Coin lesion | Radiology Signs

Answer:

A coin lesion is the legacy descriptive term for a single, round or oval, well-circumscribed intrapulmonary opacity seen on a chest radiograph and completely surrounded by aerated lung. The modern equivalent is the solitary pulmonary nodule, formally defined as an opacity 3 cm or less in diameter with no associated lymphadenopathy, atelectasis or pneumonia; a lesion above 3 cm is a mass and carries a far higher probability of malignancy. Benign causes include infectious granulomas (tuberculoma, histoplasmoma), hamartoma, arteriovenous malformation, rounded atelectasis and intrapulmonary lymph nodes, while malignant causes include primary bronchogenic carcinoma, carcinoid and a solitary metastasis. Because the plain film cannot reliably separate these, a coin lesion mandates comparison with prior imaging and characterisation by CT, with growth rate, margin, and the presence of fat or benign calcification patterns driving management. The term persists in teaching and reporting but is descriptive only and confers no diagnosis.

Why is it called so?

On the frontal radiograph the lesion is round, sharply outlined and of uniform density, resembling a coin laid flat on the lung field.

Pathophysiology

Any process that produces a discrete, roughly spherical mass of soft-tissue attenuation within otherwise aerated lung will project as a coin lesion, because the surrounding air provides near-perfect natural contrast and the sphere is tangential to the beam at every point of its rim. The appearance therefore reflects geometry alone, not aetiology, which is why granuloma, hamartoma and carcinoma are indistinguishable on the plain film and why calcification pattern, fat content and interval growth on CT carry the diagnostic weight.

Alternative names: Solitary pulmonary nodule (the current preferred term)

Other associated named signs: Popcorn calcification (hamartoma), feeding vessel sign, halo sign, and comet tail sign of rounded atelectasis