Malignant transformation of retroperitoneal inflammatory pseudotumor to undifferentiated high-grade pleomorphic sarcoma on FDG PET/CT.

We report FDG PET/CT appearance of malignant transformation of retroperitoneal inflammatory pseudotumor to pleomorphic sarcoma in a 78-year-old woman. The patient was diagnosed with left retroperitoneal pseudotumor in 2007. She subsequently suffered from recurrent disease and chronic osteomyelitis in the L5 vertebral body in July 2008, status post surgical excision. In November 2009, after the patient developed left groin pain, MRI study of lumbosacral spine and whole-body PET/CT study demonstrated 2 hypermetabolic soft-tissue masses originating from the L5 surgical bed. These masses were subsequently resected and were consistent with pleomorphic sarcoma. Malignant transformation of retroperitoneal pseudotumor to pleomorphic sarcoma is extremely rare.