Ultrastructural diagnosis of renal oncocytoma

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Renal oncocytoma is a comparatively rare tumor, having an incidence 1/20th that of renal cell carcinoma, from which it has to be differentiated. We report a case of a 61 year old man who was found to have albuminuria 100 mg/dL on a routine physical examination. A retrograde pyelogram showed splaying of the right upper pole infundibula. Two right renal tumors were shown on renal CT scan and there were multiple hypervascular masses on angiography. A right radical nephrectomy was performed. The kidney weighed 176 grains and measured 11 × 7.5 × 5 cm. There were several dark brown nodules in the upper pole, lower pole and midportion of the kidney. Some of the nodules protruded onto the cortical surface, stretching the overlying capsule, but, there was no evidence of capsular nor renal vein invasion. The largest nodule measured 4.5 × 4 × 4 cm. Histologic examination showed a distinct line of demarcation between the tumors and the normal kidney. Occasional thickened vessels were noted at the border. For the greater part, the tumor showed a tubular or acinar architecture, although the lumina of some of the tubules were narrow. The tubules were lined by large, polygonal eosinophilic cells with a granular cytoplasm. There was no evidence of nuclear atypicality nor mitotic activity. Thin walled blood vessels were present between the tubules. Transmission electron microscopy showed the presence of irregular tubules, surrounded by a well formed basement membrane. The tubules were separated by thin walled blood vessels (Fig. 1). The luminal surface of the cells lacked microvilli – Nuclei were comparatively small and irregular in shape. Lysosomes were concentrated towards the luminal surface. There were occasional lipid vacuoles. The cytoplasm contained large numbers of mitochondria with membranous cristae (Fig. 2) . These ultrastructural characteristics are in keeping with those of a renal oncocytoma. Comment: The term oncocyte1 is applied, at the light microscopic level, to large epithelial cells, which have a markedly eosinophilic and granular cytoplasm. Transmission electron microscopy shows the presence of an abundance of mitochondria filling the cytoplasm. Oncocytomas are tumors composed of a uniform population of such cells. Renal oncocytoma is considered to be a benign cortical adenoma. It has been postulated that it arises from proximal tubular epithelium. Features that distinguish renal oncocytoma from other adenomas are that they may reach a large size (7cm) and that they maybe multicentric in the same kidney (reported in 5% of cases). The dark brown bulging nodules present in our case are typical of the gross appearance of renal oncocytoma. Various theories have been proposed to account for the marked increase of mitochondria, including that it is due to a neoplastic proliferation of them, at the expense of other subcellular organelles. The diagnosis of oncocytoma is established by the ultrastructural demonstration of the abnormal abundance of mitochondria.