biostudies-literatureUnknown6(1)Morikawa REnterocolitis is a common gastrointestinal manifestation of immune-related adverse events (irAEs); however, only a few studies have reported on irAE enteritis with localized active inflammation in the small intestine. Here, we report the case of a 74-year-old man who developed diarrhea, abdominal pain, and oral intake difficulty and was subsequently hospitalized after receiving atezolizumab for pulmonary adenocarcinoma. Computed tomography and enterocolonoscopy revealed active inflammation in the small intestine but not in the colon, leading to the final diagnosis of irAE enteritis. After initiating prednisolone at a dose of 60 mg/day, his symptoms improved rapidly, and a follow-up enterocolonoscopy revealed a marked reduction in inflammation. Being a relatively rare gastrointestinal toxicDEN opene70187https://www.ebi.ac.uk/biostudies/studies/S-EPMC12415260biostudies-literatureA Rare Case of Immune-related Adverse Events Localized to the Small Intestine.PMC12415260Yamamoto KFukuda MOhtsuka KMorikawa RKanaya RShimizu HFujii TNemoto YTaguchi TOkamoto RfalseA Rare Case of Immune-related Adverse Events Localized to the Small Intestine.Enterocolitis is a common gastrointestinal manifestation of immune-related adverse events (irAEs); however, only a few studies have reported on irAE enteritis with localized active inflammation in the small intestine. Here, we report the case of a 74-year-old man who developed diarrhea, abdominal pain, and oral intake difficulty and was subsequently hospitalized after receiving atezolizumab for pulmonary adenocarcinoma. Computed tomography and enterocolonoscopy revealed active inflammation in the small intestine but not in the colon, leading to the final diagnosis of irAE enteritis. After initiating prednisolone at a dose of 60 mg/day, his symptoms improved rapidly, and a follow-up enterocolonoscopy revealed a marked reduction in inflammation. Being a relatively rare gastrointestinal toxic2026-01-01T00:00:00Z2026 Apr2026-06-02T02:45:00.245Z2026-04-13T03:12:53.821ZS-EPMC124152604092698010.1002/deo2.70187