Radiation therapy is a type of cancer treatment that uses beams of intense energy to kill cancer cells. Radiation therapy most often uses X-rays, but protons or other types of energy also can be used. The term "radiation therapy" most often refers to external beam radiation therapy. During this type of radiation, the high-energy beams come from a machine outside of your body that aims the beams at a precise point on your body. During a different type of radiation treatment called brachytherapy (brak-e-THER-uh-pee), radiation is placed inside your body.