Air crescent sign
14 year-old female with acute myeloblastic leukaemia
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Sagittal reformat from a CT scan of the chest, performed on a 14 year-old female with acute myeloblastic leukaemia.
The image shows a rounded cavity in the apical right upper lobe, with a non-dependant soft-tissue nodule within it. There is some subtle ground-glass opacity surrounding the lesion. There were several other similar lesions in the right lung (not shown).
The imaging features, combined with the clinical history, are characteristic of invasive fungal infection, most commonly due to Aspergillus spp.. Care should be taken to avoid confusion with mycetoma, which occurs in patients with pre-existing lung cavities, is gravity-dependant, and lacks the ground-glass halo.
Invasive aspergillosis occurs in immunosuppressed individuals and carries a 65-90% mortality rate if improperly treated. Cavitation occurs in the recovery phase, and thus tends to indicate a better prognosis.
Reference: Abramson S. The Air-Crescent Sign. Radiology. 2001; 218:230-232