Complementary use of cardiac magnetic resonance and 18 F-FDG positron emission tomography imaging in suspected immune checkpoint inhibitor myocarditis.

Background: Immune checkpoint inhibitor (ICI) myocarditis is an uncommon but potentially fatal complication of immunotherapy. Cardiac imaging is essential to make timely diagnoses as there are critical downstream implications for patients.

Objective: To determine the agreement of cardiac magnetic resonance (CMR) and 18 F-fluorodeoxyglucose Positron Emission Tomography (FDG-PET) in patients with suspected ICI myocarditis.

Methods: Patients with suspected ICI myocarditis, who underwent CMR and 18 F-FDG-PET imaging at a single cardio-oncology service from 2017 to 2023, were enrolled. CMR was performed according to recommended guidelines for assessment of myocarditis. 18 F-FDG-PET imaging was performed following 18 h carbohydrate-free fast. Imaging was analysed by independent reviewers to determine the presence or absence of ICI myocarditis.

Results: Twelve patients (mean age 60 ± 15 years old, 7 [58%] male) underwent both CMR and 18 F-FDG-PET imaging. Three (25%) met the 2018 Lake Louise Criteria for CMR diagnosis of myocarditis; 4 (33%) had evidence of myocardial inflammation as determined by 18 F-FDG-PET. Amongst those with positive 18 F-FDG-PET, mean standard uptake value (SUV) was 3.5 ± 1.7. There was agreement between CMR and PET in 7 cases (CMR and PET positive (n = 1), CMR and PET negative (n = 6)) and discordance in 5 cases (CMR positive and PET negative (n = 2), CMR negative and PET positive (n = 3)).

Conclusion: Both CMR and PET provide complementary clinical information in diagnostic of ICI myocarditis. CMR informs on myocardial oedema, whilst 18 F-FDG-PET provides information on glucose metabolism reflecting monocyte and lymphocytic activity. Future studies should investigate the role of hybrid PET-CMR for the timely diagnosis of ICI myocarditis.