Complete Response of High Microsatellite Instability Gastric Cancer and Synchronous Microsatellite Stability Rectal Cancer

Gastric cancer, a leading cause of cancer-related death in the world, may occur with an additional synchronous malignancy in rare cases. Of these rare cases many are colorectal cancer. Microsatellite instability is a phenomenon that may contribute to the pathogenesis of both cancers, as are field cancerization and genetic susceptibility, although none of these explain many concurrent cases. In this case, we described a patient with locally advanced microsatellite instability-high gastric cancer and synchronous microsatellite stable rectal cancer, who received a combination chemo-immunotherapy regimen and achieved complete response. This report reflects on current knowledge surrounding synchronous primary malignancies and achieving complete response.