Abstract
Chronic recurrent multifocal osteomyelitis (CRMO) is a rare and distinct form of chronic non-bacterial osteomyelitis with an unknown etiology. This 11-year-old Asian girl presented with a painful swelling in her right arm, which was suspected to be a Ewing's sarcoma based on radiological findings. The lesion appeared as a unifocal destructive bone lesion in the proximal diaphyseal region of the right humerus, accompanied by periosteal reaction and soft tissue involvement. She was otherwise clinically well, except for the elevated C-reactive protein, erythrocyte sedimentation rate and alkaline phosphatase. Due to the clinical and radiological suspicion of sarcoma, repeated bone biopsies were performed. All the biopsies revealed mature lamellar bone, normocellular marrow with moderate mixed inflammation, and fibrosis, indicative of a chronic inflammatory process. Given the discordance between the clinical, radiological, and pathological findings, surgical management was withheld and the patient was closely monitored. Over the course of several months, her symptoms gradually improved without any medical or surgical interventions. A follow-up X-ray taken 3 years after the initial diagnosis showed that the lesion had significantly reduced in size, nearly returning to a normal bone contour.