The Role of F-18 FDG PET/CT in Evaluating Therapy Response in Patients Undergoing Immuno-therapy with Immune Checkpoint Inhibitors

Background: Immune checkpoint inhibitors (ICIs) have transformed cancer treatment by stimulating anti-tumor immunity. However, evaluating therapy response remains challenging due to atypical response patterns such as pseudoprogression, which can mimic true progressive disease on imaging and therefore lead to discontinuation of a potentially effective treatment. Specific therapy response criteria were designed to better differentiate between pseudoprogression and true progressive disease. F-18 FDG PET/CT has proven itself as a valuable imaging modality in immunotherapy response assessment, offering early detection of metabolic changes before structural tumor alterations become apparent on CT or MRI. Methods: This retrospective study analyzed 93 patients undergoing immunotherapy be-tween March 2022 and March 2024. A total of 498 PET/CT scans were reviewed, with re-sponses classified according to iRECIST criteria. Therapy changes based on PET/CT find-ings were documented, and cases of immune unconfirmed progressive disease (iUCPD) were analyzed to determine pseudoprogression incidence. Results: Out of the 93 included patients, F-18 FDG PET/CT identified 9 patients with iUCPD, of which 4 showed stable disease (SD) and 5 partial response (PR) on the re-quired follow-up scan after 6–8 weeks and were therefore classified as pseudoprogres-sion. The overall incidence of pseudoprogression in our study was 9,7%. Additionally, therapy adjustments were made in 42% of the patients, highlighting the crucial role of F-18 FDG PET/CT in guiding treatment decisions. Conclusion: F-18 FDG PET/CT plays an important role in therapy monitoring in pa-tients undergoing immunotherapy. This study highlights its significance in the differ-entiation between true progression and pseudoprogression. The ability to provide met-abolic insights earlier than conventional imaging supports its integration into routine oncologic assessment.