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===Medical linacs===
[[Image:External beam radiotherapy retinoblastoma nci-vol-1924-300.jpg|thumb|left|Historical image showing Gordon Isaacs, the first patient treated for [[retinoblastoma]] with linear accelerator radiation therapy (in this case an electron beam), in 1957, in the U.S. Other patients had been treated by linac for other diseases since 1953 in the UK. Gordon's right eye was removed on January 11, 1957 because cancer had spread there. His left eye, however, had only a localized tumor that prompted [[Henry Kaplan (physician)|Henry Kaplan]] to treat it with the electron beam.]]
Linac-based [[radiation therapy]] for cancer treatment began with the first patient treated in 1953 in London, UK, at the [[Hammersmith Hospital]], with an 8 MV machine built by [[Metropolitan-Vickers]] and installed in 1952, as the first dedicated medical linac.<ref>{{Cite journal|author=Thwaites, DI and Tuohy J|title=Back to the future: the history and development of the clinical linear accelerator|journal=Phys. Med. Biol.|volume=51|year=2006|issue=13 |pages=R343–R36| doi=10.1088/0031-9155/51/13/R20|pmid=16790912 |s2cid=7672187 }}</ref> A short while later in 1954, a 6 MV linac was installed in Stanford, USA, which began treatments in 1956.
[[Medical linear accelerators]] accelerate electrons using a tuned-cavity waveguide, in which the RF power creates a [[standing wave]]. Some linacs have short, vertically mounted waveguides, while higher energy machines tend to have a horizontal, longer waveguide and a bending magnet to turn the beam vertically towards the patient. Medical linacs use monoenergetic electron beams between 4 and 25 MeV, giving an X-ray output with a spectrum of energies up to and including the electron energy when the electrons are directed at a high-density (such as [[tungsten]]) target. The electrons or X-rays can be used to treat both benign and malignant disease. The LINAC produces a reliable, flexible and accurate radiation beam. The versatility of LINAC is a potential advantage over [[cobalt therapy]] as a treatment tool. In addition, the device can simply be powered off when not in use; there is no source requiring heavy shielding – although the treatment room itself requires considerable shielding of the walls, doors, ceiling etc. to prevent escape of scattered radiation. Prolonged use of high powered (>18 MeV) machines can induce a significant amount of radiation within the metal parts of the head of the machine after power to the machine has been removed (i.e. they become an active source and the necessary precautions must be observed).
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