Invasive ductal carcinoma with a predominant intraductal component arising in a fibroadenoma of the breast

A rare case of invasive ductal carcinoma with a predominant intraductal component arising within a fibroadenoma of the breast in a 42 year old Japanese female was investigated by light microscopy. The patient, who had a well‐defined, rubbery breast tumor measuring 2.0 X 3.0 cm, had undergone a tumorectomy 21 months after she noticed the tumor. Histologically, the fundamental architecture of the tumor showed an intracanalicular‐type of fibroadenoma, but extensive proliferation of atypical cells was noticed in the lumen of the ducts. Tumor cells in the canaliculi had characteristics of ductal carcinoma, such as solid, comedo and cribriform patterns. As most of the ducts were occupied by carcinoma cells with several foci of microinvasion in the stroma of the fibroadenoma, it was diagnosed as invasive ductal carcinoma with a predominant intraductal component arising within a fibroadenoma. This coexistence of in situ proliferation and invasive lesions of carcinoma within a fibroadenoma suggest the origin of the carcinoma to have been in the epithelial component of the fibroadenoma. In addition, this study clarifies the fact that carcinoma cells may proliferate and spread along the lumen of pre‐existing ducts of the fibroadenoma and speculates that the duct system of the fibroadenoma has complete continuity.