Microcystic adnexal carcinoma: a distinct clinicopathologic entity.

Microcystic adnexal carcinoma is an unusual locally aggressive neoplasm that is important to recognize since it may be confused with benign adnexal neoplasms, particularly desmoplastic trichoepithelioma, trichoadenoma, and syringoma. Six cases are described all of which displayed benign histological features on initial biopsy. Most often these neoplasms presented as solitary flesh-colored indurated plaques on the upper lip. All patients were white, five were women, and the average age was 44-years-old. Islands of basaloid keratinocytes, some of which contained horn cysts and abortive follicles, were embedded in a desmoplastic stroma. In other foci, ducts and gland-like structures lined by a two-cell layer predominated. In deep components individual and thin strands of cells dissected collagen bundles and skeletal muscle and invaded perineural spaces. Despite this, cytologic atypia and mitotic figures were rare. The cell of origin is considered to be a pluripotential adnexal keratinocyte which is capable of both follicular and sweat gland differentiation.