"Squamoid eccrine ductal carcinoma": an unusual low-grade case with follicular differentiation. Are these tumors squamoid variants of microcystic adnexal carcinoma?

Squamoid eccrine ductal carcinoma is a rare primary cutaneous tumor exhibiting both squamous and adnexal ductal differentiation. The cell of origin of these tumors is unknown, and they have been classified both as variants of cutaneous squamous cell carcinoma and as a type of eccrine carcinoma. Only 6 of these tumors have been reported in the literature to date. We report an additional case of a slow-growing tumor, occurring on the great toe of a 61-year-old woman, which was unusual as it showed follicular differentiation in addition to squamoid and ductal areas. The lesional cells were positive for cytokeratins 7 and 17, carcinoembryonic antigen (which highlighted the ductal structures), and p63 (favoring a primary cutaneous tumor) and showed low levels of staining with Ki-67 and p53, consistent with a low-grade tumor. We postulate that these tumors may be closely related to microcystic adnexal carcinoma and similarly show differentiation along both follicular and ductal lines, likely indicating folliculosebaceous-apocrine, rather than eccrine, origin or differentiation.