Monophasic Synovial Sarcoma in an Adolescent: A Great Mimicker of Benign Pathology

Monophasic synovial sarcoma (SS) is a rare and aggressive soft-tissue malignancy that can often resemble benign conditions, making diagnosis challenging. A 13-year-old girl presented with a painless, gradually enlarging swelling in the proximal lower extremity for 4 months. Clinical examination revealed a firm, well-defined, deep-seated swelling, and imaging initially indicated a benign spindle-cell tumor, likely a schwannoma. En bloc excision of the mass was done. Histopathology and immunohistochemistry (positive for TLE1 and BCL2) revealed the diagnosis of monophasic SS. Adjuvant chemotherapy and radiotherapy were given. This case highlights the importance of maintaining a high index of suspicion for malignancy in deep-seated soft-tissue tumors. Early diagnosis, complete surgical removal, and multimodal therapy are crucial for the management of SS.