Metaplastic carcinoma of the breast with melanocytic differentiation.

We report the clinical and pathologic findings of a metaplastic carcinoma of the breast that exhibited melanocytic differentiation. The tumor possessed both in situ and invasive components. Lower grade regions of the infiltrating carcinoma had features of tubular, mucinous, and matrix-producing carcinomas. In the higher grade areas, conventional poorly differentiated ductal carcinoma merged with an anaplastic neoplasm that looked like malignant melanoma. The nonpigmented cells stained for keratin but lacked HMB-45 and S-100 proteins, whereas the cells containing melanin showed the opposite characteristics. Electron microscopic examination disclosed melanosomes in the neoplastic cells. We believe 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 with melanocytic differentiation for this neoplasm and suggest that the phenomenon of melanocytic metaplasia might underlie the formation of primary melanomas of the breast.