External beam radiotherapy
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. Kilovoltage (also known as Superficial or Deep) x-rays are used for treating skin cancer and superficial structures. 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.
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:
- diagnostic X-rays - 20 to 50 kV
- superficial X-rays - 50 to 200 kV
- orthovoltage X-rays - 200 to 500 kV
- supervoltage X-rays - 500 to 1000 kV
- megavoltage X-rays - 1 to 25 MV
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:
- 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.
- 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.
- 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.
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 energy 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.
Hadron therapy involves the theraputic use of protons, neutrons, and heavier ions (fully ionized atomic nuclei). Of these, proton therapy is by far the most common, though still quite rare compared to other forms of external beam radiotherapy.
Multi-Leaf Collimator (MLC)
A typical MLC consists of 2 sets of 20-40 leaves, each around 5mm thick and several cm in the other two dimensions. Newer MLCs now have up to 120 leaves. Each leaf in the MLC is aligned parallel to the radiation field and can be moved independently to block part of the field. This allows the dosimetrist to match the radiation field to the shape of the tumor (by adjusting the position of the leaves), thus minimizing the amount of healthy tissue being exposed to radiation. On a machine without an MLC this must be accomplished using several hand-crafted blocks.
Intensity Modulated Radiation Therapy (IMRT) to reduce damage to healthy tissue
IMRT is an advanced radiotherapy technique used to minimize the amount of normal tissue being irradiated in the treatment field. In some systems this intensity modulation is achieved by moving the leaves in the MLC during the course of treatement, thereby delivering a radiation field with a non-uniform (i.e. modulated) intensity.
Image-Guided Radiation Therapy (IGRT) also reduces damage to healthy tissue
IGRT augments radiotherapy with imaging to increase the accuracy and precision of target localization, thereby reducing the amount of healthy tissue in the treatment field.
- Radiotherapy physics in practice, edited by JR Williams and DI Thwaites, Oxford University Press UK (2nd edition 2000), ISBN 0-19-262878-X