Radiation therapy (ray-dee-AY-shun THER-ah-pee) or Radiotherapy: 

The use of high-energy radiation from x-rays, gamma rays, neutrons, and other sources to kill cancer cells and shrink tumors. Radiation may come from a machine outside the body (external-beam radiation therapy), or it may come from radioactive material placed in the body near cancer cells (internal radiation therapy, implant radiation, or brachytherapy). Systemic radiation therapy uses a radioactive substance, such as a radio labeled monoclonal antibody, which circulates throughout the body. Also called radiotherapy.