Answer: Microcystic adnexal carcinoma

Histology: This is an ill-defined proliferation of clusters of basaloid cells in a desmoplastic stroma with horn cysts (pilar differentiation) and glands (eccrine differentiation). The lesion invades into underlying cartilage, and involves perineural spaces. Cytologically, one can appreciate the biphasic nature of the lesion, with peripheral basaloid cells and central more differentiated cells.

Discussion: Metastatic breast carcinoma would lack the keratinized horn cysts of the current lesion, and would not demonstrate biphasic cytology. Desmoplastic trichoepithelioma would show the pilar differentiation seen in the current case, but would be circumscribed and not infiltrative. Syringoma would demonstrate the eccrine differentiation seen it the current case, but would also be superficial and circumscribed. Both of the latter two are easy to confuse with microcystic adnexal carcinoma on a limited biopsy. One must see the base of the lesion to be certain that the lesion is not infiltrative and thus a microcystic adnexal carcinoma.

Microcystic adnexal carcinoma typically involves the central face of middle aged individuals. 80% of cases show perineural invasion, with extension beyond the confines of what is thought clinically. The lesions are locally aggressive, but systemic metastases are generally not seen.