A 23-year-old woman with recurrent uterine endometrial stromal sarcoma underwent FDG PET/CT, which showed pelvic recurrence and multiple pulmonary metastases. In addition, increased activity in a vertical soft tissue mass along the left ureter was noted, which appears similar to retroperitoneal fibrosis on PET/CT images. However, pathological examination demonstrated that the elevated FDG activity in this mass was a result of tumor thrombus from endometrial stromal sarcoma in the left ovarian arteries and veins.