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Renal oncocytoma is a benign solid tumor of renal epithelial origin, characterized by tumor cells with abundant acidophilic granular cytoplasm. Recently differentiation from oncocytic chromophobe renal cell carcinoma has been an issue, but clear distinction by light microscope is very difficult. Therefore we analyze morphologic features observed by electron microscope in five cases of renal oncocytoma. Grossly, the tumor mass showed mahogany brown solid mass, ranged from 4.5 to 16 cm in diameters. One large mass showed focal hemorrhage and necrosis. LM findings showed sheets or alveolar arrangement of uniform eosinophilic tumor cells supported by delicate fibrovascular septa. Ultrastructurally, tumor cells formed clusters delineated by thin basal lamina. The cell membrane showed short microvilli in two cases. The cytoplasm was closely packed with round mitochondria with stacked lamellar cristae and rare small electron dense granules. Other organelles including rough endoplasmic reticulum, lipofuscin, lysosome, lipid vacuoles were rare. In all cases, no definitive microvesicles were present. The nuclei were uniform small round to oval with heterochromatin with occasion nucleoli. Tumor cells in five cases of oncocytoma we have experienced showed relatively consistent ultrastructural findings. Close packed mitochondria with stacked lamellar cristae and rare microvesicles are helpful findings in differentiating from chromophobe renal cell carcinoma, where abundant microvesicles are present. Mitochondrial cristae are of tubulovesicular type, and nuclei are markedly convoluted with irregularly folded nuclear membranes and prominent heterochromatin.
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