Radiation therapy is the use of high-energy radiation to damage cancer cells' DNA and destroy their ability to divide and grow. It may be delivered using machines called linear accelerators or via radioactive sources placed inside the patient on a temporary or permanent basis. Radiation therapy may be used to cure cancer, to relieve a cancer patient's pain or alleviate other symptoms. Preparation for radiation therapy is focused on targeting the radiation dose to the cancer as precisely as possible to minimize side effects and avoid damaging normal cells. Imaging tests may be used to help determine the exact shape and location of your tumor and define its boundaries. Your doctor will give you specific instructions based on the type of exam being performed.