this uses high-energy radiation from x-rays, gamma rays, neutrons, protons, and other sources to kill cancer cells and shrink tumours. 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 or brachytherapy where pellets of radioactive material is placed e.g. inside the prostate). Systemic radiotherapy uses a radioactive substance, such as a radio-labelled monoclonal antibody, that travels in the blood to tissues throughout the body. It is common to have radiotherapy 3 or 5 days over a period of 3-6 weeks.