Gastric cancer is among the top 5 causes of cancer-related death worldwide. Preoperative chemotherapy has been established as an option in patients with locally advanced gastric cancer. However, chemotherapy yields variable results, owing to the cellular and molecular heterogeneity of this disease. Identifying patients who did or did not respond to preoperative therapy can allow clinicians to alter treatment modalities and provide important information related to prognostication. Pathologic response to preoperative therapies, called Tumor Response Grade (TRG), has been evaluated to quantify treatment response. Multiple systems for TRG have been established. However, literature has demonstrated inconsistent results for TGR systems and prognosis, possibly due to variability in interpretation of tumor response between systems and interobserver variability. Radiographic response to preoperative therapies using RECIST 1.1 criteria and endoscopically-assessed tumor response have demonstrated association with survival; however their use in gastric cancer remains challenging given the inability to accurately and consistently identify and measure the tumor, especially in the setting of neoadjuvant therapy where treatment-related changes can obscure the gastric wall layers. This review is focused on summarizing the available literature related to evaluating TRG in gastric cancer, as well as providing a brief overview on the use of radiographic and endoscopic methods to assess response to preoperative therapies. Lastly, we outline future directions regarding the use of a universal TRG system to guide care and assist with prognosis.

The oncological outcomes are improving in gastrointestinal cancer with advancement in systemic therapies, and there is notable potential in combining immunotherapy and radiation therapy (RT)to allow for further improvements in outcomes. Various preclinical and early phase II studies have shown promising synergy with immunotherapy and RT in gastrointestinal cancer. Consolidative nivolumab in esophageal cancer after neoadjuvant chemoradiation (CRT) and complete resection has recently been approved as standard treatment after improved disease free survival in phase III study. Immunotherapy has shown promising results in hepatocellular carcinoma and colorectal cancer as well. We have reviewed the published and ongoing studies of the combinations of immunotherapy and RT in gastrointestinal cancers.