Severe gastritis due to pembrolizumab treatment in a lung cancer patient

Immune checkpoint inhibitors (ICIs) are known to induce gastrointestinal adverse events. Colitis occurs most frequently, and gastritis is less common. A few case reports of gastritis induced by ICIs have indicated that colitis induced by cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4) resembles inflammatory bowel disease (IBD) and that programmed death‐1/programmed death ligand‐1 (PD‐1/PD‐L1) inhibitor can also induce the same type of colitis. We herein encountered a case of gastritis arising after 25 cycles of pembrolizumab administration in which the pathological and endoscopic findings resembled those of IBD. ICIs may induce gastritis in a manner similar to the pathogenesis of IBD. The incidence of immune checkpoint inhibitor (ICI)‐induced gastritis is relatively rare, compared with colitis, but a massive immunoreaction caused by ICIs can induce severe gastritis. Thus, gastritis should be considered when patients taking ICIs complain of any gastrointestinal symptoms.