Microcystic adnexal carcinoma: an often misdiagnosed, locally aggressive growing skin tumor.

Microcystic adnexal carcinoma (MAC) belongs to the spectrum of locally aggressive adnexal carcinomas and most commonly occurs in the head and neck region. Recently it has been proposed that MAC is an apocrine tumor. It is characterized by slow, but locally aggressive growth with infiltration of subcutaneous fat tissue, muscles, perichondrium, periosteum, or perineurium. As a result, surgical treatment often leads to ablation defects, which are many times the size of the clinical lesion. In 1982, Goldstein and colleagues first reported MAC to be a distinct histologic entity characterized by a combination of keratinous cysts in the upper dermis, islands and strands of small basaloid, benign-appearing keratinocytes or squamous cells in the deeper dermis within a dense desmoplastic stroma, and areas of ductular differentiation. The authors report the case of a 78-year-old woman in whom a diagnosis of MAC was made when a tumor on the right cheek recurred for the second time. Previous histopathologic diagnoses were squamous cell carcinoma and desmoplastic trichoepithelioma. Local recurrences of the tumor occurred, despite histographic surgery because in hematoxylin and eosin stains, small islands of the deceptively benign-appearing small basaloid cells of MAC were not recognized as tumor cells. The reported case demonstrates the difficulties in diagnosing MAC and indicates that MAC should always be considered in the differential diagnosis of slowly growing tumors in the head and neck region. If MAC is diagnosed too late, it can be inoperable because of its infiltrative growth.