Megavolt x-ray machine installed at Los Angeles Institute of Radiotherapy, 1938. Before linacs, high voltage x-ray tubes were used to produce penetrating x-rays
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 (see external beam radiotherapy for an explanation of the maximum and mean energies as a function of voltage). Beams with the voltage range of 4-25 MV are used to treat patients because radiation oncologists find that they penetrate well to deep sites within the body.
For instance, in one particular technique of prostateradiotherapy, a patient will be treated with 5 radiation beams of 15 MV X-rays. While the beams will point into the patient from different angles, all the beams will be pointed towards one point or centre in the prostate (so called an isocentric technique). In this way the linac can rotate around the patient who does not need to move. In fact, patient motion is a source of positioning error which a radiation therapist tries to eliminate.
By crossing multiple beams, the radiation dose delivered internally in the prostate is much higher (~70-78 Gy) than the radiation dose delivered on the entry and exit tracks of each individual beam (~40 Gy). This lower dose outside the prostate rarely leads to side effects or any detectable change.
By convention, the voltage is used to characterize X- and gamma-ray beams (in volts), while electron beams are characterized by their energies (in electronvolts).