258 An Unusual Case of Retroperitoneal Poorly Differentiated Synovial Sarcoma in an Adolescent Boy

Abstract

Poorly differentiated round cell variant synovial sarcoma is a rare neoplasm that typically arises in distal extremities of adults over 30 years. Importantly, it is associated with higher metastatic rate and worse outcome (five-year survival 20%) than conventional synovial sarcoma, necessitating its accurate identification. Our aim is to report an unusual case of retroperitoneal poorly differentiated synovial sarcoma that presented clinically as a slow growing mass in an adolescent boy. Retrospective review of a case of a 14-year-old obese boy with left-sided lower back mass that has been increasing in size for the past eight years. CT imaging showed the soft-tissue mass extending from the lumbar paraspinal region into the retroperitoneum. Interventional radiology-guided biopsies demonstrated a high-grade round blue tumor. Additional testing did not confirm a specific diagnosis, and the patient underwent a wide resection to reveal a 14.5 cm tan-white lobulated soft mass with hemorrhage and necrosis extending into the skeletal muscle and fibroadipose tissue. Histologically, the tumor had small round blue cell morphology with high- grade features including increased mitotic activity, necrosis, and lymphovascular invasion. Immunohistochemical stains showed diffuse positivity to vimentin and CD56, and focal positivity to Cam5.2, CD99, WT1, and S100. It was negative for EMA and Melan A, as well as a wide panel of smooth muscle, lymphoid, and neuroendocrine markers. Differential diagnosis included Ewing sarcoma, round cell liposarcoma, desmoplastic small round cell tumor, and MPNST. G-banded chromosomal karyotyping revealed the characteristic t(X;18) translocation helping to render the final diagnosis. Post-operatively, the patient underwent seven cycles of chemotherapy and is doing well at 1.5 years’ follow-up. This case highlights how cytogenetics can help in making the correct diagnosis particularly in unusual lesions that lack typical histological and immunohistochemical findings. Recognition of the poorly differentiated variant is critical because of poor prognosis.