Answer: Microcystic adnexal carcinoma

Histology: This is a neoplastic proliferation occurring in sun damaged skin. The overlying squamous epithelium is benign. Beneath the epithelium is proliferation of fairly bland basaloid cells set in a desmoplastic stroma. There are foci of glandular, eccrine differentiation and foci of horn cysts representing pilar differentiation. The lesion is infiltrating skeletal muscle, and exhibits perineural invasion.

Discussion: The infiltrating squamous carcinoma would typically show a surface component, and greater cytologic atypia. Desmoplastic trichoepithelioma contains a densely collagenized stroma and abundant keratin microcysts. Unlike microcystic adnexal carcinoma, this lesion is generally sharply delimited at its lateral margins and is generally confined to the upper reticular dermis. Syringomas are benign eccrine proliferations composed predominantly of angular, comma-shaped, tubular aggregates. These lesions lack atypia or primitive growth, and never extend into subcutaneous tissue.

Microcystic adnexal carcinomas classically occur on the face of middle-aged patients. These lesions classically extend beyond what is expected clinically. They are locally aggressive, but generally do not give rise to systemic metastases.