Microcystic adnexal carcinoma arising in the setting of previous radiation therapy.

While there are several reports of microcystic adnexal carcinoma developing in patients within sites of previous therapeutic irradiation, this relationship is not well described in the dermatologic literature. We report a case of a 42-year-old man with a remote history of therapeutic irradiation following surgical resection of periorbital rhabdomyosarcoma. Subsequently, he developed multiple basal cell carcinomas and a microcystic adnexal carcinoma within the field of irradiation. The histologic features were those of a classic microcystic adnexal carcinoma, with well differentiated nests and cords of keratinocytes displaying follicular and ductular differentiation infiltrating diffusely into the reticular dermis. Dense fibrosis was present surrounding the neoplastic keratinocytes. Nuclear atypia and mitotic figures were not identified. A carcinoembryonic antigen (CEA) stain demonstrated glandular differentiation. It is important for dermatologists to be aware of the apparent relationship between the rare microcystic adnexal carcinoma with its innocuous scar-like clinical appearance and prior local radiation therapy.