Delayed Inflammatory Hypermetabolism in Meningioma After Immunotherapy

We report a case of malignant melanoma treated with immune checkpoint inhibitors, resulting in hypermetabolic activity in 2 meningiomas on FDG-PET, initially suggesting brain metastases. The lesions demonstrated no hypermetabolism pretreatment, faint hypermetabolism 1 month after immunotherapy discontinuation, intense hypermetabolism 2 months posttreatment, and complete resolution 6 months posttreatment. This case exemplifies delayed immune-related adverse events (DIREs), where inflammatory reactions can manifest weeks to months after immunotherapy cessation. The temporal pattern aligns with recent reports that immune-related adverse events can occur in up to 10.8% of patients more than 12 months after initial checkpoint inhibitor exposure. This case highlights the potential for false-positive FDG uptake in benign meningiomas postimmunotherapy, emphasizing the importance of recognizing delayed inflammatory reactions to avoid misdiagnosis.