In radiation therapy (also called radiotherapy), high-energy rays are used to damage cancer cells and stop them from growing and dividing. A specialist in radiation therapy is called a radiation oncologist. Like surgery, radiation therapy is a local treatment; it affects cancer cells only in the treated area. Radiation can come from a machine (external radiation). It can also come from an implant (a small container of radioactive material) placed directly into or near the tumor (internal radiation). Some patients receive both kinds of radiation therapy.