Primary Ewing sarcoma of the brain: a case report and literature review.

Ewing sarcoma, along with peripheral primitive neuroectodermal tumor, belongs to a tumor family that shares clinicopathologic and molecular genetic features, including the characteristic chromosomal translocation that results in the fusion of the EWS gene on 22q12 to either the FLI1 gene on 11q24 or other Ets family transcription factor gene, such as the ERG gene on 21q22. In contrast, such translocations are not found in central primitive neuroectodermal tumors (cPNETs), such as medulloblastoma and supratentorial PNET. Ewing sarcoma has only rarely been noted to primarily involve the central nervous system-extraosseous Ewing sarcoma (CNS-EES). We report a case of a 7-year-old girl with an anterior cranial fossa mass. Pathology showed a primitive small blue cell tumor with focal Homer Wright rosette formation. The positive membranous immunostaining for CD99 and the EWS-FLI1 fusion demonstrated by fluorescence in situ hybridization studies confirmed the diagnosis of CNS-EES. Although CNS-EES may look identical to cPNETs, these tumors differ in histogenesis, molecular characteristics, and clinical behavior. Demonstration of characteristic translocations by molecular studies differentiates CNS-EES from cPNET and help clinicians make informed decisions regarding therapy.