Immune Checkpoint Inhibitor-associated Periaortitis Detected on 18F-FDG PET/CT: A Rare Case of Nivolumab Toxicity

We report a case of a 64-year-old male with metastatic non-small-cell lung carcinoma who was undergoing neoadjuvant chemoimmunotherapy with cisplatin, pemetrexed, and nivolumab.Restaging 18 F-fluorodeoxyglucose positron emission tomography/computed tomography ( 18 F-FDG PET/CT) demonstrated segmental, linear 18 F-FDG uptake along the abdominal aortic wall with corresponding crescentic soft-tissue thickening.Differential diagnoses included intramural hematoma, focal aortitis, atherosclerotic plaque, and tumor thrombus.CT angiography revealed a mildly enhancing periaortic soft-tissue rim involving a 5 cm infrarenal aortic segment, consistent with isolated periaortitis, likely induced by immunotherapy.Steroid therapy was initiated.Follow-up 18 F-FDG PET/CT showed complete metabolic resolution, supporting an immune-related etiology.18 F-FDG PET/CT played a crucial role in early recognition of this immune-related adverse event by sensitively detecting vascular inflammation and guiding further evaluation with CT angiography.Early PET-based detection enabled timely initiation of steroid therapy, preventing potential vascular complications.Keywords: 18 F-FDG PET/