Megavoltage X-rays
（重定向自Megavoltage）
![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](/Images/godic/202501/26/Megavolt_x-ray_machine_-_Los_Angeles_Inst_of_Radiology_19385602.jpg")
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). They are used in medicine in external beam radiotherapy to treat neoplasms, cancer and tumors. Beams with the 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. Lower energy x-rays, called orthovoltage X-rays, are used to treat more superficial cancers.