External beam radiotherapy

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External beam radiotherapy
Radiation therapy.jpg
Radiation therapy of the pelvis. Lasers and a mold under the legs are used to determine exact position. ICD10 = D?0
Other namesTeletherapy
ICD-9-CM 92.21-92.26

External beam radiation therapy (EBRT) 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. A radiotherapy beam is composed of particles which travel in a consistent direction; each radiotherapy beam consists of one type of particle intended for use in treatment, though most beams contain some contamination by other particle types.

Contents

Radiotherapy beams are classified by the particle they are intended to deliver, such as photons (as x-rays or gamma rays), electrons, and heavy ions; x-rays and electron beams are by far the most widely used sources for external beam radiotherapy. Orthovoltage ("superficial") X-rays are used for treating skin cancer and superficial structures. Megavoltage X-rays are used to treat deep-seated tumors (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. 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 (radiation therapy). 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 (sealed source radiotherapy) 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

Historical image showing Gordon Isaacs, the first patient treated with linear accelerator radiation therapy (in this case an electron beam) for retinoblastoma in 1957. Gordon's right eye was removed January 11, 1957, because his cancer had spread. His left eye, however, had only a localized tumor that prompted Henry Kaplan to try to treat it with the electron beam. External beam radiotherapy retinoblastoma nci-vol-1924-300.jpg
Historical image showing Gordon Isaacs, the first patient treated with linear accelerator radiation therapy (in this case an electron beam) for retinoblastoma in 1957. Gordon's right eye was removed January 11, 1957, because his cancer had spread. His left eye, however, had only a localized tumor that prompted Henry Kaplan to try to treat it with the electron beam.

Conventionally, the energy of diagnostic and therapeutic gamma- and X-rays is on the order of kiloelectronvolts (keV) or megaelectronvolts (MeV), 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 (for diagnostic use, since the photoelectric effect offers comparatively excellent contrast with effective atomic number Z) or Compton scattering and pair production 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:

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. [1] 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, caesium-137, or cobalt-60. (Radium-226 has also been used as such a source in the past, though has been replaced in this capacity by less harmful radioisotopes.) 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. [2] 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 ("linacs"). 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.

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. [3] [4] 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.

Sources and properties of X-rays

Bremsstrahlung X-rays are produced by bombarding energetic cathode rays (electrons) onto a target made of a material with high atomic number, such as tungsten. The target acts as a sort of transducer, converting part of the electrons' kinetic energy into energetic photons. Kilovoltage X-rays are typically produced using an X-ray tube, in which electrons travel through a vacuum from a hot cathode to a cold anode, which also acts as the target. However, it is impractical to produce megavoltage X-rays using this method; instead, a linear accelerator is most commonly used to produce X-rays of such energy. X-ray emission is more forward-directed at megavoltage energies and more laterally-directed at kilovoltage energies. [5] Consequently, kilovoltage X-rays tend to be produced using a reflection-type target, in which the radiation is emitted back from the target's surface, while megavoltage X-rays tend to be produced with a transmission target in which the X-rays are emitted on the side opposite that of electron incidence. Reflection type targets exhibit the heel effect and can use a rotating anode to aid in heat dissipation.

Compton scattering is the dominant interaction between a megavoltage beam and the patient, while the photoelectric effect dominates at keV energies. Additionally, Compton scattering is much less dependent on atomic number than the photoelectric effect; while kilovoltage beams enhance the distinction between muscle and bone in medical imaging, megavoltage beams suppress that distinction to the advantage of teletherapy. Pair production and photoneutron production increase at higher energies, only becoming significant at energies on the order of 1 MeV.

X-ray energy in the keV range is described by the electrical voltage used to produce it. For instance, a 100 kVp beam is produced by a 100 kV voltage applied to an X-ray tube and will have a maximum photon energy of 100 keV. However, the beam's spectrum can be affected by other factors as well, such as the voltage waveform and external X-ray filtration. These factors are reflected in the beam's half-value layer (HVL), measured in-air under conditions of "good geometry". A typical superficial X-ray energy might be 100 kVp per 3 mmAl – "100 kilovolts applied to the X-ray tube with a measured half-value layer of 3 millimeters of aluminum". The half-value layer for orthovoltage beams is more typically measured using copper; a typical orthovoltage energy is 250 kVp per 2 mmCu. [6] For X-rays in the MeV range, an actual voltage of the same magnitude is not used in production of the beam. A 6 MV beam contains photons of no more than 1 MeV, rather than 6 MeV; the energy of such a beam is instead generally characterized by measuring the ratio of the beam's intensity at varying depths in a medium.

Kilovoltage beams do not exhibit a build-up effect and thus deposit their maximum dose at the surface, i.e. dmax = 0 or D0 = 100%. Conversely, megavoltage beams do exhibit the buildup effect deposit; they deposit their maximum dose at some depth below the surface, i.e. dmax > 0. The depth of dose maximum is governed by the range of the electrons liberated upstream during Compton scattering. At depths beyond dmax, the dose profile of all X-ray beams decreases roughly exponentially with depth. Though actual values of dmax are influenced by various factors, the following are representative benchmark values. [7]

Representative dmax
Voltage (MV)< 1469101420242534
dmax (cm)011.51.92.32.73.53.93.84.7

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 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 (in water-equivalent tissue). 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

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 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. [8]

Multi-leaf collimator

Modern linear accelerators are equipped with multileaf collimators (MLCs), 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 (OARSs), 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 (by adjusting the position of the leaves), 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

A teletherapy radiation capsule composed of the following:
A.) an international standard source holder (usually lead),
B.) a retaining ring, and
C.) a teletherapy "source" composed of
D.) two nested stainless steel canisters welded to
E.) two stainless steel lids surrounding
F.) a protective internal shield (usually uranium metal or a tungsten alloy) and
G.) a cylinder of radioactive source material, often but not always cobalt-60. The diameter of the "source" is 30 mm. Teletherapy Capsule2.svg
A teletherapy radiation capsule composed of the following:
A.) an international standard source holder (usually lead),
B.) a retaining ring, and
C.) a teletherapy "source" composed of
D.) two nested stainless steel canisters welded to
E.) two stainless steel lids surrounding
F.) a protective internal shield (usually uranium metal or a tungsten alloy) and
G.) a cylinder of radioactive source material, often but not always cobalt-60. The diameter of the "source" is 30 mm.

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. 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. [9]

Volumetric modulated arc therapy

Volumetric modulated arc therapy (VMAT) 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. [10]

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 (and is typically made from a metal such as tungsten); 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 (FFF) 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. [11] 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. [12]

Image-guided radiation therapy

Image-guided radiation therapy (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. To allow patients to benefit from sophisticated treatment techniques as IMRT or hadron therapy, patient alignment accuracies with an error margin of at most 0.5 mm are desirable. Therefore, methods such as stereoscopic digital kilovoltage imaging-based patient position verification (PPVS), [13] and alignment estimation based on in-situ cone-beam computed tomography (CT), enrich the range of modern IGRT approaches.

See also

Related Research Articles

<span class="mw-page-title-main">Radiation therapy</span> Therapy using ionizing radiation, usually to treat cancer

Radiation therapy or radiotherapy is a treatment using ionizing radiation, generally provided as part of cancer therapy to either kill or control the growth of malignant cells. It is normally delivered by a linear particle accelerator. Radiation therapy may be curative in a number of types of cancer if they are localized to one area of the body, and have not spread to other parts. It may also be used as part of adjuvant therapy, to prevent tumor recurrence after surgery to remove a primary malignant tumor. Radiation therapy is synergistic with chemotherapy, and has been used before, during, and after chemotherapy in susceptible cancers. The subspecialty of oncology concerned with radiotherapy is called radiation oncology. A physician who practices in this subspecialty is a radiation oncologist.

<span class="mw-page-title-main">Orthovoltage X-rays</span> High energy (100–500 KeV) 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 cover the upper limit of energies used for diagnostic radiography, and are used in external beam radiotherapy to treat cancer and tumors. They penetrate tissue to a useful depth of about 4–6 cm. This makes them useful for treating skin, superficial tissues, and ribs, but not for deeper structures such as lungs or pelvic organs. The relatively low energy of orthovoltage X-rays causes them to interact with matter via different physical mechanisms compared to higher energy megavoltage X-rays or radionuclide γ-rays, increasing their relative biological effectiveness.

<span class="mw-page-title-main">Linear particle accelerator</span> Type of particle accelerator

A linear particle accelerator 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 beamline. The principles for such machines were proposed by Gustav Ising in 1924, while the first machine that worked was constructed by Rolf Widerøe in 1928 at the RWTH Aachen University. Linacs have many applications: they generate X-rays and high energy electrons for medicinal purposes in radiation therapy, serve as particle injectors for higher-energy accelerators, and are used directly to achieve the highest kinetic energy for light particles for particle physics.

<span class="mw-page-title-main">Megavoltage X-rays</span> High energy (>1MeV) X-rays

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 accelerate electrons in the linear accelerator and indicates the maximum possible energy of the photons which are subsequently produced. They are used in medicine in external beam radiotherapy to treat neoplasms, cancer and tumors. Beams with a voltage range of 4-25 MV are used to treat deeply buried cancers because radiation oncologists find that they penetrate well to deep sites within the body. Lower energy x-rays, called orthovoltage X-rays, are used to treat cancers closer to the surface.

<span class="mw-page-title-main">Proton therapy</span> Medical Procedure

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 radiotherapy is that the dose of protons is deposited over a narrow range of depth; hence in minimal entry, exit, or scattered radiation dose to healthy nearby tissues.

<span class="mw-page-title-main">Radiosurgery</span> Surgical Specialty

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, it is usually used to treat cancer. Radiosurgery was originally defined by the Swedish neurosurgeon Lars Leksell as "a single high dose fraction of radiation, stereotactically directed to an intracranial region of interest".

A monitor unit (MU) is a measure of machine output from a clinical accelerator for radiation therapy such as a linear accelerator or an orthovoltage unit. Monitor units are measured by monitor chambers, which are ionization chambers that measure the dose delivered by a beam and are built into the treatment head of radiotherapy linear accelerators.

<span class="mw-page-title-main">Radiation burn</span> Damage to skin or biological tissue from radiation exposure

A radiation burn is a damage to the skin or other biological tissue and organs as an effect of radiation. The radiation types of greatest concern are thermal radiation, radio frequency energy, ultraviolet light and ionizing radiation.

<span class="mw-page-title-main">Fast neutron therapy</span>

Fast neutron therapy utilizes high energy neutrons typically between 50 and 70 MeV to treat cancer. Most fast neutron therapy beams are produced by reactors, cyclotrons (d+Be) and linear accelerators. Neutron therapy is currently available in Germany, Russia, South Africa and the United States. In the United States, one treatment center is operational, in Seattle, Washington. The Seattle center uses a cyclotron which produces a proton beam impinging upon a beryllium target.

<span class="mw-page-title-main">Radiation treatment planning</span>

In radiotherapy, radiation treatment planning (RTP) is the process in which a team consisting of radiation oncologists, radiation therapist, medical physicists and medical dosimetrists plan the appropriate external beam radiotherapy or internal brachytherapy treatment technique for a patient with cancer.

Electron therapy or electron beam therapy (EBT) is a kind of external beam radiotherapy where electrons are directed to a tumor site for medical treatment of cancer.

<span class="mw-page-title-main">Tomotherapy</span> Type of radiation therapy

Tomotherapy is a type of radiation therapy treatment machine. In tomotherapy a thin radiation beam is modulated as it rotates around the patient, while they are moved through the bore of the machine. The name comes from the use of a strip-shaped beam, so that only one “slice” of the target is exposed at any one time by the radiation. The external appearance of the system and movement of the radiation source and patient can be considered analogous to a CT scanner, which uses lower doses of radiation for imaging. Like a conventional machine used for X-ray external beam radiotherapy, a linear accelerator generates the radiation beam, but the external appearance of the machine, the patient positioning, and treatment delivery is different. Conventional linacs do not work on a slice-by-slice basis but typically have a large area beam which can also be resized and modulated.

Intraoperative radiation therapy (IORT) is radiation therapy that is administered during surgery directly in the operating room.

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, or during, a treatment fraction, the patient is localized in the treatment room in the same position as planned from the reference imaging dataset. An example of IGRT would include comparison of a cone beam computed tomography (CBCT) dataset, acquired on the treatment machine, with the computed tomography (CT) dataset from planning. IGRT would also include matching planar kilovoltage (kV) radiographs or megavoltage (MV) images with digital reconstructed radiographs (DRRs) from the planning CT.

<span class="mw-page-title-main">Cobalt therapy</span> Medical use of gamma rays

Cobalt therapy is the medical use of gamma rays from the radioisotope cobalt-60 to treat conditions such as cancer. Beginning in the 1950s, cobalt-60 was widely used in external beam radiotherapy (teletherapy) machines, which produced a beam of gamma rays which was directed into the patient's body to kill tumor tissue. Because these "cobalt machines" were expensive and required specialist support, they were often housed in cobalt units. Cobalt therapy was a revolutionary advance in radiotherapy in the post-World War II period but is now being replaced by other technologies such as linear accelerators.

Particle therapy is a form of external beam radiotherapy using beams of energetic neutrons, protons, or other heavier positive ions for cancer treatment. The most common type of particle therapy as of August 2021 is proton 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 electron energy, the dose falls off rapidly behind the target site, therefore sparing underlying healthy tissue.

A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams.

<span class="mw-page-title-main">Gamma ray</span> Energetic electromagnetic radiation arising from radioactive decay of atomic nuclei

A gamma ray, also known as gamma radiation (symbol γ or ), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically shorter than those of X-rays. With frequencies above 30 exahertz (3×1019 Hz), it imparts the highest photon energy. Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying radiation emitted by radium. In 1903, Ernest Rutherford named this radiation gamma rays based on their relatively strong penetration of matter; in 1900 he had already named two less penetrating types of decay radiation (discovered by Henri Becquerel) alpha rays and beta rays in ascending order of penetrating power.

<span class="mw-page-title-main">Gantry (medical)</span> Radiation detector holder

In a medical facility, such as a hospital or clinic, a gantry holds radiation detectors and/or a radiation source used to diagnose or treat a patient's illness. Radiation sources may produce gamma radiation, x-rays, electromagnetic radiation, or magnetic fields depending on the purpose of the device.

References

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General references