Radiation therapy, or radiotherapy, is a branch of radiology used to treat cancer. A patient is exposed to ionizing radiation
in doses designed to kill a
malignancy. Malignant tissues are more
sensitive than normal tissues to radiation exposure and can be treated if they have not spread throughout the body and are not surrounded by normal tissue that is especially sensitive to radiation, such as the spinal cord. Sophisticated physical and biological techniques are used for radiation therapy, often accompanied by computer analyses. A radiation therapist develops a treatment plan that permits the absorption of a fatal amount of radiation by all tumor cells but causes relatively minor damage to normal tissue. The usual mode of therapy is an external high-energy beam directed at the tumor site for a few minutes a day for 2 to 6 weeks, depending on the type of malignancy. X rays, gamma rays, and such isotopes as cobalt-60 and iodine-131 are often used.