Subungual keratoacanthoma masquerading as a chronic paronychia.

Subungual keratoacanthoma is a rare, benign tumor of the digits. Patients present with progressive fusiform swelling, erythema, and tenderness, usually affecting a single digit on the radial side of the hand. A cup-shaped lytic lesion of the distal phalanx is a uniform finding on radiography. Delay in diagnosis and misdiagnosis are common because of the rarity of the lesion and difficulties with histological differentiation from subungual squamous cell carcinoma. Accurate diagnosis requires a high index of suspicion, a careful history, and histological evaluation. The natural progression of the disease appears to be continued growth with ongoing destruction of the distal phalanx. Proper treatment involves surgical removal of the mass by curettage and close follow-up for at least 2 years to monitor for local recurrence.