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 ¹⁸F-fluorodeoxyglucose positron emission tomography/computed tomography (¹⁸F-FDG PET/CT) demonstrated segmental, linear ¹⁸F-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 ¹⁸F-FDG PET/CT showed complete metabolic resolution, supporting an immune-related etiology. ¹⁸F-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.