Answer: Renal cell carcinoma, chromophobe type

Histology: The lesion consists of a solid sheet of cells. Some of the cells have a pale appearance with abundant pale, lightly eosinophilic granular cytoplasm. Other cells are more eosinophilic in appearance. All of the cells show prominent cell borders. The nuclei show variation in size and shape, some being small and round while others are large and irregular. Binucleated cells are commonly seen. Perinuclear halos are easily identified throughout the tumor.

Discussion: Oncocytomas may resemble chromophobe renal cell carcinomas in that they have prominent eosinophilic cytoplasm. However, oncocytomas usually show densely eosinophilic cytoplasm throughout the entire lesion and lack the pale appearance seen in many of the cells in this case. Also, oncocytomas show uniformly round nuclei without nuclear irregularity or pleomorphism, which is seen in this case. The nuclei would also lack perinuclear halos in contrast to a chromophobe renal cell carcinoma. In addition, oncocytomas do not have such distinct cell borders as a chromophobe renal cell carcinoma. The Hale’s colloidal iron stain, which stains for acidic mucin, would be helpful in cases where it is difficult to tell the two lesions apart. A chromophobe renal cell carcinoma should show intense and diffuse staining within the cytoplasm of the tumor cells while an oncocytoma is usually negative or only shows luminal staining within some of the tubules for Hale’s colloidal iron. In contrast to the conventional or clear cell type of renal cell carcinoma, chromophobe renal cell carcinomas do not show optically clear cytoplasm, but rather have a fine granular and lightly eosinophilic appearance. The term granular renal cell carcinoma is no longer used. Tumor cells with a granular appearance may be seen in any number of renal cell carcinoma subtypes including conventional type renal cell carcinoma, papillary renal cell carcinoma, and chromophobe type renal cell carcinoma.