The medical linear accelerator (linac) is the primary workhorse for radiation oncology. While the underlying principle of a linac is remarkably simple, implementation of that principle to produce a consistent stable beam requires a precise and sophisticated design. Understanding the basics of that design is essential to ensure patient safety and machine up-time.
In the basic accelerator design, a heated filament boils off a cloud of electrons. These electrons are then accelerated by an electric field applied between the filament (cathode) and a thin metal window (anode). Clinical linacs operating in the MeV region require an Accelerating Waveguide to achieve the required acceleration over a reasonable distance. The electrons then hit a target (where they produce Bremsstrahlung X-rays) or a scattering foil (to spatially distribute the electron beam). Finally, the beam may be further shaped in the treatment head.
Components of a Clinical Linear Accelerator
Couch (Patient Positioning System): The couch supports and positions the patient during treatment. Modern couches facilitate precise patient positioning by moving along the x, y, and z axis. Advanced couches may also include the ability to adjust patient roll, pitch, and yaw.
Electronic Portal Imaging Device (EPID): The electronic portal imaging device forms an image using the MV treatment beam. EPIDs are valuable tools for monitoring patient setup and quality assurance.
Gantry: The linac is mounted on a rotating gantry which treatment from multiple angles.
kV Imaging System: The kilovoltage imaging system consists of a kV X-ray generator and an electronic imaging device. The lower energy of this imaging system improves contrast, especially when used to generate a cone-beam CT.
Stand: The stand connects the gantry to the treatment room floor and contains electronics and other systems required for linac operation.
Accelerating Waveguide: A series of microwave resonance cavities used to accelerate the electron beam to high energies.
Bending Magnet: The bending magnet is a magnetic lens used to focus and position the beam to intercept the target (for photon treatments) or scattering foil (for electron treatments). The angle of bending varies by manufacturer but may be either 90° or 270°. Magnetic focus attempts to be achromatic (does not separate by energy at point of focus).
Circulator: A device in the waveguide that is used to prevent microwave energy from reflecting backwards to the Klystron/Magnetron.
Cooling System: Production of a clinical treatment beam is an energy inefficient process due to losses in microwave generation and acceleration. A water or air cooling system is required to maintain a stable operation temperature necessary for consistent beam energy production.
Electron Gun: An electron gun produces the electrons which are accelerated in the accelerating waveguide. Electron guns consist of a heated filament (~800°C – 1100°C) which “boils off” a cloud of electrons. These electrons are immediately accelerated by a low E field (~40kV).2 Electron guns may be either of the diode or triode types. Diode electron guns consist simply of the heated cathode and an anode which set the accelerating voltage. Triode electron guns add a control grid between the cathode and anode which serves to recollect a portion of the liberated electrons. Thus, the triode design allows for variable beam current by preventing a variable fraction of electrons from reaching the accelerator.
Energy Selector: An energy selector may be placed within the bending magnet array to narrow the allowed electron energy range incident on the target/scattering foil. Typical energy band pass range in of the order of 6% (97%- 103% of desired energy).
Klystron/Magnetron: Klystrons and Magnetrons produce the microwave used to power the accelerating waveguide.
Treatment Head: The treatment head contains components required for beam production and shaping including targets, scattering foils, beam shaping collimators and the optical distance indicator.
Waveguide: The waveguide is a channel directing the microwave power from the Klystron/Magnetron to the Accelerating Waveguide. The waveguide is filled with an insulating gas (typically Sulfur Hexaflouride, SF6) to prevent electrical arcing. Microwave transparent ceramic barriers prevent the SF6 from leaking into the vacuum spaces filling the Klystron/Magnetron and the Accelerating Waveguide.
A Closer Look at Linac Components
Treatment Head Components
Dual Scattering Foils
Linear accelerators typically use a dual scattering foil design to spread the pencil electron beam into a wide uniform beam of clinically useful area.
Primary (Upper) Foil
This foil serves to both initially scatter the beam and as a vacuum/air interface.
Primary foil is comprised of a high Z material such as Tantalum (Z = 73) or Gold (Z = 79). This is because these materials have a high ratio of linear scattering power to collision stopping power. That is, they are desirable because the are efficient at scattering.
Typical thickness is approximately 0.05-0.4mm.
Secondary (Lower) Foil
The lower scattering foil further scatters the beam but is typically made of a low Z material such as aluminum (Z = 13) in order to reduce Bremsstrahlung photon production.
Typical thickness is approximately 1-3mm.
The electron applicator is a collimation device which is affixed to the treatment head for electron therapy deliveries. The applicator consists of several collimators, called scrapers, which provide collimation close to the patient surface.
Electron applicators may have a custom-shaped cutout placed at the level of the lowest scraper to provide the final beam shape. These cutouts may be molded of Cerrobend or cut, often of copper.
Key Point: Electron applicators are necessary because charged electrons scatter regularly in air. Without a collimator placed close to the patient/phantom surface, the electron beam would have a very large penumbra.
Accelerating waveguide consists of an evacuated metal cylinder separated into many sections by washer-shaped dividers. The aperture in the divider allows a channel for the accelerating electrons to pass through. A microwave electric field is induced which accelerates the electrons within a given section of the waveguide.
As the electrons first accelerate from near rest, they initially take on energy from the electric field in the form of additional velocity. Thus the first few cavities in the accelerating waveguide become progressively longer as the electrons take less and less time to traverse the cavity. The divider aperture is also large in the first section of the waveguide and progressively narrows. Narrowing of the apertures causes the electric field to act as a funnel, compressing the electron cloud into bunches. Hence this first section is known as the buncher.
As the electrons approach the speed of light, more and more of their energy gain comes in the form of additional relativistic mass rather than additional velocity. As a consequence, the cavities beyond the buncher are short and uniform in length. Accelerating waveguides may be either of the traveling-wave or standing-wave based on the time dependent variation of their electric field.
- Standing-wave designs are shorter than traveling-wave designs and, as a result, are much more common.
- Clinical accelerator waveguides may be as short as 0.3m for low energy systems or over a meter for high energy systems
- Only about 33% of electrons emitted to the accelerator waveguide are actually accelerated.
- A circulator is used to prevent reflected waves from reaching the magnetron/klystron.
As the name implies, the microwave electric field in a traveling-wave waveguide moves down the structure in a manner similar to a wave on a beach. In this analogy the electrons are accelerated through the waveguide in a manner similar to a surfer. By the far end of the waveguide, not all of the electric field’s energy has been absorbed by the electron and must be absorbed by a resistive material in the last cavity to prevent potentially damaging energy reflection. Because of the nature of the standing wave, only 1 in 4 cavities may be involved in accelerating the electrons at a given time.
The microwave electric field in a standing wave design changes its amplitude with time rather than its position. The standing-wave is set up by allowing the energy remaining at the end of the waveguide to be reflected backward. That is, the standing-wave design actually uses a forward traveling wave and a backward traveling wave to produce a net stationary waveform. Energy reflected back must be prevented by a circulator from reaching, and potentially damaging, the klystron or magnetron.
Changing amplitude of the wave rather than position is advantageous because, although the wavelength remains 4 cavities long, every other cavity may be used to accelerate the electron. Further, since every other chamber is a node, has no electric field at all any time, there is no need for the accelerating electrons to pass through it. Hence these coupling chambers may be removed from the beamline path further reducing the required waveguide length while still coupling the accelerating chambers. The net result of these techniques is a significant reduction in the accelerating waveguide length.
Magnetrons and Klystrons are used in modern linear accelerators to produce the microwaves needed to accelerate charged particles.
Magnetrons are microwave generators which accelerate thermally liberated electrons in spiral trajectories. The laws of electromagnetism dictate that accelerating charges radiate an electromagnetic wave. Inside the magnetron the spiral accelerations of the electrons are tuned, by carefully designed geometry and applied electric and magnetic fields, such that they emit microwaves.
The cylindrical cathode is heated by a filament causing emission of electrons into the drift space. The drift space must be evacuated to very low pressure to prevent burn-out. A static magnetic field is applied perpendicular to the axial slice of the magnetron shown above. An electric field, EP, is pulsed radially between the anode and the cathode. The pulsed electric field propels the electrons from the anode toward the cathode. The magnetic field induces a circular motion in these accelerating electrons such that they move in a spiraling path, S. The periodic acceleration of these electrons induces a further electric field between the poles of the anode, Em, with a microwave frequency. This process is approximately 60% efficient with the rest of the energy needing to be dissipated. Thus, for reasons of thermal and electrical conductivity, magnetrons are typically made of copper.
A klystron is a microwave amplifier used in higher energy (>12MeV) linear accelerators. Although the klystron cannot generate microwaves without input microwaves, some of the klystrons output microwave radiation can be directed back to the klystron allowing it to, once started, produce its own source microwaves.
The electron bunches arrive at the catcher resonance cavity. The catcher cavity has a shape which resonates at the frequency of the arriving bunches. This resonance allows much of the kinetic energy in the electrons to be converted to microwave power which is piped via a wave guide to the accelerating structure. The energy remaining in the electrons trapped by the collector and converted to heat. Large heat buildups require a water cooling system for removal.
- The klystron generates electrons by heating a filament within the cathode. A momentary negative pulse is applied to the cathode, propelling the electron cloud toward the anode and into the buncher cavity. Energy imparted at this stage will be used to increase the amplitude of the microwaves.
- Low-powered microwaves enter the buncher, where an alternating electric field, induced by the input low powered microwave, slows some electrons while accelerating others.
- The electrons enter a drift tube where, because of their different velocities, they are spatially compressed into bunches.
- As each bunch of electrons passes thought the catcher, they bring along a periodic change in electric field intensity (i.e. the field near the bunches is stronger than the field away from the bunches). They are timed in such a way that the catcher rings – like a bell – at its resonant frequency. This signal is collected as a high energy microwave.
- The output signal is larger than the input signal because nearly all of the electrons entering the catcher are decelerated, thus giving up energy to the catcher cavity. This is much more energy than is imparted to the electrons in the buncher because, at that stage, roughly the same number of electrons are slowed as accelerated.
- The energy used for this amplification comes from the initial acceleration between the anode and the cathode.
- Finally, an electron collector absorbs the electrons, turning their remaining kinetic energy into heat which must be dispersed.
- Magnetrons generate microwaves
- Klystons amplify microwaves so require an RF generator input
- Both Klystrons and Magnetrons operate using resonance
- Magnetrons are typically used in accelerators with energies less than 12MeV
- Klystrons are preferred for high energy applications
- Magnetrons are lower cost but also less stable than Klystrons
- Magnetrons are slightly more efficient than Klystrons (60% vs 50%)
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