Immune Checkpoint Inhibitors for Organ Preservation in Non-metastatic dMMR/MSI-H Gastric or Colon Cancers

This trial tests whether immunotherapy drugs (immune checkpoint inhibitors) can shrink tumors enough to avoid surgery in patients with stomach or colon cancer that has not spread to other organs but has a specific genetic marker called dMMR/MSI-H. These tumors respond particularly well to immunotherapy. **You may be eligible if...** - You are 18 or older - You have been diagnosed with stomach cancer or colon cancer confirmed by tissue biopsy - Your tumor has the dMMR or MSI-H genetic marker (your doctor can test for this) - Your cancer has not spread to distant organs **You may NOT be eligible if...** - Your cancer has already spread widely (metastatic) - You have a serious autoimmune disease - You have had prior chemotherapy or immunotherapy for this cancer Talk to your doctor to see if this trial is right for you.