External beam radiotherapy
External beam radiation therapy is a form of radiotherapy that utilizes a high-energy collimated beam of ionizing radiation, from a source outside the body, to target and kill cancer cells. The radiotherapy beam is composed of particles, which are focussed in a particular direction of travel using collimators. Each radiotherapy beam consists of one type of particle intended for use in treatment, though most beams contain some contamination by other particle types.
Radiotherapy beams are classified by the particle they are intended to deliver, such as photons, electrons, and heavy ions; x-rays and electron beams are by far the most widely used sources for external beam radiotherapy. Orthovoltage X-rays are used for treating skin cancer and superficial structures. Megavoltage X-rays are used to treat deep-seated tumors, whereas megavoltage electron beams are typically used to treat superficial lesions extending to a depth of approximately 5 cm. A small number of centers operate experimental and pilot programs employing beams of heavier particles, particularly protons, owing to the rapid decrease in absorbed dose beneath the depth of the target.
Teletherapy is the most common form of radiotherapy. The patient sits or lies on a couch and an external source of ionizing radiation is pointed at a particular part of the body. In contrast to brachytherapy and unsealed source radiotherapy, in which the radiation source is inside the body, external beam radiotherapy directs the radiation at the tumor from outside the body.
X-rays and gamma rays
Conventionally, the energy of diagnostic and therapeutic gamma- and X-rays is on the order of kiloelectronvolts or megaelectronvolts, and the energy of therapeutic electrons is on the order of megaelectronvolts. The beam is made up of a spectrum of energies: the maximum energy is approximately equal to the beam's maximum electric potential within a linear accelerator times the electron charge. For instance, a 1 megavolt beam will produce photons with a maximum energy around 1 MeV. In practice, the mean X-ray energy is about one-third of the maximum energy. Beam quality and hardness may be improved by X-ray filters, which improves the homogeneity of the X-ray spectrum.Medically useful X-rays are produced when electrons are accelerated to energies at which either the photoelectric effect predominates or Compton scattering and pair production predominate, for therapeutic X-ray beams. Some examples of X-ray energies used in medicine are:
- Very low-energy superficial X-rays – 35 to 60 keV
- Superficial radiotherapy X-rays – 60 to 150 keV
- Diagnostic X-rays – 20 to 150 keV ; this is the range of photon energies at which the photoelectric effect, which gives maximal soft-tissue contrast, predominates.
- Orthovoltage X-rays – 200 to 500 keV
- Supervoltage X-rays – 500 to 1000 keV
- Megavoltage X-rays – 1 to 25 MeV.
Medically useful photon beams can also be derived from a radioactive source such as iridium-192, caesium-137, or cobalt-60. Such photon beams, derived from radioactive decay, are approximately monochromatic, in contrast to the continuous bremsstrahlung spectrum from a linac. These decays include the emission of gamma rays, whose energy is isotope-specific and ranges between 300 keV and 1.5 MeV.
Superficial radiation therapy machines produce low energy x-rays in the same energy range as diagnostic x-ray machines, 20–150 keV, to treat skin conditions. Orthovoltage X-ray machines produce higher energy x-rays in the range 200–500 keV. Radiation from orthovoltage x-ray machines has been called "deep" due to its greater penetrating ability, allowing it to treat tumors at depths unreachable by lower-energy "superficial" radiation. Orthovoltage units have essentially the same design as diagnostic X-ray machines and are generally limited to photon energies less than 600 keV. X-rays with energies on the order of 1 MeV are generated in Linear accelerators. The first use of a linac for medical radiotherapy was in 1953. 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 provided using specially modified linear accelerators.
Image:Nci-vol-1819-300 cobalt 60 therapy.jpg|thumb|Cobalt-60 beam machine from 1951
Cobalt units use radiation from cobalt-60, which emits two gamma rays at energies of 1.17 and 1.33 MeV, a dichromatic beam with an average energy of 1.25 MeV. The role of the cobalt unit has largely been replaced by the linear accelerator, which can generate higher energy radiation. Nonetheless, cobalt treatment still retains some applications, such as the Gamma Knife, since the machinery is relatively reliable and simple to maintain compared to the modern linear accelerator.
Electrons
X-rays are generated by bombarding a high atomic number material with electrons. If the target is removed, a high energy electron beam is obtained. Electron beams are useful for treating superficial lesions, because the maximum dose deposition occurs near the surface and thereafter decreases rapidly with depth, sparing underlying tissue. Electron beams usually have nominal energies in the range of 4–20 MeV, corresponding to a treatment range of approximately 1–5 cm. Energies above 18 MeV are rarely used. 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.Many linear accelerators can produce both electrons and x-rays.
Hadron therapy
therapy involves the therapeutic use of protons, neutrons, and heavier ions. Of these, proton therapy is by far the most common, though still rare compared to other forms of external beam radiotherapy, since it requires large and expensive equipment. The gantry is a multi-story structure, and a proton therapy system can cost up to US$150 million.Multi-leaf collimator
Modern linear accelerators are equipped with multileaf collimators, which can move within the radiation field as the linac gantry rotates, and block the field as necessary according to the gantry position. This technology allows radiotherapy treatment planners great flexibility in shielding organs-at-risk, while ensuring that the prescribed dose is delivered to the target organs. A typical multi-leaf collimator consists of two sets of 40 to 160 leaves, each around 5–10 mm thick and several centimetres long in the other two dimensions. Each leaf in the MLC is aligned parallel to the radiation field and can be moved independently to block part of the field, adapting it to the shape of the tumor, thus minimizing the amount of healthy tissue subject to radiation exposure. On older linacs without MLCs, this must be accomplished manually using several hand-crafted blocks.Intensity modulated radiation therapy
Intensity modulated radiation therapy 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 treatment, thereby delivering a radiation field with a non-uniform intensity. Using IMRT, radiation oncologists are able to split the radiation beam into many beamlets and vary the intensity of each beamlet, and doctors are often able to further limit the amount of radiation received by healthy tissue near the tumor. Doctors have found that this sometimes allows them to safely give a higher dose of radiation to the tumor, potentially increasing the chance of successful treatment.Volumetric modulated arc therapy
Volumetric modulated arc therapy is an extension of IMRT characterized by a linear accelerator rotating around the patient. This means that rather than radiation entering the patient at only a small number of fixed angles, it can enter at many angles. This can be beneficial for some treatment sites in which the target volume is surrounded by a number, allowing directed treatment without exposing nearby organs to heightened radiation levels.Flattening filter free
The intensity of the X-rays produced in a megavoltage linac is much higher in the centre of the beam compared to the edges. To offset this central peak, a flattening filter is used. A flattening filter is cone-shaped so as to compensate for the forward bias in the momentum of incident electrons ; after an X-ray beam passes through the flattening filter, it has a more uniform profile. This simplifies treatment planning, though significantly reduces the intensity of the beam. With greater computing power and more efficient treatment planning algorithms, the need for simpler treatment planning techniques – such as "forward planning", in which the planner directly instructs the linac on how to deliver the prescribed treatment – is reduced. This has led to increased interest in flattening filter free treatments.FFF treatments have been found to have an increased maximum dose rate, allowing reduced treatment times and a reduction in the effect of patient motion on the delivery of the treatment. This makes FFF an area of particular interest in stereotactic treatments. For instance, in treatment of breast cancer, the reduced treatment time may reduce patient movement and breast treatments where there is the potential to reduce breathing motion.