Ectopic hamartomatous thymoma: a case demonstrating skin adnexal differentiation with positivity for epithelial membrane antigen, androgen receptors, and BRST-2 by immunohistochemistry.

Ectopic hamartomatous thymoma is a rare tumor of the lower neck occurring in adult men, which follows a benign course. They are triphasic with epithelial, adipocytic, and spindled elements in variable amounts. The origin of this tumor has been debated, but it is now believed to arise from remnants of the cervical sinus of His from early development. We describe a case of ectopic hamartomatous thymoma with typical features, as well as multifocal areas of skin adnexal differentiation. These included sebaceous, eccrine, and apocrine elements. We highlighted these by using immunohistochemistry to epithelial membrane antigen, BRST-2, and androgen receptors (AR). Epithelial membrane antigen stained sebaceous cells and the luminal borders of eccrine and apocrine ducts. BRST-2 and AR stained apocrine elements in a cytoplasmic and a nuclear pattern, respectively. The spindled component also expressed nuclear AR. AR positivity has not been described previously and may explain the occurrence primarily in men in postpubertal life.