Primary extraskeletal osteosarcoma of the retroperitoneum: a case report

Extraskeletal osteosarcoma arising in the retroperitoneum is exceedingly rare, and its osteoid-producing histology closely mimics dedifferentiated liposarcoma. A 33-year-old woman presented with a 1-month history of progressive abdominal distension. Imaging revealed a 15 cm right retroperitoneal mass with punctate calcifications, no macroscopic fat, and no bone attachment. The mass was resected completely en bloc with negative microscopic margins (R0). Sections showed a high-grade sarcoma weaving through lace-like osteoid. MDM2 immunohistochemistry was negative and fluorescence in situ hybridization (FISH) detected no MDM2 amplification; CDK4 immunostaining and CDK4 FISH were likewise negative. No well-differentiated liposarcoma component was found on gross or microscopic examination, and together these findings ruled out 12q13-15 amplification. The final diagnosis was primary, high-grade extraskeletal osteosarcoma of the retroperitoneum. The patient received adjuvant MAP chemotherapy (methotrexate, doxorubicin, cisplatin) and at 25 months after surgery remains free of recurrence or metastasis; surveillance imaging will continue for at least 10 years. Any osteoid-producing retroperitoneal sarcoma without a well-differentiated liposarcoma component should undergo MDM2 amplification testing, and ideally CDK4 testing, because the two diagnoses are managed differently.