Introduction/Background Sarcomas are tumors arising from mesenchymal elements. Malignant Synovial Sarcoma(MSS) also called Malignant Synovioma arises from tissues such as muscles and ligaments anywhere in the body and rarely in the abdomen. It is further classified into biphasic, monophasic or poorly differentiated. Biphasic is composed of epithelial cells, spindle fibroblast-like cells and epitheloid cells. Monophasic type exhibit abundant spindle cells, epitheloid cells lacking epithelial component. Lastly, poorly differentiated sarcomas which progress.

Methodology This is a case of nulligravid with hypogastric pain and right lower quadrant mass. Tumor markers were normal. CT scan revealed complex ill-defined mass with blood clots and moderate ascites. She underwent cytoreductive surgery with findings of pelvic mass attached to the right ovary, fallopian tube, posterior broad ligament and uterus. Histopathologic result revealed malignant spindle cell neoplasm and immunohistochemical studies showed positive on CD 99, TLE 1, Bcl-2, S100 and Desmin confirming MSS. It is classified according to AJCC staging system. Intraoperatively it is classified as Stage 1A and for adjuvant Ifosphamide-based chemotherapy.

Results MSS is a rare soft tissue neoplasm affecting adolescents and young aged 15–40 years old. It originates from primitive or uncommitted mesenchymal cells that differentiate to resemble synovial cells. It is caused by unique chromosomal translocations of t(X;18) (p11.2;q11.2) found in more than 90% of cases resulting in formation two fusion genes: SYT-SSX1 or SYT-SSX2. Immunohistochemistry has been proven to confirm diagnosis mainly of the monophasic variant. Immunohistochemical studies showed positive for TLE1, CD99, Bcl-2, S-100 and Desmin while negative for CD34. Initial surgery is an important prognostic factor, it has a 5-year survival rate of 76% following a complete cytoreductive surgery. The knowledge of its occurrence in unusual locations is necessary to avoid its misdiagnosis.

Conclusion We discussed a rare case of synovial sarcoma arising from the pelvis, including the confirmatory test and the management.

Disclosures None.