Synovial sarcomas are rare tumors that comprise only 1% of all malignancies. The retroperitoneum is an uncommon site of origin for them, accounting for 10% of all soft tissue sarcomas.

The other differential diagnoses are liposarcomas, leiomyosarcomas, malignant fibrous histiocytomas and malignant peripheral nerve sheath tumors. If there is associated lymph node enlargement, lymphoma should be considered.

Acknowledgments: Dr. Naim Qamhia, Pathologist, King Hussein Cancer Center, Amman, Jordan.