Renal cell neoplasm of oncocytosis: a type of unclassified renal cell carcinoma.

A 69-year-old woman was referred to us because a cyst in the lower pole of the left kidney had been pointed out on screening ultrasound. Computed tomography (CT) demonstrated a mural nodule of the wall of the cyst with an enhancement effect, suggesting cystic renal cell carcinoma (cT1aN0M0). The patient underwent retroperitoneoscopic non-ischemic partial nephrectomy using a microwave tissue coagulator. A hematoxylin-eosin-stained specimen showed that the wall of the cystic tumor consisted of the proliferation of large cells with rich eosinophilic granules in the cytoplasm and round-shaped nuclei, which were the characteristics of oncocytoma. This was followed by immunohistochemical studies, because of findings of local cell invasion, mitosis, and necrosis, suggesting malignancy. The results were: CK7, strongly positive; PAX2, negative; colloidal iron stain, negative; c-kit, weakly positive; vimentin, positive; and RCC marker, negative. Thus, although the tumor showed characteristics of oncocytoma and/or chromophobe renal cell carcinoma (pT1a), it could not be classified according to the known categories: it should be designated as renal cell carcinoma (neoplasm) of oncocytosis, a novel category of unclassified renal cell carcinomas. The present case indicated the fact that renal cell carcinomas resembling oncocytoma certainly exist.