Extraosseous Ewing's sarcoma/primitive neuroectodermal tumor (ES/PNET) is an uncommon, aggressive, and malignant tumor with a poor patient outcome. Its occurrence in the lesser sac is a rare event and to the best of our knowledge, has not been previously described. The present case was clinically and radiologically misdiagnosed as a pancreatic tumor/gastrointestinal stromal tumor. Histopathology revealed a tumor with "small round cells" that were positive for CD99, confirming the diagnosis of ES/PNET. This report highlights the importance of considering Ewing's sarcoma in the differential diagnosis of intraabdominal, extraintestinal masses.
Antigens, CD/analysis ; Biopsy ; Cell Adhesion Molecules/analysis ; *Diagnostic Errors ; Female ; Humans ; Immunohistochemistry ; Middle Aged ; Neuroectodermal Tumors, Primitive, Peripheral/*diagnosis/immunology/pathology/therapy ; Pancreatic Neoplasms/*diagnosis ; Peritoneal Neoplasms/*diagnosis/immunology/pathology/therapy ; Predictive Value of Tests ; Sarcoma, Ewing/*diagnosis/immunology/pathology/therapy ; Tomography, X-Ray Computed ; Tumor Markers, Biological/analysis