Radiation therapy (also called radiotherapy) uses high-energy rays to kill cancer cells in a targeted area. Similar to surgery, radiation therapy is a local treatment used to eliminate or eradicate visible tumors. Radiation can be given externally by a machine that aims radiation at the tumor area. It can also be given internally; needles, seeds, wires, or catheters containing a radioactive substance are placed directly in or near the tumor. Radiation treatments are painless. The side effects are usually temporary, and most can be treated or controlled. Patients are likely to feel very tired, especially in the later weeks of treatment. Radiation therapy may also cause a decrease in the number of white blood cells, which help protect the body against infection.  External radiation delivers high-energy rays directly to the tumor site from a machine outside the body. Internal radiation, or brachytherapy, involves the implantation of a small amount of radioactive material in or near the cancer.

Radiation may be used to cure or control cancer, or to ease some of the symptoms caused by cancer.  Sometimes radiation is used with other types of cancer treatment, such as chemotherapy and surgery, and sometimes it is used alone. Radiation therapy is not typically useful in eradicating cancer cells that have already spread to other parts of the body.

There is no risk of radiation exposure from coming in contact with a patient undergoing external radiation therapy. External radiation does not cause the body to become radioactive. With internal radiation (also called implant radiation), a patient may need to stay in the hospital, away from other people, while the radiation level is highest. Implants may be permanent or temporary. The amount of radiation in a permanent implant goes down to a safe level before the person leaves the hospital. With a temporary implant, there is no radioactivity left in the body after the implant is removed. With external radiation, it is also common to have temporary hair loss in the treated area and for the skin to become red, dry, tender, and itchy.