Carcinosarcoma of the Breast

Carcinosarcoma of the breast is rare and accounts for less than 0.1% of all breast malignancies. Though carcinosarcomas are considered a form of metaplastic breast cancer, these tumors warrant special discussion because of their aggressive and ominous prognosis. Carcinosarcomas consist of intraductal or infiltrating carcinoma contiguous or subtly merged with a highly cellular, often mitotically active malignant-appearing stroma (sarcomatous component). The macroscopic appearance of these tumors is dependent on the variety of possible epithelial and mesenchymal components. By traditional definition, a "classic" carcinosarcoma has distinct demarcation between carcinomatous and sarcomatous components in all microscopic fields. If there is obvious microscopic transition between the epithelial and sarcomatoid components, then the term "biphasic sarcomatoid carcinoma" has often been employed to distinguish this tumor from a "classic" carcinosarcoma. Although the term "carcinosarcoma" has been used inconsistently in the literature, sometimes to describe only "classic" carcinosarcomas and in other instances to describe both "classic" carcinosarcomas and other metaplastic breast sarcomatoid carcinomas, the less restrictive definition will be used in this chapter. Tumors that show both carcinomatous and sarcomatous features occur in various anatomical sites and, in spite of differences in terminology and exact microscopic composition, evidence suggests these are all similar tumors developing through a peculiar phenotypic transformation of carcinoma cells into sarcoma. In the breast, the most popular theories regarding the histogenesis of the sarcomatous component of carcinosarcomas and sarcomatoid carcinomas propose the malignant transformation of myoepithelial cells or myofibroblastic metaplasia of malignant epithelial cells.