Abstract: Small round cell mesenchymal component in breast metaplastic carcinoma is very rare and could be confused with other small round cell neoplasms of the breast. Synovial sarcoma exceptionally involves the breast and rarely may show a poorly differentiated small round cell component. These unusual small round cell components in biphasic metaplastic carcinoma and synovial sarcoma as well as collision tumor of ductal carcinoma and Ewing sarcoma might have overlapping clinical, histologic and immunohistochemical features which make distinction between these neoplasms difficult for the pathologists. Separation between these neoplasms is important for the treating oncologists because each tumor type has different prognostic implications and treatment modalities. Certain immunohistochemical markers might help, but cytogenetics study is the final confirmatory test. We report a unique example of a biphasic breast neoplasm in a 43-year-old woman that showed a combination of a minor central component of adenocarcinoma distinct from the bulky surrounding component of small round cell tumor. The histologic and immunohistochemical features were overlapping between metaplastic carcinoma, poorly differentiated synovial sarcoma and a composite collision tumor of invasive ductal carcinoma and Ewing sarcoma.
