Introduction
Radiation therapy, more commonly known as radiotherapy, is the application of radiation as a mode of treating cancer, according to Dobbs et al. (1999). As a curative therapy, the method promotes survival and can eventually cure. As a palliative medication, the therapy can not offer cure, but it controls the disease or gives the patient some relief from the symptoms of the illness.
Radiotherapy, according to Williams and Thwaites (1993) is used in many different cancer conditions and even in non-malignant cases. Likewise, it functions through several techniques, an example of which is the TBI or total body irradiation which prepares the patient for a transplant of the bone marrow. The type of radiotherapy that will be applied would depend on several factors such as the stage and type of the tumor under medication, its location in the patient’s body, and the health of the patient in general.
Radiotherapy makes use of various equipments to carry out its role in therapeutic medication of patients with tumors 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.
Description
Linear accelerators use the technology of the modern microwave, very much the same as that of the radar. Through the application of this technology, electrons located on a certain part of the linear accelerator are accelerated until a collision with a target made out of metal is reached. The collision in turn scatters the z-rays generated from the targeted metal. A fraction of the x-rays produced is then gathered and formed into a beam that is compatible with the tumor of the patient (Wikipedia, 2008).
During the course of treatment, the linear accelerator’s gantry is rotated around the ill person. The gantry is a special part of the accelerator since it is where the beam comes out of the equipment. The radiation produced can be distributed from any angle to the tumor by the rotation of the gantry. 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 design of a linear accelerator consists of several elements, each with an important function. One of these is the particle source, the design of which is highly dependent on the nature of the accelerated particle. Protons are produced in an ion source, while electrons are formed through a photocathode, cold or hot cathodes, or an RF ion source. Customized ion sources are also needed when accelerating heavier particles. Another important part of a linear accelerator is a high voltage source for particles that will be initially injected (Nave, 2005).
Linear accelerators also require a vacuum chamber similar to a hollow pipe, the length of which is varied with the application. If the x-rays generated will be used for therapeutic purposes and for observations, the required device would be about half to one and a half meters long. If for instance, the device would be used in a synchrotron to serve as an injector, the length of the vacuum chamber might reach ten meters. The vacuum chamber may also be thousands of meters long if it is to be used as a main accelerator primarily for nuclear investigative purposes (Radiologyinfo, 2005).
Cylindrical electrodes which are separated electronically are present within the vacuum chamber. The length of these electrodes would likewise vary with the length of the pipe. The shorter segments are located near the source whereas the longer ones are located nearer the target. The length of the cylindrical electrodes are largely affcted by the mass of the particles being accelerated. For example, electrons are lighter than protons and will therefore need more space or a larger portion of the electrodes, mainly because they accelerate very fast. This can be proved through the context of kinetic energy; which can be considered the same as the energy earned by the electrons as they accelerate through a potential difference commonly in the 5 kilovolts region (Wikipedia, 2008).
A linear accelerator also has a radio frequency source of energy. This is necessary so that the cylindrical electrodes will be energized. If the accelerator is powerful enough, it may employ a single source of radio frequency per electrode present. All the radio frequency sources should be precise in every operation. The operation of the sources should ensure precise, accurate, and suited to the type of particle that will be accelerated. This could result to a maximized power delivery of the device, especially when optimization is already considered (Nave, 2005).
Different targets are used in linear accelerators. 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). Different targets are also used when other nuclei, as well as protons, are being accelerated. Nave (2005) stated that when collision observations of a particle to another is taken into consideration; storage rings might be required for the process, wherein the beam will be directed while particles are maintained by magnetic fields within the ring. From the storage rings, the beams can in turn be extracted to produce collision of particles.
The crowd of particles goes through the length of the tube and is unaffected during the process as the tube serves as a Faraday cage. As this occurs, the frequency of the signal that makes the transport possible as well as the proper spacing between the electrodes are very well optimized, generating a maximum differential of the voltage. This happens as the particle goes travels through the space or gap. This would in turn make the particle accelerate, with energy being imparted to the particle itself as an increase in the velocity of its passage (TRIUMF, 2008).
When the speed of the particle reaches that of light, the increments of increase in the velocity is smaller and energy of the particles at this time will be represented by the increase in their mass. When this takes place in a portion of a linear accelerator, the lengths of the tubular electrodes can be expected to be nearly constant. Additional magnetic or electrostatic lens elements may be included to ensure that the beam remains in the center of the pipe and its electrodes (Wikipedia, 2008).
Aside from the adjustments that can be made to the series of cells, making each of them longer than the previous one in the series to make way for the increased speed of the particle, the relative phase of the speed of the particle and the alectromagnetic wave generated is also matched. Modern linear accelerators strive for the optimization of the acceleration of the particles. This is made possible by forming the waveguide cavity of the equipment in such a way that the phase velocity of the generated electromagnetic waves goes with the speed of the particle at the specific portions where acceleration exists.(Nave, 2005).
Discussion
Linear accelerators are most commonly used for treating patients suffering from cancer through the application of external beam radiation. It could likewise be employed in stereotactic radiosurgery since it can give results similar to those that can be obtained when a gamma knife is used when the tumor is within the patient’s brain. In addition, the equipment can be utilized for IMRT or Intensity-Modulated Radiation Therapy (Radiologyinfo, 2005). The equipment can also be used when the target area is outside the patient’s brain. The linear accelerator can distribute a uniform quanity of x-ray with high energy into the location of the tumor. The generated x-ray radiation can then obliterate the cancer cells without damaging the normal tissues that need to be spared (Nave, 2005). This is made possible by the mechanism of shaping the radiation beams which are in turn positioned at different angles to intersect with each other at the location of the tumor. This would give way to a considerably larger dose being exposed at the desired targeted area as compared to the amount delivered towards the healthy tissues (Radiologyinfo, 2005).
At the Stanford Linear Accelerator Center or SLAC, the largest linear accelerator can be found. It measures 3.2 kilometers in length and can accelerate electrons up to 25 GeVolts. The said machine can also act as an electron-positron collider, delivering a total energy of 50 GeVolts per beam generated (SLAC, 2008).
Linear accelerators generally have a higher capacity of accelerating heavy ions into energies not attainable by the ring-type accelerators. This is due to the magnetic fields that have a limited strength, not powerful enough to maintain the transport and passage of the heavy ions on the curved path of the ring. Moreover, high-power linear accelerators are also being designed for the generation of electrons at varying speeds in accordance with their path. This is of utmost importance because the electrons that travel fast in the path of an arc could eventually be depleted in energy through synchrotron radiation. This could restrict the highest amount of energy that is possible of being imparted to the electrons passing through a specific size of synchrotron.(Wikipedia, 2008).
A standing wave is produced by a tuned-in cavity waveguide as the linear accelerator accelerates electrons in the course of treating a patient. Linear accelerators vary in the shape and length of their waveguides. The designs are especially made to turn the beam into a position and angle appropriate for the patient’s condition. The linear accelerators used in radiotherapy utilize beams between 4 and 25 MeVolts (NCI, 2008). This gives an optimum spectrum of energy, wherein the x-rays or electrons generated can be applied in the treatment of malignant and benign tumors. The reliability, adaptability, and precision of the beams give the linear accelerator a big advantage over other alternative treatment methods. Also, the machine can simply be turned off when not being used, eliminating the need of a heavy shield (Wikipedia, 2008).
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. In addition, pregnant women are even permitted to operate the device (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).
Conclusion
A linear accelerator is a device used for the acceleration of subatomic particles such as electrons. 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 euipment is safe to use due to its internal checking systems and programs.
Bibliography
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(2008). Linear Accelerator. [online]. Web.
Williams, J. R. and Thwaites, D. L. (1993). Radiotherapy Physics. Oxford university Press.