Primary monophasic breast synovial sarcoma in a female patient

Synovial sarcoma (SS) is a rare soft tissue sarcoma usually arising in the deep soft tissues of the limbs, trunk, and head and neck region. Due to its rarity, diagnosis can be difficult to establish, especially when it involves an uncommon location like the breast. In this case report, we describe a young woman who was found to have primary SS of the breast. Initial immunohistochemistry staining was focally positive for cytokeratin and S100 and she was misdiagnosed with atypical spindle cell carcinoma. Due to the unusual presentation, further testing was performed which showed TLE1 and epithelial membrane antigen positivity, establishing the diagnosis of SS of the breast. A FISH was later sent out and was positive for SS18-SSX fusion transcript. This case highlights the importance of considering rare histopathology in breast lesions and using additional staining and cytogenetics to confirm diagnosis.