Renal tumor with overlapping distal nephron morphology and karyotype.

Although most renal epithelial tumors are derived from the proximal nephron, approximately 10% are believed to originate in the distal nephron. This latter group encompasses oncocytoma, chromophobe renal cell carcinoma, and several rare types, including collecting duct carcinoma and renal medullary carcinoma. Despite progress in the classification of renal tumors, a small subset of renal carcinomas remains unclassified (ie, renal cell carcinoma, not otherwise specified). We describe a metastatic tumor consisting of cells with overlapping distal nephron morphologies, including foci of oncocytoma, chromophobe renal cell carcinoma, and collecting duct carcinoma, as well as sarcomatoid dedifferentiation. Special stains were inconclusive, and ultrastructural study demonstrated abundant mitochondria and no microvesicles. The karyotype was hypodiploid with 41 chromosomes and abnormalities reported in all 3 phenotypes present. Rearrangements of 1p and of 11q13 previously seen in divergent subsets of oncocytomas were concomitantly present in the current tumor. Thus, this malignancy has features consistent with distal nephron derivation and demonstrates the convergence of the varied tumor morphologies arising within this site. Furthermore, this case exemplifies the value of cytogenetic analysis in the characterization of renal cell carcinoma, not otherwise specified. In view of recent advances in treatment approach, especially for collecting duct carcinoma, further categorization of this nondescript and heterogeneous group of renal cell carcinomas, not otherwise specified, at least by its derivation in relationship to the renal nephron (distal vs proximal), may be of value in the choice of treatment modality.