Immune Checkpoint Inhibitor Myocarditis on FDG PET/CT and Cardiac MR.

Immune checkpoint inhibitor-related myocarditis is one of the rare immune-related adverse effects, but with disproportionately high mortality. A 54-year-old man with stage 4 squamous cell carcinoma on pembrolizumab presented with symptoms of heart failure. FDG PET/CT revealed new cardiomegaly with intense myocardial uptake. Contrast-enhanced cardiac MRI performed concurrently demonstrated myocardial edema and gadolinium enhancement, consistent with myocarditis. The specific etiology of this case of myocarditis was further substantiated by the resolution of cardiomegaly and abnormal myocardial tracer uptake on FDG PET/CT performed after the cessation of the immunotherapy and prednisolone initiation.