Pelvic Ewing Sarcoma: The Great Mimicker.

Ewing sarcoma is the most common malignant bone tumor of the pelvis in children and young adults. Even with aggressive treatment, its survival rate is amongst the poorest. Classical presentation may not be the rule. It may simulate clinically, imagiologically and histopathologically other nonmalignant entities. Therefore, its suspicion should not be overlooked. We report two cases of pelvic Ewing sarcoma: the first mimicking eosinophilic granuloma, and the second mimicking osteomyelitis. In the latter, we also report an atypical finding of its natural history: an initial response to antibiotic and anti-inflammatory treatment. In both cases, we highlight the possibility of an inconclusive percutaneous bone biopsy and the importance of immunochemistry and cytogenetics for the definitive diagnosis.