Radiation therapy: The use of high-energy rays to damage cancer cells, stopping them from growing and dividing. Like surgery, radiation therapy is a local treatment that affects cancer cells only in the treated area. Radiation can come from a machine (external radiation) or from a small container of radioactive material implanted directly into or near a tumor (internal radiation). External radiation therapy is usually given on an outpatient basis in a hospital or clinic. Patients are not radioactive during or after external radiation therapy. For internal radiation therapy, the patient stays in the hospital for a few days. The implant may be temporary or permanent. After an implant is removed, there is no radioactivity in the body. The amount of radiation in a permanent implant goes down to a safe level before the patient leaves the hospital. Side effects of radiation therapy depend on the treatment dose and the part of the body treated. The most common side effects of radiation are fatigue, skin reactions (such as a rash or redness) in the treated area, and loss of appetite. Radiation therapy can cause inflammation of tissues and organs in and around the body site that is radiated. Radiation therapy can also cause a decrease in the number of white blood cells. Although the side effects of radiation therapy can be unpleasant, they can usually be treated or controlled. Furthermore, in most cases, they are not permanent.
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