Adjuvant PD-1 Blockade for High-risk Stage-II DMMR/MSI-H Colorectal Cancer

This trial is testing whether treatment with an immunotherapy drug (a PD-1 blocker) after surgery can prevent colorectal cancer from coming back in people with Stage II colon cancer that has a specific genetic feature called mismatch repair deficiency (dMMR) or microsatellite instability (MSI-H). **You may be eligible if...** - You are between 18 and 80 years old - You have been diagnosed with Stage II colorectal cancer (T3 or T4, no spread to lymph nodes) that is dMMR or MSI-H - You have had surgery to remove the cancer - Your cancer had at least one high-risk feature: T4 stage, vascular invasion, nerve invasion, poor differentiation, or blockage/perforation before surgery - You have not received any chemotherapy or radiation for this cancer - You are in good general health (ECOG 0 or 1) **You may NOT be eligible if...** - You have an active autoimmune disease that required treatment in the past 2 years - Cancer cells were found at the surgical margins (R1/R2 resection) - You have a serious post-surgical complication - You have another active malignancy - You have an active infection requiring treatment Talk to your doctor to see if this trial is right for you.