external radiation therapy (ERT), the therapeutic application of ionizing radiation from a source of radiation outside the body, such as a kilovoltage radiographic machine, a megavoltage cobalt 60 machine, or a supervoltage linear accelerator, cyclotron, or betatron. ERT is used most frequently in the treatment of cancer but is also used in the therapy of keloids and some dermatological conditions and in counteracting the body’s physiological rejection of transplanted organs.
