A rare case of eosinophilic gastritis induced by nivolumab therapy for metastatic melanoma.

Cancer immunotherapy using immune checkpoint inhibitors can cause immune reactions at various sites as a side effect called immune-related adverse events (irAEs). The gastrointestinal tract is susceptible to irAEs, however, the degree and presentation vary considerably from case to case. A 76-year-old woman was diagnosed with anal mucosal melanoma. She underwent radical surgery and received postoperative adjuvant therapy. However, because new metastases were also found in bilateral inguinal lymph nodes, immunotherapy with nivolumab was performed. Approximately 10 months after the initiation of nivolumab administration, she presented with epigastric discomfort and nausea, and her laboratory data showed severe eosinophilia (1938/mm 3 ). Computed tomography demonstrated a diffuse thickening of the gastric wall. Esophagogastroduodenoscopy and endoscopic ultrasonography showed mucosal thickening due to edema, and histologic examination revealed severe invasion of eosinophils in the lamina propria. Subsequently, she was diagnosed with eosinophilic gastritis due to irAEs induced by nivolumab. Oral administration of prednisolone rapidly normalized her endoscopic and histologic findings, dramatically reducing her symptoms. This is a very rare and important case report of nivolumab-induced severe eosinophilic gastritis. Although gastric lesions as IrAEs is rare, it is necessary to differentiate eosinophilic gastritis if unexplained nausea occurred during the administration of immune checkpoint inhibitors.