Primary Ewing's sarcoma of the small intestine.

17 year old female teenager with abdominal pain secondary to pelvic mass of 12 x 10 cm, which seems to depend on ovary. Surgery is scheduled for removal of the tumor, during which it is observed that the lesion originates in the ileum. The histopathological study shows a neoplasm of small round cells with nucleoli and scant cytoplasm. The tumor cells are immunoreactive to CD99 and ERG, being negative for cytokeratins, FLI1, WT1, DOG1 and lymphoid markers. By means of FISH, a rearrangement of the EWSR1 gene was demonstrated. By integrating these molecular and immunohistochemical findings with the morphology, it was diagnosed as Ewing's sarcoma. This aggressive and infrequent tumor originates from neuroectodermal cells and usually develops in the long bones of pediatric and young adult patients, although exceptionally it can occur in other locations. At the intestinal level, it mainly affects the ileum, with a non-specific pain and fatigue clinic. The treatment of choice is surgery for resection of the affected loop, followed by chemotherapy.