Mammary sarcoma is extremely rare and the diagnosis is established only after metaplastic carcinomas and malignant phyllodes tumours are excluded. A rare case of not otherwise specified-type sarcoma with CD10 expression in the left breast in a 45-year-old female was presented. It was a high-grade tumour composed of spindle cells histologically. The immunohistochemical results showed that CD10, vimentin and EGFR were positive diffusely and SMA presented focally, whereas epithelial markers and other myoepithelial or myogenic markers were all negative. The electron microscope investigation demonstrated fibroblast-like features. The exact entity of the tumour remains to be studied because it resembles undifferentiated sarcoma or sarcomatoid metaplastic carcinoma to some degree, as well as high-grade malignant phyllodes tumour in particular.
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