Subungual keratoacanthoma. A benign bone-eroding neoplasm of the distal phalanx.

Subungual keratoacanthoma is a benign tumor of the fingers and toes that can simulate malignancy and present significant diagnostic problems for both the clinician and the pathologist. It is a rapidly growing nail-eroding tumor that may cause pain, swelling, and inflammation. Radiographs consistently demonstrate a lytic cup-shaped lesion of the tip of the distal phalanx, raising the question of malignancy. Because the lesion lacks the clinical appearance of keratoacanthoma of follicular origin, the diagnosis of keratoacanthoma may not be given clinical consideration. However, the histologic appearance of a central cavity filled with keratinous debris and lined by intensely keratinized, well-differentiated neoplastic squamous epithelium mimics the appearance of keratoacanthoma of follicular origin. The subungual location of the tumor, rapid growth with pressure erosion of the tip of the distal phalanx, and typical histologic features define this keratoacanthoma of nail matrix origin as a distinct entity. Recognition of the entity is crucial if the diagnosis of squamous cell carcinoma, which would lead to an unnecessary amputation, is to be avoided. Ultrastructurally, the keratinization process resembles that of normal nail matrix.