METAPLASTIC CARCINOMA OF THE BREAST WITH MELANOCYTIC DIFFERENTIATION

We report the clinical and pathologic findings of a metaplastic carcin oma of the breast that exhibited melanocytic differentiation, The tumo r possessed both in situ and invasive components. Lower grade regions of the infiltrating carcinoma had features of tubular, mucinous, and m atrix-producing carcinomas. In the higher grade areas, conventional po orly differentiated ductal carcinoma merged with an anaplastic neoplas m that looked like malignant melanoma The nonpigmented cells stained f or keratin but lacked HMB-45 and S-100 proteins, whereas the cells con taining melanin showed the opposite characteristics. Electron microsco pic examination disclosed melanosomes in the neoplastic cells. We beli eve that these observations convincingly establish both the origin of the tumor from the mammary epithelium and the synthesis of melanin by the tumor cells. We propose the diagnosis of metaplastic carcinoma wit h melanocytic differentiation for this neoplasm and suggest that the p henomenon of melanocytic metaplasia might underlie the formation of pr imary melanomas of the breast.