High (nuclear) grade adnexal carcinoma with microcystic adnexal carcinoma-like structural features.

Microcystic adnexal carcinoma (MAC) is a slow growing, locally aggressive sweat gland tumor. It predominantly affects the face and tends to recur despite local excision. Microscopically, MAC is characterized by a stratified proliferation of microcysts, cords, and ducts of cells that show squamous or adnexal differentiation. Atypia and mitoses are almost completely absent and metastatic deposits are rare and mostly limited to the regional lymph nodes; rather than real metastases, they might be the result of local extension of the tumor through perineurial spaces. We report a case of adnexal carcinoma with architectural features of MAC that displayed also marked nuclear pleomorphism and hyperchromasia with squamous pearl formation and a widespread strong p53 immunoreaction. The lesion behaved as a high-grade neoplasm with rapid growth, carcinosarcomatous metaplastic transformation in a relapse, and what were clinically suspected to be metastases. The literature contains several other examples reported as metastatic high-grade MAC, one of them with widespread distant metastases. We therefore want to sound an alert about the possible existence of tumors displaying microscopic findings characteristic of the aggressive forms of sweat gland carcinoma (nuclear pleomorphism and hyperchromasia, vascular invasion, and necrosis) in addition to architectural features of MAC. Whether these tumors should be called high-grade MACs or belong to a separate category remains an open issue until more cases are reported and bridge cases are eventually documented.