Sintilimab Plus Hypofractionated Radiotherapy for MSI-H/dMMR Rectal Cancer

This study is testing a combination of immunotherapy (sintilimab, an anti-PD-1 drug that helps the immune system attack cancer) and a concentrated radiation treatment (hypofractionated radiotherapy) in patients with a specific subtype of rectal cancer called MSI-H or dMMR — cancers with a defect in DNA repair that makes them more likely to respond to immunotherapy. **You may be eligible if...** - You are 18 or older - You have been diagnosed with rectal cancer confirmed by biopsy - Your cancer has the MSI-H or dMMR genetic marker (a defect in DNA mismatch repair) - Your cancer has not spread to distant organs - Your tumor is within 15 cm of the anus - You have not received any prior cancer treatment - Your blood counts, liver, kidney, and heart function are within acceptable ranges **You may NOT be eligible if...** - Your cancer has spread to distant organs (metastatic) - You have received prior cancer treatment - You have active autoimmune disease or are on immunosuppressant medications - You have serious heart conditions Talk to your doctor to see if this trial is right for you.