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A 41-year-old man who was undergoing chronic hemodialysis for end-stage renal disease presented with new-onset hemoptysis. His chest radiograph showed an opacified middle lobe (Fig. 1), a change from earlier radiographs. Two weeks later, the middle lobe contained a large cavity with a nonmobile, intracavitary, rounded mass (Fig. 2). Core biopsy of the cavitary lesion disclosed necrotic tissue with intravascular fungal elements (Fig. 3); culture grew Aspergillus fumigatus.
The air crescent sign (ACS), also called the meniscus or cap sign,1 appears on radiographs or computed tomographic scans of the chest as air interposed between an intracavitary, ball-like mass and the cavity wall. Its shape ranges from crescent-like to that of total encapsulation,2 and its volume varies greatly.
By far the most common cause of the ACS is the fungus ball of angioinvasive aspergillosis.1,2 Formed of condensed hyphae, the ball can be large or small, solitary or multiple.1,2 It may move when the patient's position changes3,4 or remain fixed, projecting into the cavity's lumen like a polyp.1 Occasionally, the ball partially calcifies,1,5 or it may disintegrate and exit as particles in the sputum.1 Although often indolent with few or no symptoms,5 the process frequently presents with hemoptysis, which can sometimes be fatal.3
Additional causes of the ACS include pulmonary hydatid cyst,1,6 other fungi,1,2,5 blood clot or Rasmussen aneurysm in a tuberculous cavity,1 lung abscess with inspissated pus,1 staphylococcal pneumonia,6 nocardial infection,1,2 carcinoma of the lung,4 and pulmonary gangrene or hematoma.1
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