Reviewers: Dharam M. Ramnani, MD
In metaplastic carcinomas, the predominant cell type is other than epithelial, usually a mesenchymal element. They make up less than 1% of all mammary carcinomas. The most common metaplastic elements are spindle cell and squamous, whereas heterologous elements including chondroid, osseous, and other sarcomas are seen in a minority of tumors. The epithelial component is usually DCIS and/or invasive ductal carcinoma, usually high-grade. In rare cases, lobular carcinoma-in-situ or atypical ductal hyperplasia is the epithelial component. Chronic inflammation within and around the tumor is a frequent finding. The extent of metaplastic change ranges from microscopic foci to complete replacement of the tumor by the metaplastic phenotype. There are no established criteria regarding the amount of metaplastic component required to make the diagnosis of metaplastic carcinoma. Regardless of the extent, one should mention the specific component/s present. The morphologic range of metaplasic carcinomas is broad but some of the common subtypes are as follows: spindle cell carcinoma (fibromatosis-like; fibrosarcoma-like), sarcomatoid carcinoma/carcinosarcoma (chondrosarcoma, osteosarcoma, matrix producing carcinomas), metaplastic high-grade adenosquamous carcinoma, carcinoma with osteoclast-like giant cells, and metaplastic carcinoma with choriocarcinoma. The prognosis varies depending upon the specific subtype present. Low-grade spindle cell carcinomas (fibromatosis-like) have a low-risk of recurrences and distant metastases. Carcinosarcomas and matrix-producing metaplastic breast carcinomas are highly aggressive tumors and usually require mastectomy and chemotherapy.