Extrarenal Retroperitoneal Angiomyolipoma Masquerading as Retroperitoneal Liposarcoma : A Report of Two Cases

We report two patients with extrarenal retroperitoneal angiomyolipoma masquerading as perinephric liposarcoma. Patient 1 : A 66-year-old man was diagnosed with a retroperitoneal tumor near the right renal hilum on an abdominal computed tomography (CT) performed before surgery for gastric cancer. A diagnosis of extrarenal retroperitoneal angiomyolipoma was made on the basis of negative uptake of fluorine- 18 2-deoxy-2-fluoro-D-glucose positron emission tomography (18F-FDG PET)/CT. However, because the tumor was found to have gradually enlarged at 18 months afterward, he underwent resection of the extrarenal fat tissue together with the right kidney. Patient 2 : A 56-year-old man underwent abdominal ultrasound during a periodic medical examination, which revealed a right retroperitoneal tumor. Because of the findings in the contrast-enhanced CT and positive uptake of 18F-FDG PET/CT, he underwent resection of the extrarenal fat tissue together with the right kidney. The pathological examination of the two tumors confirmed extrarenal angiomyolipoma. The differential diagnosis of extrarenal retroperitoneal angiomyolipoma from retroperitoneal liposarcoma is difficult even with the use of 18F-FDG PET/CT.