Abstract
Pembrolizumab has been associated with a high tumor response rate among high microsatellite instability (MSI-H) cancer patients. The efficacy and safety of pembrolizumab in the treatment of MSI-H gastric cancer (GC) patients aged ≥ 85 years have not been reported. This study reports the case of an 89-year-old woman diagnosed with stage IIA MSI-H GC based on her chief complaint of abdominal pain. We considered surgery, but it was contraindicated due to the patient's age and cardiovascular comorbidity. Therefore, we administered pembrolizumab after receiving approval from the ethics committee, and no significant adverse events were noted. The tumor was markedly responsive to pembrolizumab, and the computed tomography and endoscopic findings revealed a complete response. This is the first report on the efficacy and safety of pembrolizumab in the treatment of GC in an "oldest old" patient with MSI-H.