Ewing's sarcoma/peripheral primitive neuroectodermal tumor (pPNET) arising in the omentum as a multilocular cyst with intracystic hemorrhage.

A rare case of Ewing's sarcoma/peripheral primitive neuroectodermal tumor arising in the greater omentum in a 41-year-old man is reported. The patient presented with a hemorrhagic mesenteric cyst that was disclosed by the results of an abdominal echogram, a computed tomography scan, and magnetic resonance imaging. A laparotomy showed a multilocular cyst with intra-cystic hemorrhage. Histologically, the tumor wall consisted of sheets of small round cells separated by thick desmoplastic stroma. Rosette formations or ribbon-like cell arrangements were absent. Further pathological examination revealed that the membrane of the tumor cells was positive for MIC-2, and negative for epithelial membrane antigen, cytokeratin, and desmin, which are usually positive in intra-abdominal desmoplastic small round-cell tumors. An EWS/FLI1 fused transcript was detected by reverse transcription-polymerase chain reaction. These findings confirmed the diagnosis of Ewing's sarcoma/peripheral primitive neuroectodermal tumor. The patient died of tumor recurrence 4 months after his first admission. The autopsied tumor tissue exhibited neural differentiation in certain regions. To our knowledge, this is the first case to be reported of Ewing's sarcoma/peripheral primitive neuroectodermal tumor arising in the omentum with unique pathological features and the occurrence of partial neural differentiation during the clinical course. This case pointed out to us, as gastroenterologists, that only thorough examination confirms a definitive diagnosis of small round-cell tumor of the abdomen, it also shows that Ewing's sarcoma/peripheral primitive neuroectodermal tumor should be included in the differential diagnosis of cystic lesions in the omentum.