Primary Synovial Sarcoma of the Breast Initially Mimicking Dermatofibrosarcoma Protuberans: A Case Report.

Primary synovial sarcoma of the breast is an extremely rare malignancy, representing less than 1% of breast tumors. Its clinical presentation can mimic other spindle cell neoplasms, including dermatofibrosarcoma protuberans (DFSP), making diagnosis challenging. We report the case of a 60-year-old postmenopausal woman presenting with a firm, mobile 10 cm mass in the upper outer quadrant of the left breast, classified on imaging as breast ımaging reporting and data System 4. A core needle biopsy initially suggested DFSP because of strong CD34 positivity. A simple mastectomy revealed a highly cellular, spindle-cell tumor arranged in fascicles, forming a characteristic whorled pattern. Immunohistochemistry showed CD34 negativity, transducin-like enhancer of split 1 positivity, B-cell lymphoma 2 positivity, and focal epithelial membrane antigen positivity. Molecular analysis confirmed SYT-SSX1 fusion, establishing the diagnosis of monophasic synovial sarcoma. Surgical margins were negative, and one-year follow-up showed that the patient remained disease-free. This case highlights the importance of immunohistochemistry and molecular testing for accurate diagnosis and appropriate management.