Radiation therapy is the use of high-dose X-rays to destroy cancer cells and shrink tumours. Radiation may come from a machine outside the body (external radiation therapy) or from the placement of thin plastic tubes containing radiation (radioisotopes) into the area where the cancer cells are found (internal radiation therapy, or brachytherapy).

Radiation therapy is standard treatment for many types of cancer. It may be used in combination with surgery, chemotherapy, or hormonal therapy.