Trichilemmal carcinoma arising in seborrheic keratosis: a case report and published work review.

The secondary skin malignancies arising in seborrheic keratosis (SK) are uncommon, and the causal association between the pre-existing lesion and subsequent malignant transformation remains uncertain. Among these enigmatic conditions, trichilemmal differentiation and/or neoplasms in SK have rarely been reported thus far. Herein, we describe a case of invasive trichilemmal carcinoma arising in a long-standing SK of the abdominal skin, and clinicopathologically review this rare complication with a computerized medical published work search (PubMed) and citations from earlier reports. To our knowledge, only four cases of trichilemmal tumors arising in SK have been observed, and, interestingly, all cases, including ours, were Japanese. Four of five cases (80%) developed the tumors in non-sun-exposed SK, and indeed had no apparent actinic damage in the histology. The pre-existing SK itself is more likely to act as the primary pathogenic event for developing the secondary trichilemmal tumors than a coincidental phenomenon and a consequence of skin damage by cumulative sun exposure.