megavoltage X-rays

{{Use American English|date = February 2019}}

{{Short description|High energy (>1MeV) X-rays}}

{{Infobox medical intervention |

Name = Megavoltage X-rays |

Image = File:Megavolt x-ray machine - Los Angeles Inst of Radiology 1938.jpg |

Caption = Early megavolt x-ray machine installed at Los Angeles Institute of Radiotherapy, 1938. Before linacs, high voltage x-ray tubes (left column) powered by million volt transformers (right column) were used to produce penetrating x-rays|

image_size = 140px|

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ICD9unlinked = {{ICD9proc|92.24}} |

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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.{{cite book|last1=Podgorsak|first1=E B|title=Radiation Oncology Physics: A Handbook for Teachers and Students|date=2005|publisher=International Atomic Energy Agency|location=Vienna|isbn=92-0-107304-6|page=125|url=http://www-pub.iaea.org/books/iaeabooks/7086/Radiation-Oncology-Physics-A-Handbook-for-Teachers-and-Students|chapter=Treatment Machines for External Beam Radiotherapy}} 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.Camphausen KA, Lawrence RC. [http://www.cancernetwork.com/cancer-management-11/chapter02/article/10165/1399960 "Principles of Radiation Therapy"] in Pazdur R, Wagman LD, Camphausen KA, Hoskins WJ (Eds) [http://www.cancernetwork.com/cancer-management-11/ Cancer Management: A Multidisciplinary Approach]. 11 ed. 2008. Lower energy x-rays, called orthovoltage X-rays, are used to treat cancers closer to the surface.{{cite book|last1=Herrmann|first1=Joerg|title=Clinical Cardio-oncology|date=2016|publisher=Elsevier Health Sciences|isbn=9780323462396|page=81|url=https://books.google.com/books?id=KWtjDQAAQBAJ&pg=PT81|language=en|doi=10.1016/B978-0-323-44227-5.00003-X}}

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.{{cite journal|last1=Buzdar|first1=SA|last2=Rao|first2=MA|last3=Nazir|first3=A|title=An analysis of depth dose characteristics of photon in water.|journal=Journal of Ayub Medical College, Abbottabad|date=2009|volume=21|issue=4|pages=41–5|pmid=21067022}}{{cite journal|last1=Sixel|first1=Katharina E.|title=Buildup region and depth of dose maximum of megavoltage x-ray beams|journal=Medical Physics|date=1999|volume=21|issue=3|pages=411|doi=10.1118/1.597305|bibcode=1994MedPh..21..411S}}{{cite book|last1=Pazdur|first1=Richard|title=Cancer management : a multidisciplinary approach : medical, surgical, & radiation oncology|date=2005|publisher=Oncology Group|location=New York|isbn=9781891483356|edition=9th ed., 2005-2006.|chapter=Principles of radiation therapy}} Megavoltage X-rays also have lower relative biological effectiveness than orthovoltage x-rays.{{cite journal|last1=Amols|first1=H. I.|last2=Lagueux|first2=B.|last3=Cagna|first3=D.|title=Radiobiological Effectiveness (RBE) of Megavoltage X-Ray and Electron Beams in Radiotherapy|journal=Radiation Research|date=January 1986|volume=105|issue=1|pages=58|doi=10.2307/3576725|bibcode=1986RadR..105...58A}} These properties help to make megavoltage x-rays the most common beam energies typically used for radiotherapy in modern techniques such as IMRT.{{cite book|last1=Levitt|first1=Seymour H. Levitt|last2=Purdy|first2=James A|last3=Perez|first3=Carlos A|last4=Poortmans|first4=Philip|title=Technical basis of radiation therapy practical clinical applications|date=2012|publisher=Springer|location=Heidelberg|isbn=9783642115721|page=96|edition=5th|chapter=Physics of Radiotherapy Planning and Delivery}}