Megavoltage X-rays
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Megavoltage X-rays are produced by linear accelerators ("linacs") operating at voltages in excess of 1000 kV (1 MV) range, and therefore have an energy in the MeV range. The voltage in this case refers to the voltage used to accelerate electrons in the linear accelerator and indicates the maximum possible energy of the photons which are subsequently produced.<ref>Template:Cite book</ref> They are used in medicine in external beam radiotherapy to treat neoplasms, cancer and tumors. Beams with a voltage range of 4-25 MV are used to treat deeply buried cancers because radiation oncologists find that they penetrate well to deep sites within the body.<ref>Camphausen KA, Lawrence RC. "Principles of Radiation Therapy" in Pazdur R, Wagman LD, Camphausen KA, Hoskins WJ (Eds) Cancer Management: A Multidisciplinary Approach. 11 ed. 2008.</ref> Lower energy x-rays, called orthovoltage X-rays, are used to treat cancers closer to the surface.<ref>Template:Cite book</ref>
Megavoltage x-rays are preferred for the treatment of deep lying tumours as they are attenuated less than lower energy photons, and will penetrate further, with a lower skin dose.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite book</ref> Megavoltage X-rays also have lower relative biological effectiveness than orthovoltage x-rays.<ref>Template:Cite journal</ref> These properties help to make megavoltage x-rays the most common beam energies typically used for radiotherapy in modern techniques such as IMRT.<ref>Template:Cite book</ref>
HistoryEdit
The use of megavoltage x-rays for treatment first became widespread with the use of Cobalt-60 machines in the 1950s.<ref>Template:Cite journal</ref> However prior to this other devices had been capable of producing megavoltage radiation, including the 1930s Van de Graaff generator and betatron.<ref>Template:Cite book</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
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