Oncocytic renal cell carcinoma with immunohistochemical properties of renal oncocytoma

Renal oncocytoma (RO) is a characteristic benign renal tumor. The existence of malignant RO is controversial and anecdotal, partly due to a lack of specific markers for RO. With recent advances in immunohistochemistry, RO can be distinguished from other renal neoplasms with routine stains and with the aid of immunostaining. We report two cases of renal neoplasms with similar histopathological appearances. They were characterized by oncocytic cytoplasm, numerous intra-cytoplasmic vacuoles, uniform round to oval hyperchromatic nuclei with remarkably thick nuclear membranes and prominent nucleoli. The tumor cells were closely packed and disposed in an alveolar pattern. The neoplastic cells were diffusely reactive for CD117 and progesterone receptor, and diffusely or focally reactive for cytokeratin AE1/AE3, and focally reactive for cytokeratin 7, CD10, and racemase. The cells were non-reactive for renal cell carcinoma (RCC) antigen, vimentin, S100, and neuroendocrine markers. One tumor showed lymph node metastasis. Due to the remarkable cytological atypia, lymph node metastasis, and similar immunological features of RO, these two tumors likely represent a distinct subtype of RCC related to RO.