Squamous cell carcinoma arising in keratoacanthoma: a neglected phenomenon in the elderly.

Keratoacanthoma is a unique clinicopathologic entity with a behavior and clinical outcome that differs markedly from that of squamous cell carcinoma. The development of squamous cell carcinoma in a keratoacanthoma alluded to by Rook and Whimster in 1979 and by Reed in 1993 was confirmed by Sánchez Yus et al in 2000. We found this phenomenon in 5.7% of keratoacanthomas in a series of 3465 cases. Its incidence in patients older than 90 years was 13.9%. The incidence of perineural invasion in this series of keratoacanthomas was 0.2%.