Machines For External Beam Radiotherapy
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Link to IAEA Radiation Oncology Physics: A Handbook for Teachers and Students
Slides: Chapter 5. MACHINES FOR EXTERNAL BEAM RADIOTHERAPY
Text: Chapter 5. MACHINES FOR EXTERNAL BEAM RADIOTHERAPY
[edit] INTRODUCTION
Since the inception of radiotherapy soon after the discovery of x-rays by Roentgen in 1895, the technology of x-ray production has first been aimed toward ever higher photon and electron beam energies and intensities, and more recently toward computerization and intensity-modulated beam delivery. During the first 50 years of radiotherapy, the techno-logical progress has been relatively slow and mainly based on x-ray tubes, Van de Graaff generators and betatrons.
The invention of the cobalt-60 teletherapy unit by H.E. Johns in Canada in the early 1950s provided a tremendous boost in the quest for higher photon energies, and placed the cobalt unit into the forefront of radiotherapy for a number of years. The concurrently developed medical linear accelerators (linacs), however, soon eclipsed the cobalt unit, moved through five increasingly sophisticated generations, and became the most widely used radiation source in modern radiotherapy. With its compact and efficient design, the linac offers excellent versatility for use in radiotherapy through isocentric mounting and provides either electron or megavoltage x-ray therapy with a wide range of energies.
In addition to linacs, electron and x-ray radiotherapy is also carried out with other types of accelerators, such as betatrons and microtrons. More exotic particles, such as protons, neutrons, heavy ions, and negative π mesons, all produced by special accelerators, are also sometimes used for radiotherapy; however, most of the contemporary radiotherapy is carried out with linacs or teletherapy cobalt units.
[edit] X-RAY BEAMS AND X-RAY UNITS
5.2 X-RAY BEAMS AND X-RAY UNITS
- Clinical x-ray beams typically range in energy between 10 kVp and 50 MV, and are produced when electrons with kinetic energies between 10 keV and 50 MeV are decelerated in special metallic targets.
- In the target, most of the electron's kinetic energy is transformed into heat and a small fraction of the energy is emitted in the form of x-ray photons which are divided into two groups: characteristic x-rays and bremsstrahlung x-rays.
- Characteristic x-rays
- Bremsstrahlung (continuous) x-rays
- X-ray targets
- Clinical x-ray beams
- X-ray beam quality specifiers
- X-ray machines for radiotherapy
[edit] GAMMA RAY BEAMS AND GAMMA RAY UNITS
5.3 GAMMA RAY BEAMS AND GAMMA RAY UNITS
- Basic properties of gamma rays
- Teletherapy machines
- Teletherapy sources
- Teletherapy source housing
- Dose delivery with teletherapy machines
- Collimator and penumbra
[edit] PARTICLE ACCELERATORS
Numerous types of accelerators have been built for basic research in nuclear and high-energy physics, and most of them have been modified for at least some limited use in radiotherapy. Irrespective of the accelerator type two basic conditions must be met for particle acceleration:
- Particle to be accelerated must be charged
- Electric field must be provided in the direction of particle acceleration.
- The various types of accelerators differ in the way they produce the accelerating electric field and in how the field acts on the particles to be accelerated.
- As far as the accelerating electric field is concerned there are two main classes of accelerators: electrostatic and cyclic.
- In electrostatic accelerators the particles are accelerated by applying an electrostatic electric field through a voltage difference, constant in time, whose value fixes the value of the final kinetic energy of the particle. Since the electrostatic fields are conservative, the kinetic energy that the particle can gain depends only on the point of departure and point of arrival and, hence, cannot be larger than the potential energy corresponding to the maximum voltage drop existing in the machine. The energy that an electrostatic accelerator can reach is limited by the discharges that occur between the high voltage terminal and the walls of the accelerator chamber when the voltage drop exceeds a certain critical value (typically 1 MV).
- The electric fields used in cyclic accelerators are variable and non-conservative, associated with a variable magnetic field and resulting in some close paths along which the kinetic energy gained by the particle differs from zero. If the particle is made to follow such a closed path many times over, one obtains a process of gradual acceleration that is not limited to the maximum voltage drop existing in the accelerator. Thus, the final kinetic energy of the particle is obtained by submitting the charged particle to the same, relatively small, potential difference a large number of times, each cycle adding a small amount of energy to the kinetic energy of the particle.
- Examples of electrostatic accelerators used in medicine are: superficial and orthovoltage x-ray tubes and neutron generators. The most known example of a cyclic accelerator is the linear accelerator (linac); other examples are microtrons, betatrons and cyclotrons.
[edit] LINEAR ACCELERATORS
- Medical linear accelerators (linacs) are cyclic accelerators which accelerate electrons to kinetic energies from 4 MeV to 25 MeV using non-conservative microwave RF fields in the frequency range from 103 MHz (L band) to 104 MHz (X band), with the vast majority running at 2856 MHz (S band).
- In a linear accelerator the electrons are accelerated following straight trajectories in special evacuated structures called accelerating waveguides. Electrons follow a linear path through the same, relatively low, potential difference several times; hence, linacs also fall into the class of cyclic accelerators just like the other cyclic machines that provide curved paths for the accelerated particles (e.g., betatron)
- The high power RF fields, used for electron acceleration in the accelerating waveguides, are produced through the process of decelerating electrons in retarding potentials in special evacuated devices called magnetrons and klystrons.
- Various types of linacs are available for clinical use. Some provide x-rays only in the low megavoltage range (4 MV or 6 MV) others provide both x-rays and electrons at various megavoltage energies. A typical modern high energy linac will provide two photon energies (6 MV and 18 MV) and several electron energies (e.g., 6, 9, 12, 16, 22 MeV)
- Linac generations
- Safety of linac installations
- Components of modern linacs
- Configuration of modern linacs
- Injection system
- RF power generation system
- Accelerating waveguide
- Microwave power transmission
- Auxilliary system
- Electron beam transport
- Linac treatment head
- Production of clinical photon beams in a linac
- Beam collimation
- Production of clinical electron beams in a linac
- Dose monitoring system
[edit] RADIOTHERAPY WITH PROTONS, NEUTRONS AND HEAVY IONS
5.6 RADIOTHERAPY WITH PROTONS, NEUTRONS AND HEAVY IONS
External beam radiotherapy is carried out mainly with machines that produce either x-rays or electrons. In a few specialized centers around the world, external beam radiotherapy is also carried out with heavier particles such as:
- Neutrons produced by neutron generators and cyclotrons,
- Protons produced by cyclotrons and synchrotrons, and
- Heavy ions (helium, carbon, nitrogen, argon, neon) produced by synchro-cyclotrons and synchrotrons.
These particles offer some distinct advantages over the standard x-ray and electron modalities, such as:
- Considerably lower oxygen enhancement ratio (OER) for neutrons (see Section 14.10)
- Improved dose-volume histograms (DVHs) for protons and heavy ions (see Section 7.6).
However, equipment for production of protons, neutrons and heavy ions is considerably more expensive than standard radiotherapy equipment, both in capital costs as well as in maintenance and servicing costs, thus precluding a wide-spread use in standard radiotherapy departments. The decreasing costs of proton cyclotrons are likely to result in a wider use of proton beam therapy in the future.
[edit] SHIELDING CONSIDERATIONS
External beam radiotherapy is carried out mainly with three types of equipment that produces either x-rays or electrons:
- X-ray machines (superficial and orthovoltage);
- Teletherapy (cobalt-60) machines;
- Linear accelerators (linacs).
All radiotherapy equipment must be housed in specially shielded treatment rooms to protect the personnel and general public in areas adjacent to the treatment rooms. The treatment rooms must comply not only with structural building codes but also with national and international regulations that deal with shielding requirements to render an installation safe from the radiation protection point-of-view. During the planning stage for a radiotherapy machine installation, a qualified medical physicist determines the required thickness of primary and secondary barriers and provides the information to the architect and structural engineer for incorporation into the architectural drawing for the treatment room.
- Superficial and orthovoltage x-ray therapy rooms are shielded either with ordinary concrete (2.35 g/cm3) or lead. In this energy range, the photoelectric effect is the predominant mode of photon interaction with matter, making the use of lead very efficient for shielding purposes.
- Megavoltage treatment rooms (often referred to as bunkers or vaults because of the large barrier thickness required for shielding) are most commonly shielded with ordinary concrete in the interest of minimizing the construction costs. The Compton effect is the predominant mode of photon interaction with shielding material in this energy range. To conserve space other higher density materials may be used with the required wall thickness inversely proportional to the density of the shielding material. Thus, the use of high density concrete (5 g/cm3) will cut the required thickness of ordinary concrete barrier to approximately one half, however, it will also increase the construction material cost by a factor of 30.
- Shielding issues related to linear accelerator bunkers are discussed in more detail in Section 16.17.
[edit] COBALT-60 TELETHERAPY UNIT VERSUS LINAC
5.8 COBALT-60 TELETHERAPY UNIT VERSUS LINAC
The important features of cobalt-60 teletherapy machines can be summarized as follows:
- Relatively high energy gamma ray emission;
- Relatively long half-life;
- Relatively high specific activity;
- Relatively simple means of production.
Figure 5.8 shows a cobalt-60 teletherapy machine (left side) and a stamp issued by Canada Post commemorating Canada’s role in the development of the cobalt-60 machine (right side).
Linear accelerators (linacs) were developed concurrently by two groups: W.W. Hansen’s group at Stanford University in the U.S.A. and D.D. Fry’s group at Telecommunications Research Establishment in the U.K. Both groups were interested in linacs for research purposes and profited heavily from the microwave radar technology developed during World War II and using 3000 MHz as the design frequency.
The potential for the use of linacs in radiation therapy has become apparent in the 1950s and the first clinical linac was installed in 1950s at the Hammersmith Hospital in London, U.K. During subsequent years, the linac eclipsed the cobalt unit and became the most widely used radiation source in modern radiotherapy with several thousand units in clinical practice around the world today. In contrast to a cobalt-60 unit that provides essentially only one gamma energy of 1.25 MeV, a linac can provide either megavoltage electron or x-ray therapy with a wide range of energies. Figure 5.9 shows a modern dual energy linear accelerator.
In comparison to cobalt-60 machines, linacs have become very complex in design:
- In part because of the multimodality capabilities that have evolved and are available on most modern machines,
- In part because of an increased use of computer logic and microprocessors in the control systems of these machines, and
- In part because of added features, such as high dose rate modes, multileaf collimation, electron arc therapy, and the dynamic motion while the beam is ON of the collimators (dynamic wedge), multileaf collimator leaves (intensity-modulated radiotherapy), gantry and couch.
Despite the clear technological and practical advantages of linacs over cobalt-60 machines, the latter still occupy an important place in radiotherapy armamentarium, mainly because of considerably lower capital, installation and maintenance costs of cobalt-60 machines compared to linacs. In the developing world, the cobalt-60 machines, because of their relatively lower costs, simplicity of design, and ease of operation, are likely to play an important role in cancer therapy for the foreseeable future.
Many modern features of linacs, such as multileaf collimators, dynamic wedges and dynamic operation, could also be installed on modern cobalt-60 machines to allow, at a lower cost, a similar sophistication in treatment as linacs do. It is unfortunate that manufacturers of cobalt-60 units are very slow in reacting to new technological developments in radiotherapy, conceding pre-eminence to linac manufacturers even in jurisdictions that would find it much easier and more practical to run cobalt-60 machines than linacs.
[edit] SIMULATORS AND CT-SIMULATORS
5.9 SIMULATORS AND CT-SIMULATORS
Simulators and CT-simulators are an important component of equipment used in radiation therapy. They cover several crucial steps in the radiotherapeutic process that are not related to the actual dose delivery but are nonetheless very important as they deal with the determination of target location, treatment planning and spatial accuracy in dose delivery. The determination of the target volume that is related to the extent of the disease (See Section 7.2) and its position relative to adjacent critical normal tissues can be achieved with various methods. These range from a simple clinical examination through planar x-ray imaging to the use of complex modern imaging equipment such as CT scanners in conjunction with MR, and PET scanners. Both simulators and CT-simulators incorporate three major systems: (i) mechanical, (ii) x-ray tube, and (iii) imaging equipment.
The major steps in the target localization and field design are:
- Acquisition of patient data set.
- Localization of target and adjacent structures.
- Definition and marking of patient coordinate system.
- Design of treatment fields.
- Transfer of data to treatment planning system.
- Production of image for treatment verification.
The six steps above can be achieved either with a conventional simulator or with a CT-simulator; however, the CT-simulator provides for the more elegant, reliable and practical means to achieve the six steps, in addition to providing reliable external and internal contours and electron density information.
[edit] TRAINING REQUIREMENTS
The increased complexity of radiotherapy equipment demands that equipment be used only by highly trained and competent staff in order to minimize the potential for accidents. A recent report by the International Atomic Energy Agency (IAEA) summarized the lessons learned from accidental exposures in radiation therapy and a report by the American Association of Physicists in Medicine specifically addressed medical accelerator safety considerations.
Of vital importance in purchasing, installation and clinical operation of radiotherapy equipment are the following:
- Preparation of equipment specification document
- Design of treatment room and radiation safety
- Acceptance testing of equipment
- Commissioning of equipment
- Quality assurance program
Items (1), (3) and (4) are addressed in detail in Chapter 10, item (5) in Chapter 12, and item (2) in Chapter 16.
[edit] BIBLIOGRAPHY
AMERICAN ASSOCIATION OF PHYSICISTS IN MEDICINE (AAPM), “Medical accelerator safety considerations”, Task Group 35 Report; Med. Phys. 20, 1261-1275 (1993).
COIA, L., SCHULTHEISS, T.E., HANKS, G.E., “A practical guide to CT-simulation”, Advanced Medical Publishing, Madison, Wisconsin, U.S.A. (1995).
INTERNATIONAL ATOMIC ENERGY AGENCY (IAEA), “Lessons learned from accidental exposures in radiotherapy”, IAEA, Vienna, Austria (2000).
GREENE, D. and WILLIAMS, P.C., “Linear accelerators for radiation therapy”, Institute of Physics Publishing, Bristol, United Kingdom (1997).
INTERNATIONAL ELECTROTECHNICAL COMMISSION (IEC), “Medical electrical equipment: Particular requirements for the safety of electron accelerators in the range 1 MeV to 50 MeV”, Document 60601-2-1, IEC, Geneva, Switzerland (1998).
JOHNS, H.E. and CUNNINGHAM, J.R.,“The physics of radiology”, Thomas, Springfield, Illinois, U.S.A. (1984).
KARZMARK, C.J., NUNAN, C.S. and TANABE, E., “Medical Electron Accelerators”, McGraw-Hill, New York, New York, U.S.A. (1993).
KHAN, F., “The physics of radiation therapy”, Williams and Wilkins, Baltimore, Maryland, U.S.A. (1994).
PODGORSAK, E.B., METCALFE, P., VAN DYK, J., “Medical accelerators”, in "The Modern Technology in Radiation Oncology: A compendium for Medical Physicists and Radiation Oncologists", edited by J. Van Dyk, Chapter 11, pp. 349-435, Medical Physics Publishing, Madison, Wisconsin, U.S.A. (1999).
