Adaptive Radiation Boost for Rectal Cancer

The goal of this clinical trial is to find out if giving extra adaptive radiation therapy after standard chemoradiation treatment is safe and helpful for people with rectal cancer. The main questions the study aims to answer are: * Can this approach help target the most aggressive cancer cells more accurately, while protecting nearby healthy tissue? * Can it reduce the side effects that people may experience during treatment? Participants will: * First receive standard treatment: radiation (45 Gy in 25 sessions) along with a chemotherapy pill called capecitabine. * Then get extra radiation using MRI scans every two weeks to adjust the treatment based on how the tumor responds. * Use a small balloon during treatment to help aim the radiation and protect healthy areas. * Finally, receive additional chemotherapy (such as FOLFOX) for four months.