Ewing sarcoma/peripheral primitive neuroectodermal tumor (pPNET) is a rare primary tumor of the kidney with morphologic features similar to those of other primitive tumors. Previous studies have shown that these tumors frequently stain positively with immunostains against CD99 and FLI-1 and negatively with stains against WT-1, suggesting that these markers may be used for the distinction between Wilms tumor and pPNET. We present 30 cases of primary malignant neuroepithelial tumor with immunohistochemical profiles and reverse transcriptase polymerase chain reaction (RT-PCR) analysis and show that immunophenotypic overlap exists between Wilms tumor and pPNET. A subset of 30 neuroepithelial tumors from the National Wilms Tumor Study originally categorized as putative pPNETs of the kidney was stained with FLI-1, WT-1, and thyroid transcription factor-1. Bicolor fluorescence in situ hybridization studies were performed on 19 of the cases. Other data on these tumors were available from a previous study (Am J Surg Pathol 2001;25:133). Of 7 primary tumors that had the EWS/FLI-1 fusion transcript by RT-PCR, 6 exhibited strong immunopositivity for FLI-1. Nine that were negative by RT-PCR stained positively with the FLI-1 stain. Five fusion-negative cases stained with both FLI-1 and WT-1. Three fusion-negative cases were negative for FLI-1 but positive for WT-1. Five fusion-negative cases were negative for both FLI-1 and WT-1. Of the 30 cases, 29 were positive for CD99. Seven cases that were negative for the EWS-FLI-1 fusion by RT-PCR were positive by fluorescence in situ hybridization. All cases were negative for thyroid transcription factor-1. Reliance upon immunohistochemistry as the sole means of ancillary diagnosis in renal pPNET can lead to confusing results. We recommend molecular fusion studies for clarification of primitive renal tumors with unexpected immunophenotypic results.
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