Abstract

Microcystic adnexal carcinoma (MAC) is a rare locally aggressive neoplasm with a propensity for local recurrence. However, the nodal and/or metastatic potential is low. MAC most commonly presents on the face/upper lip. The diagnosis of MAC can be challenging when the pathologic sampling is partial or when presentation occurs in an unusual site. We present a 70-year old female with a 5.5 cm mass of the right breast. Histopathologic examination reveals an infiltrative carcinoma which closely approximates the epidermis and extends through the dermis into subcutaneous tissue/breast tissue. The neoplasm is comprised of keratocysts, and aggregates of slightly apocrine to squamoid appearing cells with ductular differentiation. There was prominent perineural invasion. The neoplastic cells demonstrated positivity for CK5/6, P40, and P63, with weak focal ER expression, and negativity for PR and Her2neu. Initially this neoplasm was interpreted as a primary breast carcinoma (adenosquamous) and was considered high risk due to its triple negative status and mammaprint score. However, when the case underwent review the histologic features including positive staining for CK5/6, P40 and P63, prominent perineural invasion and keratocysts was felt to better represent a microcystic adnexal carcinoma (MAC) extending into breast tissue rather than primary breast carcinoma. The distinction between primary breast carcinoma and cutaneous adnexal carcinoma is of high importance as the management is different. This patient should undergo local excision which could include Mohs micrographic surgery and possibly adjuvant radiotherapy due to the presence of perineural invasion rather than mastectomy with adjunct chemotherapy for primary breast carcinoma. Sometimes it is hard to differentiate adnexal carcinoma from breast carcinoma by histology and/or immunohistochemistry due to overlapping features. However, the presence of significant CK5/6, P40, and/or P63 favors cutaneous adnexal carcinoma.

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