What causes cannonball metastases on chest radiography and CT?
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Answer:
Cannonball metastases are multiple large, round, sharply marginated pulmonary nodules produced by haematogenous spread of tumour to the lungs. They are typically bilateral, randomly distributed with a basal and peripheral predominance reflecting pulmonary blood flow, and measure from about 1 cm to several centimetres, the largest lesions dwarfing the miliary or micronodular patterns of other metastatic disease. Their smooth, well-defined outline reflects expansile growth of a well-vascularised tumour deposit within otherwise normal lung, with no surrounding consolidation or ground-glass halo. The classic primaries are renal cell carcinoma, choriocarcinoma and endometrial carcinoma, with prostate carcinoma, sarcomas (including chondrosarcoma and synovial sarcoma), colorectal and bladder carcinoma, and testicular tumours also described. Cavitation is uncommon and should raise the possibility of a squamous primary or a sarcoma. In a patient with no known malignancy the pattern warrants an urgent search for a renal, gonadal or uterine primary, including a serum beta-hCG.
Why is it called so?
The nodules are large, round, dense and sharply defined, scattered through the lung fields like a volley of cannonballs frozen in flight.
Pathophysiology
Tumour emboli reaching the lung through the pulmonary arterial circulation lodge in small arterioles and grow centrifugally as spherical deposits. Because these tumours are richly vascular and expand by pushing rather than infiltrating the surrounding alveoli, the nodules stay round and keep a sharp interface with aerated lung, and because arterial flow is greatest to the lung bases the deposits cluster peripherally and inferiorly. Their large size reflects sustained growth from a well-perfused deposit rather than any different route of spread.
Alternative names: Cannonball secondaries, cannonball lesions
Other associated named signs: Miliary pattern (much smaller haematogenous nodules), halo sign, feeding vessel sign, and the Golden S sign of a central obstructing mass
References
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