Collision tumor: a peripheral neuroepithelioma and a transitional-cell carcinoma occurring simultaneously in the renal pelvis.

We report a unique case of a collision tumor of the kidney composed of a peripheral neuroepithelioma (PNE) and a transitional cell carcinoma (TCC) that occurred in a 67-year-old man with a long history of superficial TCC of both the bladder and bilateral upper urinary tracts. The PNE was composed of hyperchromatic, small, round cells, arranged in clusters and poorly cohesive sheets. Immunohistochemical studies revealed the small cells to be strongly positive for O13 (ie, a MIC2 gene product stain [CD99]) and vimentin. Electron microscopic examination found numerous interdigitating cell processes, multiple poorly formed cell-cell junctions, and dense-core granules. The latter were most conspicuous at the tips of the cell precesses. The PNE cells were clearly negative for cytokeratin, and they showed no transitional or intermediate features that would suggest evolution of the PNE from the adjacent TCC. The proximity of these two tumors appears to be serendipitous; thus, it can be considered a true collision tumor of the kidney.