[RENAL ONCOCYTOMA] – Renal oncocytomas are benign neoplasms composed exclusively of oncocytes that are arranged in nests or alveoli separated from each other by loose fibrous stroma. Solid sheet-like or papillary growth patterns are inconsistent with an oncocytoma. Cystic changes and hemorrhage can be seen but necrosis is not a feature. Most are solitary, incidental and about 2-3 cms in diameter but symptomatic ones are often 5-6 cms in maximum dimension.
[RENAL ONCOCYTOMA] – The cells of oncocytoma are brightly eosinophilic and arranged in nests embedded in a loose fibrous stroma. Clear cells are not seen in a pure oncocytoma. The nests are closely packed at the periphery of the tumor but widely separated from each other in the center. The central region of larger tumors often has a grossly identifiable scar composed of a large area of fibrosis. Smaller tumors may not have a central scar.
[RENAL ONCOCYTOMA] – The tumor cells are arranged in nests and may occasionally form tubules and trabeculae but large solid sheets of cells are not seen. Gland formation with mucin production and papillary structures are also incompatible with a diagnosis of oncocytoma. Cytogenetic findings of loss of chromosome 1 and Y also help in the differential diagnosis with chromophobe renal cell carcinoma. Mitoses are either not seen or are very rare and true nuclear anaplasia is also not a feature.
[RENAL ONCOCYTOMA].The brightly eosinophilic appearance is due the presence of cytochrome in numerous mitochondria that fill these cells. The granularity observed histologically corresponds to engorged rounded mitochondria that almost totally replace the cytoplasm. The tumor cells are thought to be related to the intercalated cells in renal collecting ducts but are not called collecting duct carcinoma. Note the presence of abundant granular cytoplasm, distinct cell membranes, small round and regular euchromatic nuclei, and only small nucleoli.