small cell variant of renal oncocytoma – a case report of unusual histopathologic entity

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.