A positive hybrid (HMW-CK and E-cadherin) carcinoma in situ arising in a phyllodes tumor of the breast -: A case report

Malignant transformation in phyllodes tumor (PT) is uncommon and almost always confined to the stromal component. Epithelial changes like hyperplasia, metaplasia, and varying degrees of atypia are not uncommon in PT, whereas carcinomatous change is extremely rare. We report a 37-year-old woman with carcinoma in situ (CIS) arising in a benign PT. Grossly, it was a well circumscribed, 4.5 cm-sized mass. The CIS component was confined to the PT and showed overlapping ductal and lobular features with coexpression of E-cadherin and high molecular weight cytokeratin (HMW-CK). The present case emphasizes that careful investigation of multiple microscopic sections is mandatory to find a small carcinomatous lesion within PT. Expression of E-cadherin and HWM-CK in this hybrid CIS suggests that intraepithelial neoplasia of the breast arising in PT may be derived from a common progenitor of the terminal duct-lobular unit.