High FDG Uptake on Oncocytoma Located in the Retroperitoneum Mimicking Malignancy

Oncocytoma is a benign tumor composed histologically of epithelial cells with abundant acidophilic, large granular cytoplasm packed with accumulating mitochondria, and can be arranged in alveolar, tubular, or solid patterns. Oncocytoma is commonly found in the kidneys, salivary glands, and endocrine organs. Oncocytoma arising in retroperitoneal heterotopic adrenal tissue is very rare. We report a case of oncocytoma that showed a significantly high uptake of F-18 fluorodeoxyglucose (F-18 FDG) on positron emission tomography (PET) study. The lesion was located in the retroperitoneum and was strongly suspected of arising from retroperitoneal heterotopic adrenal tissue. High FDG uptake usually indicates existing malignant potential. Oncocytoma should be considered in the differential diagnosis in cases of high FDG uptake in a solid mass.