Advanced Gastric Cancer with Immune-related Adverse Events-induced Severe Colitis and Rapid Disease Progression during Nivolumab-based Combination Chemotherapy: A Case Report

A 73-year-old woman developed peritoneal dissemination after undergoing distal gastrectomy for gastric cancer and was receiving first-line chemotherapy combined with nivolumab. Four cycles achieved significant reduction in the disseminated lesions, prompting continuation of the treatment. After completing eight cycles, she developed persistent watery diarrhea and was hospitalized. A CT scan at the time of hospitalization revealed a significant increase in the disseminated lesions. Further examination led to a diagnosis of nivolumab-associated immune-related adverse event (irAE) colitis, and the diarrhea gradually improved with oral steroid treatment. However, a CT scan on the 16th day of hospitalization showed further enlargement of the disseminated lesions, and the patient’s condition deteriorated, leading to her death on the 19th day of hospitalization. This is the report of immune-related adverse events and rapid disease progression occurring simultaneously when nivolumab-based combination therapy was administered as first-line treatment. When an irAE and disease progression occur simultaneously during administration of a nivolumab combination as first-line chemotherapy, selection of optimal therapy can be challenging.