Epithelioid Malignant Peripheral Nerve Sheath Tumor of the Breast with Breast Cancer in the Contralateral Breast: A Case Report

Reports of MPNSTs in the breasts are rare. The pathological analysis showed atypia of spindle cells, positive staining for vimentin, S100, SOX10, and epithelial-like cells, and positive staining for cytokeratin AE1/AE3. H3K27me3 is reportedly absent in typical MPNSTs; however, it is expressed in epithelioid types. It sometimes occurs in patients with a germline mutation in <i>SMARCB1/INI1</i>, where INI1 is absent in tumor cells. In this case, the tumor cells expressed H3K27me3 and also maintained INI1.