Microcystic adnexal carcinoma. Immunohistologic observations suggesting dual (pilar and eccrine) differentiation.

Microcystic adnexal carcinoma (MAC) is a locally aggressive neoplasm that has recently been recognized as a clinicopathologic entity. Its histologic appearance includes both pilar and eccrine differentiation. We initially treated two patients with MAC of the cheek and of the nasolabial fold, respectively; by microscopically controlled excision because of the contiguous growth of the tumors. Despite the benign histologic appearance, there was deep and extensive infiltration of the subcutaneous tissue. Both patients responded favorably to initial treatment with microscopically controlled excision. In addition, immunoperoxidase staining for carcinoembryonic antigen supported the dual differentiation of this unusual neoplasm. We speculate that previous radiotherapy may be an important predisposing factor in the pathogenesis of MAC.