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External beam radiotherapy

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 color 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 colors 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

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