A 90-year-old woman experienced gastritis due to an immune-related adverse event (irAE). Two years earlier, she had been diagnosed with recurrent postoperative lung adenocarcinoma (cT2aN0M1, Stage IV) and had received three courses of pembrolizumab, which she responded well to. However, she suffered from intense abdominal pain and acid reflux after eating for over a year. During a repeated esophagogastroduodenoscopy, the examination revealed stiffness in the gastric wall in the antrum, raising concerns about scirrhous carcinoma. However, biopsy results did not indicate malignancy; instead, they showed severe inflammation, and CD8 antibody staining was positive. Based on clinical and immunopathological assessments, she was diagnosed with irAE gastritis. After initiating steroid treatment, her condition improved significantly, and she fully recovered from her difficulty with oral intake.
