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Strictly speaking, the term megavoltage therapy should be applied to the use of x rays and gamma rays which have an effective energy of 1 million electron volts or more. However, for our purposes the term can be applied to cover all x rays generated by voltages greater than 1 megavolt and terminating, for medical purposes, at 43 megavolts, as well as cobalt 60 gamma radiation.
From the practical point of view, the apparatus in regular use for providing high energy radiation are cobalt teletherapy units (1.2 megavoltage gamma radiation), Van de Graaf generators (1 to 2 megavoltage x rays), linear accelerators (4 to 8 megavoltage x rays), and betatrons (18 to 43 megavoltage x rays). Both the linear accelerator and the betatron have advantages over a cobalt unit in the sense that they can produce both electrons and x rays; they are capable of carrying a higher patient load
Lederman M. Megavoltage Advances vs the Orthovoltage Era. JAMA. 1972;220(3):398-400. doi:10.1001/jama.1972.03200030056015