Low-Grade Oncocytic Renal Tumor With Massive Liver Metastasis: Do Not Trust a Negative CK7 and Misdiagnose Oncocytoma: A Cautionary Tale and Current Classification of Oncocytic Renal Tumors

We report a renal tumor originally diagnosed as oncocytoma that recurred with biopsy-proven multiple liver metastases. Review of the initial tumor revealed a predominantly CK7-negative neoplasm that had areas resembling oncocytoma, areas resembling eosinophilic chromophobe renal cell carcinoma, and areas with intermediate features. The case illustrates that the distinction between eosinophilic chromophobe renal cell carcinoma and “oncocytic tumor of uncertain malignant potential” is in some cases subjective. Criteria need better definition and should not be based on cytokeratin 7 staining pattern alone. A discussion of the current classification of oncocytic renal tumors is included, as well as a discussion of the differential diagnosis of renal neoplasms with eosinophilic cytoplasm.