A 59-year-old man presented with a history of a painful, rapidly growing mass on the anteromedial aspect of his thigh. On examination, he had a mobile, hard, and nontender mass. Magnetic resonance imaging revealed a well-encapsulated, heterogeneous mass with high-intensity internal septations. Blood-filled spaces were observed within the mass with no involvement of the underlying normal bony structures. The mass was excised, and the diagnosis of an extraosseous aneurysmal bone cyst was confirmed histologically.
Extraosseous aneurysmal bone cysts are rare benign lesions that can mimic malignant tumors. Careful clinical and pathological assessment should be performed in such cases.