Microcystic Adnexal Carcinoma of the Skin. A Reappraisal of the Differentiation and Differential Diagnosis of an Underrecognized Neoplasm

Background: Microcystic adnexal carcinoma (MAC) is a locally aggressive adnexal neoplasm whose histogenesis is disputed. Many cases referred to us had been misdiagnosed.

Objective: Our purpose was to clarify the differential diagnosis and differentiation of MAC.

Methods: We sought follow-up data and examined routinely stained sections from 17 cases. We performed immunoperoxidase stains for carcinoembryonic antigen, pilar keratin (AE13), proliferating cell nuclear antigen (PCNA), type IV collagen, p53, and CD34 on selected cases.

Results: Nine biopsy specimens had initially been misinterpreted. Cysts containing compact keratin or shadow cells were present in 11 cases, which we interpret as evidence of follicular differentiation. Sebaceous gland and duct as well as inner root sheath structures were seen in one case each. CD34 did not mark the clear cells as it does those of the outer root sheath. Staining for PCNA, type IV collagen, and p53 did not distinguish MAC from benign adnexal neoplasms.

Conclusion: MAC can be distinguished from its simulants in adequate biopsy specimens. Incompletely excised lesions usually recur. Both follicular and sudoriforous differentiation is present. Type IV collagen, PCNA, and p53 antisera were not useful in differential diagnosis.