Testing Immunotherapy (Atezolizumab) With or Without Chemotherapy in Locoregional MSI-H/dMMR Gastric and Gastroesophageal Junction (GEJ) Cancer

This phase II trial compares atezolizumab in combination with chemotherapy (docetaxel, oxaliplatin, leucovorin calcium, fluorouracil, capecitabine) to atezolizumab alone for controlling the growth and/or spreading of the disease in patients with gastric or gastroesophageal junction (JEG) cancer that has not spread from where it first started (local) or only has spread to nearby lymph nodes or tissue (locoregional) and has high microsatellite instability (MSI-H) and mismatch repair deficiency (dMMR). The mismatch repair (MMR) system in the body corrects errors made during the copying of DNA and serves as a proofreading function. If this system isn't working correctly, mutations (changes) in DNA occur which can allow the cancer to grow or spread. This is called dMMR (deficient mismatch repair) . MSI-H describes cancer cells that have a high number of mutations within microsatellites. For example, microsatellite testing that shows mutations in 30% or more microsatellites is called microsatellite instability-high (MSI-H). Microsatellites are short, repeated sequences of DNA. There is evidence that MSI-H/ dMMR gastric or GEJ tumors respond well to immunotherapy. Immunotherapy with monoclonal antibodies, such as atezolizumab, may help the body's immune system attack the cancer, and may interfere with the ability of tumor cells to grow and spread. Docetaxel is in a class of medications called taxanes. It stops tumor cells from growing and dividing and may kill them. Oxaliplatin is in a class of medications called platinum-containing antineoplastic agents. It damages the cell's DNA and may kill tumor cells. Capecitabine is in a class of medications called antimetabolites. It is taken up by tumor cells and breaks down into fluorouracil, a substance that kills tumor cells. Chemotherapy drugs such as leucovorin calcium and fluorouracil work in different ways to stop the growth of tumor cells, either by killing the cells, by stopping them from dividing, or by stopping them from spreading. Using atezolizumab as immunotherapy with and following chemotherapy versus atezolizumab alone prior to and after surgery may shrink or stabilize the tumor in patients with MSI-H/dMMR localized gastric or GEJ cancer and may increase the length of time after treatment that cancer does not come back or get worse.