Radiotherapy is an effective treatment for many types of cancer. It uses high energy X-rays to kill off cancer cells. However, these also damage normal cells so the aim in radiotherapy is to get maximum radiation to cancer cells without too much damage to normal ones. To achieve this the dose of radiotherapy must be carefully calculated and the exact area to be irradiated must be carefully planned. Radiotherapy also includes the controlled use of radioactive materials which release radiation similar to X-rays.