External beam radiotherapy (EBRT) is the most common form of radiotherapy (
radiation therapy
Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radia ...
). The patient sits or lies on a couch and an external source of
ionizing radiation
Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
is pointed at a particular part of the body. In contrast to
brachytherapy (sealed source radiotherapy) and
unsealed source radiotherapy
Radionuclide therapy (RNT, also known as unsealed source radiotherapy or molecular radiotherapy) uses radioactive substances called radiopharmaceuticals to treat medical conditions, particularly cancer. These are introduced into the body by vario ...
, in which the radiation source is inside the body, external beam radiotherapy directs the radiation at the tumour from outside the body.
Orthovoltage ("superficial") X-rays are used for treating skin cancer and superficial structures.
Megavoltage X-rays are used to treat deep-seated tumours (e.g. bladder, bowel, prostate, lung, or brain), whereas megavoltage electron beams are typically used to treat superficial lesions extending to a depth of approximately 5 cm (increasing beam energy corresponds to greater penetration). X-rays and electron beams are by far the most widely used sources for external beam radiotherapy. A small number of centers operate experimental and pilot programs employing beams of heavier particles, particularly
protons
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
, owing to the rapid dropoff in absorbed dose beneath the depth of the target.
X-rays and gamma rays
![External beam radiotherapy retinoblastoma nci-vol-1924-300](https://upload.wikimedia.org/wikipedia/commons/5/59/External_beam_radiotherapy_retinoblastoma_nci-vol-1924-300.jpg)
Conventionally, the energy of diagnostic and therapeutic
gamma- and
X-rays
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...
is expressed in
kilovolts
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defi ...
or
mega
Mega or MEGA may refer to:
Science
* mega-, a metric prefix denoting 106
* Mega (number), a certain very large integer in Steinhaus–Moser notation
* "mega-" a prefix meaning "large" that is used in taxonomy
* Gravity assist, for ''Moon-Eart ...
volt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defi ...
s (kV or MV), whilst the energy of therapeutic electrons is expressed in terms of mega
electronvolt
In physics, an electronvolt (symbol eV, also written electron-volt and electron volt) is the measure of an amount of kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defi ...
s (MeV). In the first case, this voltage is the maximum electric potential used by a
linear accelerator
A linear particle accelerator (often shortened to linac) is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear beam ...
to produce the
photon
A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless, so they always ...
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
X-ray filter An X-ray filter is a material placed in front of an X-ray source in order to reduce the intensity of particular wavelengths from its spectrum and selectively alter the distribution of X-ray wavelengths within a given beam.
When X-rays hit matter, p ...
s, 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
The photoelectric effect is the emission of electrons when electromagnetic radiation, such as light, hits a material. Electrons emitted in this manner are called photoelectrons. The phenomenon is studied in condensed matter physics, and solid st ...
predominates (for diagnostic use, since the photoelectric effect offers comparatively excellent contrast with effective atomic number ''Z'') or
Compton scatter
Compton scattering, discovered by Arthur Holly Compton, is the scattering of a high frequency photon after an interaction with a charged particle, usually an electron. If it results in a decrease in energy (increase in wavelength) of the photon ...
and
pair production
Pair production is the creation of a subatomic particle and its antiparticle from a neutral boson. Examples include creating an electron and a positron, a muon and an antimuon, or a proton and an antiproton. Pair production often refers specific ...
predominate (at energies above approximately 200 keV for the former and 1 MeV for the latter), for therapeutic X-ray beams. Some examples of X-ray energies used in medicine are:
*
Very low-energy superficial X-rays
Very may refer to:
* English's prevailing intensifier
Businesses
* The Very Group, a British retail/consumer finance corporation
** Very (online retailer), their main e-commerce brand
* VERY TV, a Thai television channel
Places
* Véry, a c ...
– 35 to 60 keV (mammography, which prioritizes soft-tissue contrast, uses very low-energy kV X-rays)
*Superficial radiotherapy X-rays – 60 to 150 keV
*
Diagnostic X-rays
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine " cause and effect". In systems enginee ...
– 20 to 150 keV (mammography to CT); this is the range of photon energies at which the photoelectric effect, which gives maximal soft-tissue contrast, predominates.
*
Orthovoltage X-rays
Orthovoltage X-rays are produced by X-ray tubes operating at voltages in the 100–500 kV range, and therefore the X-rays have a peak energy in the 100–500 keV range. Orthovoltage X-rays are sometimes termed "deep" X-rays (DXR). They ...
– 200 to 500 keV
*
Supervoltage X-rays – 500 to 1000 keV
*
Megavoltage X-rays – 1 to 25 MeV (in practice, nominal energies above 15 MeV are unusual in clinical practice).
Megavoltage X-rays are by far most common in radiotherapy for the treatment of a wide range of cancers. Superficial and orthovoltage X-rays have application for the treatment of cancers at or close to the skin surface. Typically, higher energy megavoltage X-rays are chosen when it is desirable to maximize "skin-sparing" (since the relative dose to the skin is lower for such high-energy beams).
Medically useful photon beams can also be derived from a radioactive source such as
iridium-192
There are two natural isotopes of iridium (77Ir), and 34 radioisotopes, the most stable radioisotope being 192Ir with a half-life of 73.83 days, and many nuclear isomers, the most stable of which is 192m2Ir with a half-life of 241 years. All othe ...
,
caesium-137
Caesium-137 (), cesium-137 (US), or radiocaesium, is a radioactive isotope of caesium that is formed as one of the more common fission products by the nuclear fission of uranium-235 and other fissionable isotopes in nuclear reactors and nucl ...
or
radium
Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rather t ...
-226 (which is no longer used clinically), or
cobalt-60
Cobalt-60 (60Co) is a synthetic radioactive isotope of cobalt with a half-life of 5.2713 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisoto ...
. Such photon beams, derived from radioactive decay, are more or less
monochromatic
A monochrome or monochromatic image, object or color scheme, palette is composed of one color (or lightness, values of one color). Images using only Tint, shade and tone, shades of grey are called grayscale (typically digital) or Black and wh ...
and are properly termed
gamma ray
A gamma ray, also known as gamma radiation (symbol γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...
s. The usual energy range is between 300 keV to 1.5 MeV, and is specific to the isotope. Notably, photon beams deriving from radioisotopes are approximately monoenergetic, as contrasted with the continuous
bremsstrahlung
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
spectrum from a linac.
Therapeutic radiation is mainly generated in the radiotherapy department using some of the following equipment:
#''Superficial radiation therapy'' (SRT) machines produce low energy x-rays in the same energy range as diagnostic x-ray machines, 20 - 150 kV, to treat skin conditions.
#''
Orthovoltage'' X-ray machines, which produce higher energy x-rays in the range 200–500 kV. This radiation was called "deep" because it could treat tumors at depths at which lower energy "superficial" radiation (above) was unsuitable. Orthovoltage units have essentially the same design as
diagnostic X-ray machines
Diagnosis is the identification of the nature and cause of a certain phenomenon. Diagnosis is used in many different disciplines, with variations in the use of logic, analytics, and experience, to determine "cause and effect". In systems engineer ...
. These machines are generally limited to less than 600 kV.
#''
Linear accelerator
A linear particle accelerator (often shortened to linac) is a type of particle accelerator that accelerates charged subatomic particles or ions to a high speed by subjecting them to a series of oscillating electric potentials along a linear beam ...
s'' ("linacs") which produce
megavoltage
Megavoltage X-rays are produced by linear accelerators ("linacs") operating at voltages in excess of 1000 kV (1 MV) range, and therefore have an energy in the MeV range. The voltage in this case refers to the voltage used to accelerat ...
X-rays. The first use of a linac for medical radiotherapy was in 1953 (see also
Radiation therapy
Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radia ...
). 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
Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...
alloy, which produces an X-ray spectrum via
bremsstrahlung
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
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 use radiation from the
radioisotope
A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess nuclear energy, making it unstable. This excess energy can be used in one of three ways: emitted from the nucleus as gamma radiation; transferr ...
cobalt-60
Cobalt-60 (60Co) is a synthetic radioactive isotope of cobalt with a half-life of 5.2713 years. It is produced artificially in nuclear reactors. Deliberate industrial production depends on neutron activation of bulk samples of the monoisoto ...
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 largely been replaced by the linear accelerator, which can generate higher energy radiation. Cobalt treatment still has a useful role to play in certain applications (for example the
Gamma Knife
Radiosurgery is surgery using radiation, that is, the destruction of precisely selected areas of tissue using ionizing radiation rather than excision with a blade. Like other forms of radiation therapy (also called radiotherapy), it is usually u ...
) and is still in widespread use worldwide, 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 (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.
Many linear accelerators can produce both electrons and x-rays.
Hadron therapy
Hadron
In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ele ...
therapy involves the therapeutic use of
proton
A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mass ...
s,
neutron
The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...
s, and heavier
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...
s (fully ionized atomic nuclei). Of these,
proton therapy
In medicine, proton therapy, or proton radiotherapy, is a type of particle therapy that uses a beam of protons to irradiate diseased tissue, most often to treat cancer. The chief advantage of proton therapy over other types of external beam ra ...
is by far the most common, though still quite rare compared to other forms of external beam radiotherapy, since it requires large and expensive equipment. The gantry (the part that rotates around the patient) is a multi-story structure, and a proton therapy system can cost (as of 2009) up to US$150 Million.
Multi-leaf collimator
Modern linear accelerators are equipped with
multileaf collimator
A multileaf collimator (MLC) is a Collimator or beam-limiting device that is made of individual "leaves" of a high atomic numbered material, usually tungsten, that can move independently in and out of the path of a radiotherapy beam in order to sh ...
s (MLCs) which can move within the radiation field as the linac gantry rotates, blocking the field as necessary according to the gantry position. This technology allows radiotherapy treatment planners great flexibility in shielding organs-at-risk (OARSs), while ensuring that the prescribed dose is delivered to the target(s). A typical multi-leaf collimator consists of two sets of 40 to 80 leaves, each around 5 mm to 10 mm thick and several centimetres 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 older linacs without MLCs, this must be accomplished manually using several hand-crafted blocks.
Intensity modulated radiation therapy
![Teletherapy Capsule2](https://upload.wikimedia.org/wikipedia/commons/d/d1/Teletherapy_Capsule2.svg)
Intensity modulated radiation therapy (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 treatment, thereby delivering a radiation field with a non-uniform (i.e. modulated) intensity. With IMRT, radiation oncologists are able to break up the radiation beam into many "beamlets". This allows
radiation oncologists
A radiation oncologist is a specialist physician who uses ionizing radiation (such as megavoltage X-rays or radionuclides) in the treatment of cancer. Radiation oncology is one of the three primary specialties, the other two being surgical and me ...
to vary the intensity of each beamlet. With IMRT, doctors are often able to further limit the amount of radiation received by healthy tissue near the tumor. Doctors have found this sometimes allowed them to safely give a higher dose of radiation to the tumor, potentially increasing the chance of a cure.
Volumetric Modulated Arc Therapy
Volumetric modulated arc therapy (VMAT) is an extension of IMRT where in addition to MLC motion, the linear accelerator will move around the patient during treatment. This means that rather than radiation entering the patient through only a small number of fixed angles, it can enter through many angles. This can be beneficial for some treatment sites where the target volume is surrounded by a number of organs which must be spared radiation dose.
Flattening Filter Free
The intensity of the X-rays produced in an megavoltage linac is much higher in the centre of the beam compared to the edge. To counteract this a flattening filter is used. A flattening filter is a cone of metal (typically tungsten); after the X-ray beam has passed through the flattening filter it will have a more uniform profile, since the flattening filter is shaped so as to compensate for the forward bias in the momentum of the electrons incident upon it. This makes treatment planning simpler but also reduces the intensity of the beam significantly. With greater computing power and more efficient treatment planning algorithms, the need for simpler treatment planning techniques ("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).
The advantage of FFF treatments is the increased maximum dose rate, by a factor of up for four, 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.
, where the reduced treatment time may reduce patient movement, and breast treatments, where there is the potential to reduce breathing motion.
Image-guided radiation therapy
Image-guided radiation therapy
Image-guided radiation therapy is the process of frequent imaging, during a course of radiation treatment, used to direct the treatment, position the patient, and compare to the pre-therapy imaging from the treatment plan. Immediately prior to, o ...
(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. The more advanced the treatment techniques become in terms of dose deposition accuracy, the higher become the requirements for IGRT. In order to allow patients to benefit from sophisticated treatment techniques as IMRT or hadron therapy, patient alignment accuracies of 0.5 mm and less become desirable. Therefore, new methods like stereoscopic digital kilovoltage imaging based patient position verification (PPVS)
[Boris Peter Selby, Georgios Sakas et al. (2007) 3D Alignment Correction for Proton Beam Treatment. In: Proceedings of Conf. of the German Society for Biomedical Engineering (DGBMT). Aachen.] to alignment estimation based on in-situ cone-beam
computed tomography (CT) enrich the range of modern IGRT approaches.
See also
*
Brachytherapy
Brachytherapy is a form of radiation therapy where a sealed radiation source is placed inside or next to the area requiring treatment. ''Brachy'' is Greek for short. Brachytherapy is commonly used as an effective treatment for cervical, prosta ...
*
Cyberknife
*
Gamma Knife
Radiosurgery is surgery using radiation, that is, the destruction of precisely selected areas of tissue using ionizing radiation rather than excision with a blade. Like other forms of radiation therapy (also called radiotherapy), it is usually u ...
, a type of
radiosurgery
Radiosurgery is surgery using radiation, that is, the destruction of precisely selected areas of tissue using ionizing radiation rather than excision with a blade. Like other forms of radiation therapy (also called radiotherapy), it is usually u ...
*
Intraoperative electron radiation therapy
Intraoperative electron radiation therapy is the application of electron radiation directly to the residual tumor or tumor bed during cancer surgery. Electron beams are useful for intraoperative radiation treatment because, depending on the e ...
*
Intraoperative radiation therapy
Intraoperative radiation therapy (IORT) is radiation therapy that is administered during surgery directly in the operating room (hence ''intraoperative'').
Usually radiation therapy, therapeutic levels of radiation are delivered to the cancer, tu ...
*
Neutron capture therapy of cancer
*
Radiation therapy
Radiation therapy or radiotherapy, often abbreviated RT, RTx, or XRT, is a therapy using ionizing radiation, generally provided as part of cancer treatment to control or kill malignant cells and normally delivered by a linear accelerator. Radia ...
*
Tomotherapy
Tomotherapy is a radiation therapy modality, in which the patient is scanned across a modulated strip-beam, so that only one “slice” (Greek prefix “tomo-”) of the target is exposed at any one time by the linear accelerator (linac) beam. ...
References
General references
*''Radiotherapy physics in practice'', edited by JR Williams and DI Thwaites, Oxford University Press UK (2nd edition 2000),
Linear Particle Accelerator (Linac) Animation by Ionactive**http://www.myradiotherapy.com
National Institute of Radiological Science(Japan)
{{DEFAULTSORT:External Beam Radiotherapy
Electron beams in medical applications
Radiation therapy procedures