Radiation therapy is the use of high energy radiation to kill cancer cells and shrink tumors. Radiation therapy for gastrointestinal cancer patients is generally administered via a machine that targets the cancer from outside the body (external beam). As with chemotherapy, radiation therapy can also damage normal, healthy cells and cause minor side effects such as hair loss, skin irritation, and fatigue. Radiation therapy is often used in conjunction with surgery and/or chemotherapy in complex gastrointestinal cancer cases.