Ductal eccrine carcinoma with intraductal squamous metaplasia: case report and critical review of diagnostic criteria.

Ductal eccrine carcinomas with squamous metaplasia are extremely rare. A 54-year-old woman presented with a 1-year lasting slowly growing nodular lesion on the right tibial region. The lesion measured 12 x 11 mm. An excisional biopsy was obtained. The tumor was well circumscribed and showed an infiltrative appearance at the periphery, consistent with a low-grade malignancy. Histologically, the lesion was composed of numerous tubular structures, lined by one or several layers of atypical basaloid cells with focal finding of a cuticular luminal border, admixed with clear myoepithelial cells. Diffuse intraductal squamous metaplasia was present. The tumor was associated with a fairly dense sclerotic stroma. The patient is free of recurrence or metastasis, 12 months after excision. The term “squamoid eccrine ductal carcinoma” (SEDC) has been used as synonymous of ductal eccrine carcinoma with squamous differentiation. All reported cases of SEDC are not convincing because the intraductal squamous metaplasia mixed with carcinomatous component has not been documented in the histological descriptions and in the illustrations. For this reason, we prefer the terminology of “eccrine ductal carcinoma with intraductal squamous metaplasia” in order to emphasize that two components are intermingled and that the nature of the squamous differentiation is benign.