Metaplastic breast carcinoma is very rare, and metaplastic carcinoma with chondroid differentiation is even rarer. Here, we report a case of metaplastic carcinoma with extensive chondroid differentiation mimicking chondrosarcoma that was challenging to diagnose. The tumor was characterized by an abundant chondromyxoid matrix. The definitive area of classic invasive ductal carcinoma was minimal. The peripheral portion of the tumor showed increased cellularity with pleomorphism and definitive invasive growth. Tumor cells in the chondrosarcomatous areas were diffusely immunoreactive for S-100 protein, patchy positive for cytokeratin, but negative for epithelial membrane antigen (EMA). Tumor cells in carcinomatous areas were diffusely positive for cytokeratin, S-100 protein, and patchy positive for EMA. In both areas, tumor cells were negative for smooth muscle actin (SMA) and CD34, while oncoprotein p53 was overexpressed. When pathologists encounter breast tumors with chondroid differentiation, careful sampling and immunohistochemistry for cytokeratin and SMA are most helpful to differentiate metaplastic carcinoma from malignant phyllodes tumor and malignant adenomyoepithelioma.
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