Solid Tumour Section

Macroscopically, renal oncocytomas are solitary, wellcircumscribed, slightly lobulated solid tumors with generally mahogany brown or dark red cut surface. A central scar is frequently observed. Necrosis and hemorrhage are uncommon. Microscopically, tumor cells with abundant, granular cytoplasm are arranged in nests, tubulocystic, solid, or trabecular patterns within edematous, myxomatous or hyalinized stroma. On low power, the epithelial cell nests surrounded by myxomatous stroma gives the appearance of an island archipelago, which is a characteristic feature of oncocytomas. The nucleus is homogenous, round and centrally located, but large bizarre nuclei are frequently observed. Ultrastructurally, tumor cells contain many mitochondria with predominantly lamellar cristae. Some special stains are helpful for differential diagnosis with the eosinophilic or granular form of clear cell carcinoma: most RO show negative vimentin and RCC antigen staining, although positive staining for vimentin in up to 72% has been reported in some series. The immunohistochemical patterns of oncocytoma and chromophobe carcinoma are virtually identical, which often makes distinctions of these tumors with overlapping morphologic features quite difficult. Recently, claudin 8 positivity in oncocytomas has been reported as tool to differentiate these two neoplasms since chromophobe carcinomas are negative for claudin 8 and positive for claudin 7. Some authors have suggested that oncocytoma can progress to chromophobe carcinoma, based on shared abnormalities in chromosome 1, although the occurrence of this transformation has not been conclusively proven.