External beam radiotherapy is the most common form of radiotherapy where a patient lies on a couch and an external source of X-rays is pointed at a particular part of the body. The radiation interacts with tissues and is absorbed, damaging the DNA of the cell.

The source of the X-rays can be from a radioactive source such as cobalt-60, iridium-137, caesium or radium-226 (which is no longer available). Such X-rays are monochromatic and called gamma rays. The usual energy range is in the 300keV to 1.5MeV range.

The other source of X-rays are from machines that generate them, and there are two basic varieties used now:

1. conventional X-ray generators which produce X-rays called 'superficial' X-rays and 'orthovoltage' X-rays. These machines are limited to less than 500,000 electron-volts (or 500 kiloelectronvolts or 500 keV, as it is written in its shortened form).
2. linear accelerators or linacs which produce X-rays called megavoltage X-rays. These X-rays have an energy range from 500 keV up to any number but the highest available at present is around 25 MeV (25 million electron volts).

A third variety called the 'betatron' was used previously but was unable to match the linac for stability or ease of use and they were really noisy and frightened the patients.

The energy of some X-rays is expressed in MeV (mega electron volts) and others in MV (megavolts). In X-rays produced from radioactive materials, the rays have a single energy (called a monoenergetic beam) or are made up of combinations of a few X-rays of different energies. Since the energy of the X-ray (or rays) can be given as a discrete value, it is expressed in units of energy (MeV). In contrast, the photon beam produced by a linear accelerator is made up of a nearly-continuous spectrum of energies. Since it is thus incorrect to give a single energy value, the maximum electric potential used to produce the beam is thus used and is given in terms of MV, or megavolts. The maximum X-ray energy across the spectrum, given in MeV, is approximately equal to the potential MV. However the mean X-ray energy is only 1/3 of the maximum energy.

In the medical field, useful X-rays are produced when electrons are accelerated to a large number of electron volts. Some examples of X-ray energies used in medicine are:

• superficial X-rays - 20-50 thousand electron volts or keV
• diagnostic X-rays - 50-150 thousand electron volts or keV
• orthovoltage X-rays - 200-500 thousand electron volts or keV
• megavoltage X-rays - 1-25 million electron volts or MeV