DETECT: Target Volume for Rectal Endoluminal Radiation Boosting

This study is exploring how to best target radiation therapy in rectal cancer patients who still have some remaining tumor after initial chemotherapy and radiation, before they go on to have surgery. **You may be eligible if...** - You are 18 or older - You have been diagnosed with rectal cancer (a type of cancer in the lower part of the large intestine) confirmed by biopsy - You have already completed chemotherapy and/or radiation treatment, with at least 6 weeks having passed since finishing - You are scheduled to have surgery to remove the rectum **You may NOT be eligible if...** - You have not had prior treatment for your rectal cancer - Less than 6 weeks have passed since completing your radiation or chemoradiation Talk to your doctor to see if this trial is right for you.