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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:
- 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 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 patient.
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-raysOrthovoltage X-rays have an energy in the range of 200-500 kV. When used to treat patients, radiation oncologists find that they penetrate to a useful depth of about 4-6cm. That makes them good for ribs, but not for lungs. Long ago, orthovoltage X-rays we - 200-500 thousand electron volts or keV
- megavoltage X-rays - 1-25 million electron volts or MeV
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