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, cesium 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:
- 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).
- linear accelerators or linacs which produce X-rays called megavoltage X-rays. These X-rays have an energy range from 500keV up to any number but the highest available at present is around 25MeV (25 million electron volts).
Some X-rays are measured in MeV and others in MV. The difference is because MeV beams (gamma rays) have a single beam type - like a laser has a single colour and so the energy can be described as a single number. The MV beams of linacs have a spectrum of energies - like a torch and the sun have a spectrum of colours and so the energy can be described as the value of the most energetic X-ray beam.
In the medical area, useful X-rays are produced when electrons are accelerated to high numbers of electron volts. Some examples of X-ray electron volt figures below:
- 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
