Extraocular well‐differentiated sebaceous tumors with overlying cutaneous horns: four tumors in three patients

Background Sebaceous tumors are adnexal neoplasms showing sebocytic differentiation. They range from benign to malignant and are associated with Muir‐Torre syndrome ( MTS ). Several clinical and histopathological features associated with MTS have been described. Sebaceous tumors with an overlying cutaneous horn are extremely rare. Methods Hematoxylin and eosin‐stained slides were retrospectively reviewed to identify sebaceous tumors with marked hyperkeratosis, a condition that is often associated with cutaneous horns. Clinical correlation and mismatch repair protein immunohistochemical studies were then conducted. Results Four tumors from three patients were identified in our archive. Three were classified as sebaceous adenomas, and the fourth was considered as a borderline sebaceous tumor favoring well‐differentiated sebaceous carcinoma. All cases showed loss of expression of mismatch repair proteins (three tumors from two patients exhibited lost expression of MSH2 and MSH6 , and the fourth exhibited lost expression of MLH1 and PMS2 ). Additionally, one patient presented characteristic clinical manifestations of MTS , including multiple sebaceous adenomas and visceral carcinomas. Conclusions We suggest that extraocular well‐differentiated sebaceous neoplasms with overlying cutaneous horns may be an indication of underlying mismatch repair protein deficiency and potential MTS . This distinctive morphology might be an exaggerated combination of other features associated with MTS , i.e. keratoacanthoma‐like architecture and extensive holocrine secretion.