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Renal oncocytoma (RO) accounts for 5% of renal cancers and generally behaves as a benign tumor with favorable long-term prognosis. It is difficult to confidently distinguish between benign RO and other renal malignancies, particularly chromophobe renal cell carcinoma (chRCC). Therefore, RO is often managed aggressively with surgery. We sought to identify molecular biomarkers to distinguish RO from chRCC and other malignant renal cancer mimics. In a 44-patient discovery cohort, we identified a significant differential abundance of nine genes in RO relative to chRCC. These genes were used to train a classifier to distinguish RO from chRCC in an independent 57-patient cohort. The trained classifier was then validated in five independent cohorts comprising 89 total patients. This nine-gene classifier trained on the basis of differential gene expression showed 93% sensitivity and 98% specificity for distinguishing RO from chRCC across the pooled validation cohorts, with a c-statistic of 0.978. This tool may be a useful adjunct to other diagnostic modalities to decrease the diagnostic and management uncertainty associated with small renal masses and to enable clinicians to recommend more confidently less aggressive management for some tumors. PATIENT SUMMARY: Renal oncocytoma is generally a benign form of kidney cancer that does not necessarily require surgical removal. However, it is difficult to distinguish renal oncocytoma from other more aggressive forms of kidney cancer, so it is treated most commonly with surgery. We built a classification tool based on the RNA levels of nine genes that may help avoid these surgeries by reliably distinguishing renal oncocytoma from other forms of kidney cancer.

Renal oncocytoma (RO) accounts for 3–7% of all renal cells tumors. It typically consistes of large eosinophilic cells (oncocytes) with abundant cytoplasm, which constitute the crucial diagnostic feature. In 2001, the Czeck authors first described an unusual small cell variant of RO and untill now, only a few reports of such cases have been published. In the current article, the author presents an additional new case. 40-year old male with macroscopic hematuria as a clinical symptom was diagnosed to have solitary tumor in the upper third of the right kidney. He underwent a nephrectomy. On light microscopy, the tumor was predominantly composed of uniform small cells („oncoblasts“) with scant cytoplasm, hyperchromic nuclei and high nuclear-to-cytoplasmic ratio. In addition, it also contained characteristic oncocytes typical for oncocytoma. Tumor was strongly immunoreactive for EMA, sporadically positive for CK7 and negative for RCC antigen, vimentin, S100, WT1, chromogranin and synaptophysin. Proliferative activity did not exceed 1% and mitotic activity was virtually absent. No necrosis or aggressive growth features were found. The spectrum of histopathologic and immunohistochemical findings was consistent with a diagnosis of small cell variant of RO. The author focus on histopathological aspects and differential diagnostic pitfalls of this unique lesion.

A 50-year old woman was admitted to our hospital for further evaluation of the right renal tumor. She had been on maintenance hemodialysis for 12 years. Computed tomography (CT) and magnetic resonance imaging (MRI) revealed a large right renal tumor. There was no central stellate scar. Angiography showed hypervascularity and tumor staining . With the pre-operative diagnosis of renal cell carcinoma, right nephrectomy was performed. The pathological diagnosis was renal oncocytoma., Acta medica Nagasakiensia. 1997, 42(1-2), p.60-62