Acute development of fatal myocarditis after pembrolizumab in non-small-cell lung cancer with thymoma

We herein report the case of a sixty-four-year-old woman with stage IV lung adenocarcinoma with thymoma. The adenocarcinoma showed 95% tumor proportion scores of PD-L1 (Programmed death-ligand 1) expression without driver mutations. She was treated with pembrolizumab as 1st-line chemotherapy. Creatine kinase elevation was observed on the 15th day, and fatal myocarditis developed on the 20th day despite steroid therapy. The autopsy showed lymphocyte-infiltrated cardiac muscle, and pembrolizumab-induced myocarditis was diagnosed.Our case indicates that lung cancer combined with thymoma should alert clinicians to monitor the early onset of myocarditis in patients on immune checkpoint inhibitor therapy.