Mesenteric Panniculitis Mimicking Metastases From Melanoma on FDG PET/CT in a Pediatric Patient Receiving Immunotherapy

Inflammatory immune-related adverse events are known complications of immunotherapy, particularly checkpoint inhibitors. A 16-year-old girl with metastatic melanoma, treated with nivolumab, dabrafenib, and trametinib, presented with abdominal pain and vomiting. Initial CT suggested metastases, showing mesenteric stranding and targetoid lesions. FDG PET/CT confirmed increased FDG uptake in the same region, but subsequent MRI and follow-up findings pointed to mesenteric panniculitis.