Cutaneous adnexal tumor with an unusual presentation--discussion of a potential diagnostic pitfall.

The clinical presentation of skin adnexal tumors is nonspecific, and histologically; the differential diagnosis between primary cutaneous adnexal malignant carcinomas and metastatic tumors with a visceral origin can be challenging. We report a patient with history of invasive ductal carcinoma of the breast who presented with a 1-cm erythematous palpable lesion on her right calf. The biopsy showed an intradermal proliferation of malignant epithelioid cells with ductal differentiation, histologically compatible with metastatic breast carcinoma. However, the tumor cells labeled strongly and diffusely not only for pancytokeratin and cytokeratin (CK7) but also with p63 and CK5/6; carcinoembryonic antigen highlighted the ductal structures. No labeling was seen for mammoglobin, estrogen/progesterone, Her2-neu, S-100 protein, CK20, thyroid transcription factor-1 (TTF-1), and CDX-2. Based on the p63 and CK5/6 positivity, the differential diagnosis also included the possibility of a primary adnexal neoplasm and a complete excision was advised. The reexcision specimen revealed residual infiltrating dermal tumor and an overlying intraepithelial component with marked cytologic atypia and focal duct formation, diagnostic of a primary cutaneous adnexal tumor with ductal differentiation (porocarcinoma). Immunohistochemical studies (like p63 and CK5/6) can help to differentiate a primary cutaneous neoplasm from a metastatic lesion. This discrimination is of a paramount importance because a diagnostic error can result in profound implications for patient's assumed prognosis and subsequently applied therapy.