Sclerosing Microcystic Adenocarcinoma of the Head and Neck Mucosa: A Neoplasm Closely Resembling Microcystic Adnexal Carcinoma.

Microcystic adnexal carcinoma (MAC) occurs predominantly in the centrofacial skin and has been only rarely reported in mucosal surfaces. We here present a 5 case series of tumors closely resembling MAC occurring in the mucosal surfaces of the head and neck, which we have termed sclerosing microcystic adenocarcinomas. These tumors showed a predilection for women (4:1) with an average patient age of 52.6 years (range 41-73 years). Location included the tongue (n = 2), the floor of the mouth (n = 2), and the nasopharynx/clivus (n = 1). One occurred after radiation therapy and another occurred in the setting of immune compromise. Immunohistochemistry highlighted a dual cell population with luminal cells showing positivity for high and low-molecular weight keratins and surrounding myoepithelial cells showing S100 and smooth muscle actin staining. No cases had nodal involvement, and the single patient with clinical follow-up was alive and free of disease 34 months after diagnosis and definitive radiochemotherapy. Differential diagnoses for all cases diverged from those provoked by MAC in the skin and included a variety of salivary gland neoplasms such as adenoid cystic carcinoma, polymorphous low grade adenocarcinoma, and mucoepidermoid carcinoma. Recognition of sclerosing microcystic adenocarcinoma in the head and neck mucosa is critical given its bland appearance and subtle infiltration pattern, infrequency of nodal involvement, and behavioral differences from the other entities on the differential.