rare; submucosal deposits cause airway obstruction as dominant radiological feature
Leads to collapse, consolidation, or obstructive pneumonitis distal to the deposit
Primary tumours: melanoma, renal, colorectal, breast carcinoma

6. Tumour Emboli

Radiologically recognisable pulmonary arterial hypertension from tumour emboli blocking small pulmonary arteries
Tumours: hepatoma, breast, kidney, stomach, prostate, choriocarcinoma
Rare but high-yield PG MCQ association

7. Pleural Effusion

Pleural effusion may accompany any pattern
Especially common with lymphangitic spread (~30%)
May also result from lymphatic obstruction, direct pleural seeding, or hypoalbuminaemia

Summary Table - High-Yield for PG Exam

Pattern	Key Primary Tumours	Key Radiological Clue
Multiple discrete nodules	Breast, kidney, colon, sarcoma	Peripheral, basal, bilateral, well-defined
Cannon-ball	RCC, testis, choriocarcinoma	Large rounded bilateral nodules
Cavitating nodule	Squamous cell carcinoma	Thick-walled cavity
Calcified nodule	Osteosarcoma, chondrosarcoma	Dense calcification in nodule
Lymphangitic carcinomatosis	Breast, stomach, prostate, bronchus	Kerley B lines + reticulonodular shadowing; HRCT = septal thickening
Miliary	Thyroid, RCC, choriocarcinoma	TB-like tiny bilateral nodules
Endobronchial	Melanoma, RCC, colorectal, breast	Collapse/consolidation distal to obstruction
Tumour emboli	Hepatoma, choriocarcinoma	Pulmonary hypertension pattern

Role of Imaging Modalities

CXR (PA + lateral, high-kV): First-line; detects nodules >1 cm; misses lesions behind cardiac shadow, diaphragm, hilum
MDCT: Detects lesions >3 mm; gold standard for staging and follow-up; essential for lymphangitic pattern (HRCT)
PET-CT: FDG uptake; best for detecting distant metastases and monitoring treatment response
MRI: Not routine for pulmonary metastases; useful for chest wall/pleural invasion assessment

Solitary Pulmonary Nodule in a Known Malignancy

Features favouring metastasis:

Smooth, round borders on CT
Temporal proximity to the primary lesion
History of RCC, sarcoma, germ cell tumour

Features favouring new primary lung cancer:

Spiculated edge
History of head and neck (94%), lung (92%), bladder (89%), or uterine (74%) carcinoma - these carry highest risk of new primary

Schwartz's Principles of Surgery, 11e, p. 19-16

Sources:

Grainger & Allison's Diagnostic Radiology, Chapter 8 (Lung Metastases, Lymphangitic Carcinomatosis, Unusual Patterns)
Schwartz's Principles of Surgery, 11e, Chapter 19 (Metastatic Lesions to the Lung)

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