A Study on the Accurate Evaluation of Pseudoprogression of Rectal Cancer Immunotherapy Based on Multitemporal and Multiparameter MRI

This study uses advanced MRI imaging taken at multiple time points to distinguish between "pseudoprogression" (where a tumor appears to grow on scans but is actually responding to treatment) and true progression in rectal cancer patients receiving immunotherapy — helping doctors make better treatment decisions. **You may be eligible if...** - You have rectal cancer confirmed by biopsy - Your cancer is at clinical stage II, III, or IV - Your treatment team has decided you will receive immunotherapy (a PD-1 or PD-L1 inhibitor) - You have not received any prior treatment for this cancer - You can undergo MRI scans before and after starting immunotherapy - Your expected survival is more than 3 months **You may NOT be eligible if...** - Your primary tumor has already been surgically removed at initial diagnosis - You have a history of another malignant tumor - You are pregnant - You are unable to provide informed consent due to physical or psychological illness Talk to your doctor to see if this trial is right for you.