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Radiotherapy makes use of various equipments to carry out its role in therapeutic medication of patients with tumours or other disease. One of these equipments is the linear accelerator.
Wideroe showed in the year 1928 that it is possible to accelerate electrons using a tube through application of voltage to various segments of the tube. As a result, the electrons passing the hollow tube created an electric field which was accelerated in the course of passage. The linear accelerator was merely based on this idea, generating an extended linear arrangement of cells accelerated by radio frequency (Nave, 2005).
This paper aims to describe the design and operation of a typical linear accelerator, as well as its methods of x-ray generation and beam definition. It also aims to relate the equipment's design to the physics of its operation, to discuss its safe and effective delivery of treatment and imagery; and explain its suitability for its function.
During the course of treatment, the linear accelerator's gantry is rotated around the patient; this means that the beam can be distributed from any angle to the tumour. This procedure is also assisted by the use of a moveable treatment bed or couch. Use of lasers is also employed to ensure proper position of the patient as the treatment is being delivered (Radiologyinfo, 2005).
The first component of the Linear Accelerator it the Electron Gun. This is a tungsten filament, which is heated and therefore emits electrons by Thermoinic emission. An electrical field accelerates the electrons to the beginning of the accelerator structure. (Bomford and Kunkler 2003)
Klystron and Magnetron
The radiofrequency generator known as the Klystron (used for high energy) or the Magnetron (for low energy), (Cherry and Duxbury 1998).
It is composed of an electron gun which produces a flow of electrons, Bunching cavities that regulate the speed of electrons so that they arrive in bunches at the output cavities, and a Vacuum Chamber. The bunching electrons excite microwaves in the output cavity; the microwaves flow into the waveguide and get transported to the accelerator. (Saldin et al). The electrons are absorbed by an electron catcher e.g., cooling water which dissipates the energy (Bomford and Kunkler 2003)
The Waveguide is another important component of the Linear Accelerator. The microwaves travel up this hollow copper pipe. Inside the waveguide is a vacuum maintained by a vacuum pump this is to allow electrons to be propagated without collisions with gas molecules. The waveguide also contains sulphur hexafluoride to prevent arching of electrons. (Cherry and Duxbury 1998). There are 2 sections, the bunching cavity which allow the electrons to bunch together and the accelerating cavity which spreads the electrons apart using metal plates known as Irises and therefore accelerates the electrons.
The Bending Magnet
The Bending Magnet which can either be electromagnetic or permanent, is to change the direction of the electrons before they strike the target. It bends the electron stream so it hits the target at 90 ° or 270°°(Weideman, 2002). The lower energy electrons will travel through the bending magnet at an arc of smaller radius whereas higher energy electrons travel at an arc of larger radius. However, all electrons will all reach the target at the same time due to the shape of the bending magnet, and therefore resulting in homogenous energies. (Cherry and Duxbury 1998).
The fifth component is the Target. A target made up of tungsten is chosen when x-rays are to be produced through the acceleration of the electrons, as discussed by Chin and Regine (2008). Fast moving electrons hit a high atomic number material (Tungsten), and therefore loose enery quikly which results in the formation of xrays. To avoid delivering a significantly higher radiation dose to the centre of the beam a flattening filter is used (Cherry and Duxbury 2008).
Beam Flattening Filter
The Beam Flattening filter is conical shaped where the thickest portion is in line with the central axis of the beam. Therefore the filter absrobs the photons in the central axis resulting in a more uniform dose (Cherry and Duxbury 1998).
The ionisation chambers monitors the amount of radiation leaving the linear accelerator in the form of Monitor Units. (Cherry and Duxbury 1998). The chamber is round, flat and is made up of a number of segments which also determine the flatness and symmetry profile of the beam
Beam Collimator System
The eighth component is the Beam collimation. This includes the primary and secondary collimation. The primary collimator ensures that the majority of the xray that leaves the target moves in a forward direction (Cherry and Duxbury 1998). Whereas the secondary collimator includes the Jaws and Multi leaf collimators (MLC's) which define the shape and size of the beam . The jaw system includes a upper and lower jaw where the upper jaw is closest to the target. An assymetric field size can be achived using Assymetric jaws where one jaw moves but not the other. This technique is normally used when half beam blocking is required. MLC's are large numbers of finger like sub units known as leafs which can be controled independently and therefore, permit the field size to be shaped more closley to the PTV (Cherry and Duxbury 1998).
Many Linear Accelerators include Electrol Portal imaging devices which are devices for takin images with high energy radiation beams. Electronic portal imaging devices is becoming a more crucial tool for the verification of the patient setup (Vetterli. D 2004).
The gantry and Collimator can rotate through an angle of 360° so that many different angles of the patient can be reached instead of the patient changing positions leading to inaccurate angles and inducing patient stress.
The electron gun produces electrons by a current passing through the filament and causing it to heat. The electrons gain heat and move from their orbital shells. A tungsten wire is used because it had a high atomic number but has a high resistance to electricity and this causes it to heat when a current passes through it. Tungsten is ductile so it can be easily shaped into a spiral, which creates a larger surface area and therefore potentially more electrons can be emitted. Also the electron gun is small and efficient at producing electrons.
The Magnetron and Klystron produce microwaves to carry and accelerate electrons. The bunching cavities in the klystron bunch the electrons together for higher power, the electrons then travel and cause the output cavity in the klystron to vibrate and produce microwaves (Bomford and Kunkler 2003). A vacuum cylinder is present so that the electrons do not interact with particles in the air and therefore be deflected reducing the strength of the microwave (Saldin et al, 2002). Klystron is the most common source of microwave power on Linear accelerators due to its higher energy microwaves accelerating the electrons at a higher speed then a Magnetron. This speeds up the process and creates less cluttering within the machine.
The waveguide is a pathway for microwaves to travel through so the electrons can reach the target. The microwaves carry bunched electrons at the entrance of the waveguide and then begins to accelerate, carrying the electrons to the target (Cherry and Duxbury 1998). A vacuum in the waveguide is maintained so that electrons do not interact with gases, this would cause no or very little x-ray production and hardly any electrons reaching the target. Along the waveguide are 2 types of coils, focussing coil where electrons are stopped from diverging and, Centring coil, which insures electrons are not drifting from the central axis. Further more this insures the electron stream reaches the bending magnet in one axis and can be easily bent (Wiedemanm 1999) Due to the coils the electrons do not bombarding the wall of the waveguide damaging and wearing out the machine.
The linear accelerator with a horizontally mounted accelerator structures requires changing the direction of the electrons before they hit the target. This is achieved by the principle that any charged particle moving in a magnetic field will experience a force at right angles to its direction of travel, this is known as the Bending Magnet. (Cherry and Duxbury 1998). Interactions between electrons and target atoms causes electrons to be produced on the other side of the target in a forward direction. Electromagnets are most commonly used because they can switch on and off at any required time, furthermore there power can also be altered to accommodate the energy of the electrons. A tungsten target is used because it has a high melting point ensuring the target wont melt due to the high temperatures and change shape. Also because it has a high atomic number which would give a bigger chance of interactions occurring. Furthermore Tungsten is a good conductor of heat and therefore stays hot increasing the productivity of the x-rays.
Jaws are projections that move in and out of the beam shaping the beam of x-rays. These jaws block parts of the beam that need to be blocked by moving the Y axis only. MLC's further collimate the beam by moving independently on the X-axis. Most MLC systems consist of 40 leaves restricting the transmission through the leaves to approximately 1% of the open field dose and therefore, irradiating less tissue outside the Target Volume. (Cherry and Duxbury 1998).
The x-rays pass through the ionisation chamber before they reach the MLC's , the x-rays ionise the gas causing a flow of charge which can be detected and converted into radiation intensity. Pressure inside the chamber can change according to outside environment.
On treatment images are taken using Electron Portal Imaging devices. These are taken to be compared with bony landmarks and reference images usually taken at simulator, this checks that the patient is in the right position and that the right area is being treated (Langmack KA 2000).
The use of linear accelerators in radiotherapy is considered safe and effective. In terms of safety, the therapist monitors the patient through a television. A microphone is also present in the room where the patient is located to give way for communication between the patient and the therapist. The films are also checked on a regular basis to ensure that the position or angle of the beam is in accordance with the original preparation specific for the patient (TRIUMF, 2008).
The machine is placed in a room design with concrete and lead walls to contain the x-rays so that they would not escape. The equipment is turned on using a switch located outside the treatment room to avoid unnecessary exposure of the therapist to the radiation. This makes the risk of unwanted exposure particularly low, since the linear accelerator emits radiation only when turned on (Nave, 2005).
More modern designs of linear accelerators are equipped with a built-in checking program which aims to add to safety measures. This system ensures first that the all the presciptions of the phycisian are considered. When the checking and matching are assured of, this is the only time that the machine will turn on for treatment (ROS, 2008).
All the components discussed work together to form a individual x-ray beam for patients individual target volumes to destroy the tumour. The shape and size of the tumour is achieved by MLC's and the angle differentiation the machine can achieve. The direction of the beam should be considered because normal tissue should not be irradiated and therefore the location of the tumour effects the treatment. Overall Due to its design and mechanism of function, the linear accelerator is commonly used in hospitals for radiation treatment. Its design is dependent on the nature of the accelerated particle and specific function of the machine. The equipment is safe to use due to its internal checking systems and programs.
The future of radiotherapy is looking at Intensity Modulated Radiotherapy and Image Guided Radiotherapy as machine has excelled in many ways delivering high quality does distribution to the target Volume and therefore irradiation less normal tissue.
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