Incidence of Reported Pseudoprogression on 18 F ‐ FDG PET / CT in Metastatic Melanoma Patients Undergoing Immune Checkpoint Inhibitor Immunotherapy

ABSTRACT Aim To evaluate the incidence of pseudoprogression on 18 F‐FDG PET/CT scans in metastatic melanoma patients commencing immune checkpoint inhibitor therapy and to determine the average time to confirmed 18 F‐FDG PET/CT response in the pseudoprogression cohort identified. Methods Patients with metastatic melanoma who underwent baseline and follow‐up 18 F‐FDG PET/CT scans after commencing immune checkpoint inhibitor therapy at Alfred Hospital (2012–2023) were retrospectively reviewed. Cases of pseudoprogression were identified by a keyword search of reports and confirmed on image review by a nuclear medicine physician. Data on timing of confirmed response, immune‐related adverse events, and autoimmune history were also collected. Results 10/401 (2.49%) metastatic melanoma patients were confirmed as having pseudoprogression on PET/CT. 8/10 (80%) had new FDG‐avid lesions and 2/10 (20%) had an increase in size and SUVmax of original disease. 8/10 (80%) of patients achieved complete metabolic remission on subsequent PET/CT scans with an average time to confirmed reduction of disease on PET/CT of 28.4 weeks. 4/10 (40%) received dual agent immunotherapy. 8/10 (80%) developed irAEs of varying types and severity. 2/10 (20%) had a history of pre‐existing autoimmune disease. 9/10 (90%) of pseudoprogression patients are alive at last review. Conclusion Pseudoprogression on 18 F‐FDG PET/CT occurred in approximately 2.5% of metastatic melanoma patients commencing immunotherapy with an average time to subsequent confirmed response of ~6 months. This highlights the importance of not ceasing immunotherapy prematurely based on early 18 F‐FDG PET/CT findings.