Cancer is the growth of abnormal cells in the body. These extra cells grow together and form masses, lumps, or tumors. In colorectal cancer, these growths usually start as harmless (benign) polyps in the large intestine (colon or rectum).
Radiation therapy uses X-rays to destroy colorectal cancer cells. It is often combined with surgery or chemotherapy. Radiation therapy may also be used to reduce the cancer's size when it is blocking the colon or rectum or to relieve pain.