<jats:title>Abstract</jats:title><jats:sec> <jats:title>Purpose</jats:title> <jats:p>This study aimed to determine the organ-specific accuracy of [<jats:sup>18</jats:sup>F]FDG-PET/CT in identifying immune-related adverse events (irAEs) in patients with high-risk (stage III/IV) surgically resected melanoma treated with an adjuvant immune checkpoint inhibitor (ICI) and determine the incidence of irAEs within the first year after starting treatment.</jats:p> </jats:sec><jats:sec> <jats:title>Materials and methods</jats:title> <jats:p>This registry-based study included individuals who had undergone surgical removal of melanoma and were undergoing adjuvant ICI treatment (either nivolumab or pembrolizumab). The study specifically enrolled patients who had undergone both a baseline and at least one subsequent follow-up [<jats:sup>18</jats:sup>F]FDG-PET/CT scan. Follow-up scans were performed every third month in the first year after surgery to screen for disease recurrence. We retrospectively compared the follow-up scans with baseline scans to identify irAEs. Clinical information on irAEs was obtained from medical records and served as a reference standard for determining the accuracy of [<jats:sup>18</jats:sup>F]FDG-PET/CT.</jats:p> </jats:sec><jats:sec> <jats:title>Results</jats:title> <jats:p>A total of 123 patients with 363 [<jats:sup>18</jats:sup>F]FDG-PET/CT scans were included, and 65 patients (52.8%) developed irAEs. In decreasing order, the organ-specific incidences of irAEs were: skin 26/65 (40%), muscle and joints 21/65 (32.3%), intestines 13/65 (20%), thyroid gland 12/65 (18.5%), lungs 4/65 (6.2%), and heart 2/65 (3.1%). The sensitivities and specificities of [<jats:sup>18</jats:sup>F]FDG-PET/CT for diagnosing irAEs were: skin 19% (95% CI: 7–39%) and 95% (88–98%), muscles and joints 71% (48–89%) and 83% (75–90%), intestines 100% (75–100%) and 85% (77–91%); thyroid gland 92% (62–99%) and 95% (89–98%), lungs 75% (19–99%) and 90% (83–95%), and heart 50% (13–99%) and 97% (92–99%), respectively.</jats:p> </jats:sec><jats:sec> <jats:title>Conclusion</jats:title> <jats:p>[<jats:sup>18</jats:sup>F]FDG-PET/CT generally had moderate to high sensitivities (except for skin and heart) and specificities in diagnosing irAEs in patients receiving adjuvant ICI; this could be suggested to be systematically assessed and reported in scan reports.</jats:p> </jats:sec>