[A case of renal oncocytoma with cystic formation].

A 60-year-old man was admitted to our hospital because of a right renal mass incidentally detected by ultrasonography during a general health check. Computerized tomography (CT) and magnetic resonance imaging (MRI) revealed a solid mass measuring 6 cm diameter containing a cystic portion in the right kidney. Cystic renal cell carcinoma was suspected, and right nephrectomy was performed. The gross appearance showed a well circumscribed mahogany brown tumor in the lower portion. On cross section of the tumor, a solid mass was seen in the cystic portion. Microscopically, oncocytic cells with eosinophilic granular cytoplasm were scattered on the cystic wall, and tumor cells formed microcystic structures in the solid part. Therefore, the tumor was diagnosed as renal oncocytoma with cystic formation. Only twelve cases have previously been reported in the literature.