Magnetic resonance imaging of a cystic pilomatricoma in an ankle joint.

Pilomatricoma, also known as calcifying epithelioma of Malherbe, is an asymptomatic, slow-growing, benign dermal neoplasm that originates from hair cortex cells. The most common sites are the head and neck (68.2%), followed by the trunk (14.4%) and the upper (15.3%) and lower (2%) extremities. The majority (60%-70%) of tumors develop in the first 2 decades of life. A typical pilomatricoma presents as a firm, calcified, dermal nodule appearing as a solid mass in subcutaneous tissue on computed tomography or magnetic resonance imaging.This article describes the case of a 42-year-old man who presented with nontender soft tissue swelling in the right ankle joint. Radiographs and magnetic resonance imaging findings showed an inflamed cystic mass with calcification in the lateral malleolar area of the right ankle joint. These atypical magnetic resonance imaging features around the ankle joint led to confusion with other disease entities, including chronic bursitis, pigmented villonodular synovitis, rheumatoid nodules, and gout. After complete excision of the mass, histological examination revealed a cystic pilomatricoma.Although rare, a cystic pilomatricoma should be considered in the differential diagnosis of soft tissue masses with calcification.