On chest CT, a rounded, well-defined lung mass with an air-fluid level and thick irregular inner wall is identified in a patient with a prior cavitary lesion that has now changed character. What does development of an air crescent sign within a pre-existing cavity most specifically indicate?
- A A fungus ball (aspergilloma) within a pre-existing cavity ✓
- B Lung abscess with partial drainage
- C Cavitary squamous cell carcinoma
- D Pneumatocele following staphylococcal pneumonia
Correct answer: A. A fungus ball (aspergilloma) within a pre-existing cavity
Explanation
The air crescent (Monod) sign — a crescentic lucency between the wall of a cavity and a contained soft-tissue mass — is the hallmark of aspergilloma (fungus ball). The mobile ball of mycelium separated from the wall by a crescent of air is pathognomonic. Lung abscesses show dependent air-fluid levels without a separate intracavitary mass. Carcinoma cavities have irregular walls. Pneumatoceles are thin-walled and typically lack intracavitary contents.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.