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

A rare case of Ewing's sarcoma/peripheral primitive neuroectodermal tumor a rising 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 res ults of an abdominal echogram, a computed tomography scan, and magnetic res onance imaging. A laparotomy showed a multilocular cyst with intra-cystic h emorrhage. Histologically, the tumor wall consisted of sheets of small roun d cells separated by thick desmoplastic stroma. Rosette formations or ribbo n-like cell arrangements were absent. Further pathological examination reve aled that the membrane of the tumor cells was positive for MIC-2, and negat ive for epithelial membrane antigen, cytokeratin. and desmin, which are usu ally positive in intra-abdominal desmoplastic small round-cell tumors. An E WS/FLI1 fused transcript was detected by reverse transcription-polymerase c hain reaction. These findings confirmed the diagnosis of Ewing's sarcoma/pe ripheral primitive neuroectodermal tumor. The patient died of tumor recurre nce 4 months after his first admission. The autopsied tumor tissue exhibite d neural differentiation in certain regions. To our knowledge, this is the first case to be reported of Ewing's sarcoma/peripheral primitive neuroecto dermal tumor arising in the omentum with unique pathological features and t he occurrence of partial neural differentiation during the clinical course. This case pointed out to us, as gastroenterologists, that only thorough ex amination confirms a definitive diagnosis of small round-cell tumor of the abdomen, it also shows that Ewing's sarcoma/peripheral primitive neuroectod ermal tumor should be included in the differential diagnosis of cystic lesi ons in the omentum.