Histiocytoid Ductal Carcinoma In Situ of the Breast: Not All Intraductal Foamy Cells Are Macrophages!

With the advent of screening mammography, the incidence of ductal carcinoma in situ (DCIS) has increased. DCIS accounts for around 20% of breast cancers diagnosed at present. The histiocytoid variant of clear cell DCIS is a rare subtype of DCIS, characterized by epithelial cells with a clear and foamy cytoplasm. Histiocytoid DCIS serves as a potential diagnostic pitfall, since it can be easily mistaken for intraductal collections of foamy macrophages. In this article, we report a particular case of biphasic DCIS, characterized by an extensive histiocytoid-type clear cell component and a conventional "non-clear" cell component. Both components presented with HER2 protein overexpression. We discuss the diagnostic challenge and differential diagnosis of clear cell DCIS, as well as the role of HER2 overexpression in DCIS pathogenesis.