A 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 toxicity, irAE enteritis often goes unrecognized due to diagnostic challenges, but can lead to serious AEs such as perforation. Therefore, even if colonoscopy findings are normal, a thorough examination of the small intestine is essential for patients who develop gastrointestinal symptoms while undergoing immune checkpoint inhibitor therapy. We herein report a rare case of irAE enteritis confirmed through endoscopic and pathological examination, which has not been previously reported.