hydatid disease

Pulmonary echinococcal cysts are composed of three layers: an exocyst (chitinous layer), which is a protective membrane; an inner endocyst, which produces the daughter cysts‌; and a surrounding capsule of compressed, fibrotic lung known as the pericyst.
Pulmonary echinococcal cysts characteristically present as well-circumscribed, spherical soft tissue masses. In distinction to hepatic cysts, lung cysts do not have calcified walls. The cysts range in size from 1 to 20 cm, with a predilection for the lower lobes and the right side. While most cysts remain asymptomatic, patients may present when the cyst develops a communication with the bronchial tree. If the pericyst ruptures, a thin crescent of air will be seen around the periphery of the cyst, producing the meniscus‌ or crescent sign. If the cyst itself ruptures, the contents of the cyst are expelled into the airways, producing an airfluid level. On occasion, the cyst wall may be seen crumpled and floating within an uncollapsed pericyst, producing the pathognomonic sign of the camalote‌ or water lily sign. Rarely, a cyst will rupture into the pleural space, producing a large pleural effusion.