Answer: Invasive ductal carcinoma, histiocytid

Histology: This breast contains an irregular proliferation of epithelioid cells with abundant foamy to finely granular pale pink cytoplasm. Some of these same cells appear to be filling ducts in the pattern of ductal carcinoma in situ. The nuclei are generally round though focally irregular, but lack prominent vacuolization. These cells were immunoreactive for cytokeratin.

Discussion: Granular cell tumor is in the differential of histiocytoid ductal carcinoma. Granular cell tumors, like many breast carcinomas, express S100 protein, but they lack labeling for cytokeratin. Histiocytic reaction is excluded by the nuclear atypia and, most importantly, by the immunoreactivity for cytokeratin. Lipid-rich carcinoma may resemble histiocytoid ductal carcinoma. However, lipid-rich carcinomas feature tumor cells with a lipoblast-like appearance; namely, the cells have multivacuolated lipid-rich cytoplasm with indentation of the nuclei.

Many cases of histiocytoid breast carcinoma have been lobular carcinomas, which lack the expression of E-cadherin. An unusual proclivity to metastasize to the eyelid has been described. However, it is now clear that ductal carcinomas may have a histiocytoid appearance. These cases will be immunoreactive for e-cadherin. Many histiocytoid carcinomas express the apocrine marker GCDFP.

Reference(s):
– Ann Diagn Pathol. 2002; 6:141-147.