Primary retroperitoneal synovial sarcoma (RSS): A case report

INTRODUCTION: Leiomyosarcoma and lipomsarcoma account for the majority of retroperitoneal soft tissue sarcomas. Synovial sarcoma (SS) in this location is exceptionally uncommon, with fewer than 30 cases reported. Misdiagnosis is frequent due to its rarity and overlapping histological features with other sarcomas. CASE PRESENTATION: We reported the case of a 38-year-old woman who presented with a retroperitoneal mass. Initial imaging revealed a large, heterogeneous tumor displacing adjacent organs. A fine-needle biopsy was inconclusive, and laparoscopic resection was performed. Histopathology initially suggested leiomyosarcoma based on spindle cell morphology and immunopositivity for caldesmon and SMA. Two years later, the tumor recurred, exhibiting increased cellularity and mitotic activity. Further immunohistochemistry revealed positive staining for BCL2, EMA, and TLE1, raising suspicion of SS. Molecular analysis confirmed the SS18-SSX translocation, diagnosing high-grade monophasic synovial sarcoma. Despite surgical interventions, the patient experienced rapid recurrence and passed away within 6-months. DISCUSSION: RSS poses significant diagnostic challenges due to its rarity and overlapping features with more common sarcomas like leiomyosarcoma. Immunohistochemical panels including TLE1, BCL2, and EMA are critical for differential diagnosis. Molecular confirmation via the SS18-SSX fusion is necessary for definitive diagnosis. In our case, initial misdiagnosis delayed appropriate management. This highlights the importance of considering SS in young adults with retroperitoneal tumors. CONCLUSION: RSS, though rare, should be part of the differential diagnosis for spindle cell tumors in this location. Morphology, aided by thorough immunohistochemical and molecular analyses, is crucial to avoid diagnostic errors and improve patient outcomes.