MRI Simulation-guided Boost in Short-course Preoperative Radiotherapy for Unresectable Rectal Cancer

This trial is testing whether adding an extra targeted radiation boost — guided by MRI imaging — to a short-course radiation treatment before surgery can improve outcomes for patients with advanced rectal cancer (cancer of the lower bowel) that is difficult to remove due to its closeness to surrounding structures. **You may be eligible if...** - You have been diagnosed with rectal cancer (adenocarcinoma) confirmed by biopsy - The tumor is located within 10 cm of the anal opening - Your cancer is locally advanced (T3, T4, or spread to nearby lymph nodes), confirmed by MRI - You have not yet received any treatment (surgery, chemotherapy, or radiation) for this cancer - Your blood counts and organ function meet minimum thresholds - You are well enough to receive chemotherapy (ECOG 0–1) **You may NOT be eligible if...** - Your cancer has spread to distant organs (metastases) - Your rectal cancer has come back after prior treatment - You have active Crohn's disease or ulcerative colitis - You are allergic to fluorouracil or platinum-based drugs - You cannot have an MRI - You are pregnant or breastfeeding - You have symptoms of numbness or nerve damage in your hands or feet Talk to your doctor to see if this trial is right for you.