External beam radiotherapy
（重定向自Radioisotope teletherapy）
![Radiation therapy of the pelvis. Lasers and a mould under the legs are used to determine exact position.](/Images/godic/202502/05/Radiation_therapy5027.jpg")
![Historical image showing Gordon Isaacs, the first patient treated with linear accelerator radiation therapy (in this case an electron beam) for retinoblastoma in 1957. Gordon's right eye was removed January 11, 1957, because the cancer had spread. His left eye, however, had only a localized tumor that prompted Henry Kaplan to try to treat it with the electron beam.](/Images/godic/202502/05/External_beam_radiotherapy_retinoblastoma_nci-vol-1924-3005027.jpg")
![A teletherapy radiation capsule composed of the following: A.) an international standard source holder (usually lead), B.) a retaining ring, and C.) a teletherapy](/Images/godic/202502/05/Teletherapy_Capsule2.svg5027.png")
External beam radiotherapy (EBRT) or teletherapy is the most common form of radiotherapy (radiation therapy). The patient sits or lies on a couch and an external source of ionizing radiation is pointed at a particular part of the body. In contrast to internal radiotherapy (brachytherapy), in which the radiation source is inside the body, external beam radiotherapy directs the radiation at the tumour from outside the body. Orthovoltage ("superficial") X-rays are used for treating skin cancer and superficial structures. Megavoltage ("deep") X-rays are used to treat deep-seated tumours (e.g. bladder, bowel, prostate, lung, or brain).