Primary ovarian synovial sarcoma in an adolescent mimicking a germ cell tumor: A case report and diagnostic challenge

Background Primary ovarian synovial sarcoma is an exceedingly rare malignancy that may mimic more common ovarian tumors, particularly in adolescents, posing significant diagnostic challenges. Case presentation A 14-year-old girl presented with progressive abdominal distension and respiratory compromise. Imaging revealed a large mixed solid–cystic adnexal mass with ascites and pleural effusion. Serum CA-125 was elevated, while AFP and CEA were normal. The patient underwent right salpingo-oophorectomy and omentectomy. Histopathology showed a monophasic spindle cell neoplasm with focal cartilaginous differentiation. Immunohistochemistry demonstrated diffuse nuclear positivity for SS18 and TLE1, with co-expression of BCL-2, CD99, and EMA, while inhibin, calretinin, and WT-1 were negative. The positive SS18 immunohistochemical staining provided additional diagnostic confirmation of synovial sarcoma, supporting the diagnosis in conjunction with the characteristic histopathologic findings. The patient deteriorated rapidly and died before initiation of chemotherapy. Conclusion Primary ovarian synovial sarcoma is a rare and aggressive tumor that can mimic germ cell neoplasms. Accurate diagnosis requires careful integration of histopathology and immunohistochemistry. Although molecular confirmation remains the diagnostic gold standard, SS18 fusion-specific immunohistochemistry provides a reliable diagnostic surrogate when molecular testing is unavailable.