A 42 year old woman presented with abrupt onset of cough and minimal hemoptysis

A large, sharply circumscribed, homogeneous mass in the lower part of the right lung. A thin crescent-shaped radiolucency was observed at the top of the mass.

The air seeps between the two layers of the capsule ("meniscus sign" or "air cap" or air crescent). The same sign may be also observed in cases of a fungus ball, blood clot in a tuberculous cavity, bronchial adenoma, carcinoma, Rasmussen aneurysm, hamartoma, lung abscess (inspissated pus), pulmonary gangrene, sarcoma or tuberculoma.