Microcystic adnexal carcinoma arising within a nevus sebaceus.

Nevus sebaceus (NS) is a congenital skin lesion arising on the face and scalp that has been linked to the development of various carcinomas. We describe a case of microcystic adnexal carcinoma (MAC) arising in an NS on the scalp of a 62-year-old man. Excisional skin biopsy and hematoxylin and eosin stains were performed to examine the specimen. Serial sections revealed papillomatosis typical of NS, with focal changes consistent with syringocystadenoma papilliferum. Adjacent to the syringocystadenoma papilliferum was an area containing small epithelial islands that extended focally into the subcutaneous layer. The cystic islands were embedded in a desmoplastic stroma with poor circumscription, consistent with MAC. This case presents a rare finding of MAC within an NS.