Renal epithelial neoplasms consist of a group of distinct genetic and clinical entities that occasionally have overlapping morphological features. Pronounced cytoplasmic granularity or eosinophilia may be seen in a number of tumor types, including conventional (clear-cell) carcinomas, papillary carcinomas, chromphobe carcinoma, collecting duct carcinomas, and oncocytomas. Mesenchymal neoplasms such as angiomyolipomas as well as metastatic lesions such as malignant melanoma may have marked epithelial features and cytoplasmic granularity, thus mimicking a renal epithelial tumor. The same can be said for adrenal cortical neoplasms, which sometimes may be confused clinically, radiographically, and pathologically with a renal neoplasm. Close attention to morphological and cytologic detail will solve the differential diagnosis in the majority of cases, although some will require ancillary studies such as histochemistry, immunohistochemistry, and election microscopy. In a small percentage of cases molecular genetic studies are required to properly classify the tumor.
