Watch and Wait in PD-1 Monoclonal Antibody Treated dMMR/MSI-H Distal Rectal Cancer

This study is exploring a "watch and wait" approach for patients with a specific type of rectal cancer (dMMR or MSI-H) who achieve a complete clinical response after immunotherapy (anti-PD-1 treatment). Instead of proceeding to surgery after treatment, these patients are closely monitored to see if surgery can be avoided altogether. **You may be eligible if...** - You have been diagnosed with rectal adenocarcinoma confirmed by biopsy - Your cancer has the dMMR or MSI-H genetic marker (a DNA repair defect that makes it respond well to immunotherapy) - Your tumor is in the lower rectum (below the peritoneal reflection) - You have not received any prior cancer treatment - You achieved a complete clinical response after anti-PD-1 immunotherapy **You may NOT be eligible if...** - Your cancer has spread to distant organs - You have a bowel obstruction that cannot be relieved - You have received prior surgery, chemotherapy, or radiation for rectal cancer - You have not responded to immunotherapy treatment Talk to your doctor to see if this trial is right for you.