Xanthogranulomatous osteomyelitis of the humerus in a pediatric patient with Alagille syndrome: a case report and literature review.

Xanthogranulomatous osteomyelitis (XO) is an exceedingly rare disease characterized by infiltration of histiocytes and foamy macrophages. Both on gross examination and on radiographs, XO can mimic malignancy. We describe the case of a 5-year-old female with Alagille syndrome who presented with a pathologic fracture of the right humerus. Initial radiographs revealed multiple osteolytic lesions in the distal humerus while MRI showed a large soft tissue mass. Biopsy confirmed the diagnosis of XO, which has hitherto not been described in a patient with Alagille syndrome. The patient was admitted for IV antibiotics and taken to the operating room for an incision and debridement. Tissue cultures were obtained and grew Salmonella. Antibiotic therapy was tapered, and the patient responded to treatment. She was doing well at her 6-month follow-up visit. In the discussion section, we explore how osteopenia and immune dysregulation caused by Alagille syndrome can affect the development of XO. We summarize all previously reported cases of XO and conclude that XO presents as an osteolytic lesion that expands rapidly over the course of a few weeks. We highlight that XO can mimic sarcoma because of its mass effect but can be distinguished radiographically by the presence of surrounding sclerosis. Given the rapid onset of XO, we classify it as an acute osteomyelitis. We discuss how leukemia and Ewing sarcoma can present similarly to acute osteomyelitis. We then emphasize key features that can be used to distinguish these malignancies from acute osteomyelitis.