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. In contrast to internal radiotherapy (brachytherapy), in which the radiation source is placed inside the body, external beam radiotherapy directs the radiation at the tumor from outside the body. Kilovoltage (also known as, superficial) x-rays are used for treating skin cancer and superficial structures. Megavoltage (or deep) X-rays are used to treat deep-seated tumors (e.g. bladder, bowel, prostate, lung, brain).
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:
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 lesions because the maximum of dose deposition occurs near the surface. The dose then decreases rapidly with depth, sparing underlying tissue. Electron beams usually have nominal energies in the range 4-20 MeV. Depending on the energy this translates to a treatment range of approximately 1–5 cm (in water-equivalent tissue). Energies above 18 MeV are used very rarely. Although the X-ray target is removed in electron mode, the beam must be fanned out by sets of thin scattering foils in order to achieve flat and symmetric dose profiles in the treated tissue.
Hadron therapy involves the therapeutic 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 160 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.