Extraosseous Ewing sarcoma with foci of neuroblastoma-like differentiation associated with EWSR1(Ewing sarcoma breakpoint region 1)/FLI1 translocation without prior chemotherapy.

Peripheral primitive neuroectodermal tumor/Ewing sarcoma and neuroblastoma are distinct malignant tumors belonging to the group of undifferentiated solid pediatric tumors. We report a case of a 14-year-old adolescent girl who presented with a right lower quadrant mass. At surgery, a mobile retroperitoneal mass was entirely removed. Histologic evaluation revealed 2 distinct components; the first, consisting of sheets of undifferentiated cells, was CD99+ and CD56-, whereas the second, consisting of multiple foci of neuropil and maturing neuroblasts, was CD99- and CD56+. Fluorescence in situ hybridization analysis revealed the presence of EWSR1/FLI1 translocation in both histologic distinct components. MYCN (myelocytomatosis viral related oncogene, neuroblastoma derived) was not amplified. The tests for t(11;22) and t(21;22) performed by reverse transcription-polymerase chain reaction were negative. The final diagnosis corresponds to an extraosseous Ewing sarcoma with foci of neuroblastoma-like differentiation. This is the first case, documented by molecular studies, in which neuroblastoma-like differentiation has been noted in primitive neuroectodermal tumor/Ewing sarcoma without prior chemotherapy.