A 60-year-old man with prior treated pulmonary tuberculosis presents with recurrent hemoptysis. CXR shows a mobile round mass within a pre-existing upper lobe cavity, surrounded by a crescent of air. What is this radiological sign called?

• A Water lily sign
• B Halo sign
• C Reversed halo sign
• D Air crescent sign (Monod sign) ✓

Explanation

The air crescent sign (Monod sign) represents a fungus ball (aspergilloma) within a pre-existing cavity, classically from prior tuberculosis. The mobile mass surrounded by air creates this crescent. The halo sign is ground-glass opacity around a nodule seen in invasive aspergillosis. The reversed halo sign is seen in organizing pneumonia. Water lily sign is for hydatid cyst.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

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