Malignant adenomyoepithelioma of the breast: A non‐tubular and matrix‐producing variant

Two women, aged 82 and 58 years old, cases 1 and 2, respectively, with the non-tubular and matrix-producing variant of malignant adenomyoepithelioma (MAM) of the breast are described. The tumors were 20 and 35 mm in diameter, respectively, and had cut surfaces with a tan-white-colored appearance and vague lobulation. Although both tumors showed marked central necrosis and a high level of mitoses, the tumor cells had relatively monomorphous nuclei and exhibited only mild atypia. The invasive component was predominantly trabecular and lobular, and the intraductal component demonstrated a focal-comedo pattern. The cytoplasm of the tumor cells was rather scanty, vacuolar or pale with ill-defined borders. The tumor in case 2 contained intermingled spindle-shaped cells. The stroma of both tumors resembled that of pleomorphic adenoma, containing a myxoid and chondroid matrix and, in case 2, cartilage and mature bone. Immunohistochemical and ultrastructural analyses of both tumors revealed dual cytological differentiation, predominantly of myoepithelial cells with secretory epithelial cells intermingled haphazardly. Although these tumors resembled metaplastic carcinomas, particularly matrix-producing carcinomas, they showed marked myoepithelial differentiation without overt tubular differentiation, a pattern quite different from matrix-producing carcinomas and from the adenomyoepitheliomas reported so far. MAM of the breast, non-tubular and matrix-producing variant, resembles epithelioid myoepithelial carcinoma of the salivary gland and has not been reported previously in the English literature.