Microcystic adnexal carcinoma (sclerosing sweat duct carcinoma).

Microcystic adnexal carcinoma is a relatively uncommon tumor first described in 1982. Most commonly presenting as a facial plaque or nodule, this highly invasive tumor is characterized by slow growth and relentless local destruction. We report the presence of microcystic adnexal carcinoma in the neck, a previously unreported site. The patient was a 48-year-old man who had received radiation therapy at age 5 to shrink the thymus gland and subsequent radiation therapy after excision of a thyroid tumor at age 35. The results of immunohistochemical stains were consistent with the hypothesis that this tumor develops from pluripotential cells capable of follicular and eccrine differentiation.