Intensity-Modulated Radiation Therapy With Incorporated Boost and Capecitabine Before Surgery in Treating Patients With Locally Advanced Rectal Cancer

RATIONALE: Radiation therapy uses high-energy x-rays to damage tumor cells. Intensity-modulated radiation therapy (radiation directed at the tumor more precisely than in standard radiation therapy) with incorporated boost (an increase in the amount of radiation given during treatment) may cause less damage to normal tissue. Drugs used in chemotherapy, such as capecitabine, work in different ways to stop tumor cells from dividing so they stop growing or die. Giving radiation therapy together with chemotherapy before surgery may shrink the tumor so it can be removed. PURPOSE: This phase I trial is studying the side effects and best dose of neoadjuvant intensity-modulated radiation therapy with incorporated boost when given together with capecitabine in treating patients with locally advanced rectal cancer.