External beam radiotherapy

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External beam radiotherapy otherwise known as teletherapy, is the most frequently used form of radiotherapy. The patient sits or lies on a couch and an external source of radiation is pointed at a particular part of the body. Superficial x-rays are used for treating skin cancer, and are particularly useful when treating near to an eye. Megavoltage X-rays are used to treat deep-seated tumours (eg bladder, bowel, prostate, lung, brain). Megavoltage electrons are mainly used for treating superficial tumours.

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[edit] Photons

Conventionally, the energy of diagnostic and therapeutic gamma- and X-rays is expressed in kilovolts or megavolts (kV or MV), whilst the energy of therapeutic electrons is expressed in terms of megaelectronvolts (MeV). In the first case, this voltage is the maximum electric potential used by a linear accelerator to produce the photon beam. The beam is made up of a spectrum of energies: the maximum energy is approximately equal to the beam's maximum electric potential times the electron charge. Thus a 1 MV beam will produce photons of no more than about 1 MeV. The mean X-ray energy is only about 1/3 of the maximum energy. Beam quality and hardness may be improved by special filters, which improve the homogeneity of the X-ray spectrum.

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

Of these energy ranges, megavoltage X-rays are by far most common in radiotherapy. Orthovoltage X-rays do have limited applications, and the other energy ranges are not typically used clinically.

Medically useful photon beams can also be derived from a radioactive source such as cobalt-60, iridium-192, caesium-137 or radium-226 (which is no longer used clinically). Such photon beams, derived from radioactive decay, are more or less monochromatic and are properly termed gamma rays. The usual energy range is between 300 keV to 1.5 MeV, and is specific to the isotope.

Therapeutic radiation is mainly generated in the radiotherapy department using the following equipment:

  1. Orthovoltage units. These are also known as "deep" and "superficial" machines depending on their energy range. Orthovoltage units have essentially the same design as diagnostic X-ray machines. These machines are generally limited to less than 600 kV.
  2. Linear accelerators ("linacs") which produce megavoltage X-rays. Commercially available medical linacs produce X-rays and electrons with an energy range from 4 MeV up to around 25 MeV. The X-rays themselves are produced by the rapid deceleration of electrons in a target material, typically a tungsten alloy, which produces an X-ray spectrum via bremsstrahlung radiation. The shape and intensity of the beam produced by a linac may be modified or collimated by a variety of means. Thus, conventional, conformal, intensity-modulated, tomographic, and stereotactic radiotherapy are all produced by specially modified linear accelerators.
  3. Cobalt units which produce stable, dichromatic beams of 1.17 and 1.33 MeV, resulting in an average beam energy of 1.25 MeV. The role of the cobalt unit has partly been replaced by the linear accelerator, which can generate higher energy radiation. Cobalt treatment still has a useful role to play in certain applications and is still in widespread use worldwide, since the machinery is relatively reliable and simple to maintain compared to the modern linear accelerator.

[edit] Electrons

X-rays are generated by bombarding a high atomic number material with electrons. If the target is removed (and the beam current decreased) a high eneregy electron beam is obtained. Electron beams are useful for treating superficial disease because they deposit a uniform dose near the surface, which then decreases rapidly with depth, sparing underlying tissue. An energy in the range 6-9MeV is typically used. In water this gives a practical range of 3-4.5cm. Energies above 18MeV are used very rarely.

[edit] Neutrons

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[edit] IMRT and MLCs to reduce damage to "innocent" tissue

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[edit] References

    See also Gamma knife.

    [edit] General References

    • Radiotherapy physics in practice, edited by JR Williams and DI Thwaites, Oxford University Press UK (2nd edition 2000), ISBN 0-19-262878-X
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