Primitive neuroectodermal tumor of the kidney confirmed by fluorescence in situ hybridization.

Peripheral primitive neuroectodermal tumors (PNETs) are rare lesions that form part of the Ewing family of tumors, which includes osseous and extraosseous Ewing's sarcoma and Askins tumor of the thorax. All are characterized by translocations involving the EWS gene at 22q12, usually the translocation t(11;22)(q24;12). PNETs usually occur in soft tissues but occasionally arise within a visceral organ. We describe a PNET of the kidney that showed characteristic microscopic and immunohistochemical appearances of a small, round, dark blue cell tumor with focal rosette formation and strong membrane positivity for the MIC2 gene product. Interphase fluorescence in situ hybridization on touch imprints prepared from frozen tissue using cosmid probes flanking the EWS gene at 22q12 and the FLI1 gene at 11q24 indicated the presence of t(11; = +22)(q24; = +q12), confirming the diagnosis of PNET. This is the first reported PNET of the kidney supported by cytogenetic analysis. We also review the literature on this fascinating tumor in this unusual location.