Rectal primitive neuroectodermal tumor.

We report the clinical, histologic, immunohistochemical, and molecular findings of a Ewing's sarcoma/primitive neuroectodermal tumor (ES/PNET) localized to the rectum in a 17-year-old boy. Notably the 4.5 x 4 x 4-cm sessile mass was spontaneously eliminated through the anus, producing an episode of hemorrhagic shock. Histologically the tumor presented as a proliferation of membrane CD99-positive, small round cells. EWS/FLI1 chimeric mRNA was demonstrated by nested reverse-transcription polymerase chain reaction (RT-PCR) analysis in the tumor tissue. The patient is alive and well 2 years after initial symptoms. We also review reported cases of ES/PNET of the digestive system.