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.