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Nowadays, there are quite a few medical devices that are used in the treatment of cancer. One of these is a machine called linear accelerator, or LINAC, for short. Now, it has a fancy name but what does a linear accelerator really do? Accelerate something linearly? Well, technically, yes that is what it does. Before looking at its application in medical engineering, let's take a look at its basic principles.
As seen by its name, a LINAC accelerates particles along linear paths instead of circular orbits. The LINAC is based on the principle of resonance. It is set up in the form of a series of metal tubes (they are named drift tubes) and placed in a vacuum vessel. The tubes are arranged one-by-one and connected to alternate terminals of a radio frequency oscillator. This can be seen in Figure 1. Looking at our figure with the fields present, the electric field will accelerate the positive ions from the source towards the first drift tube. Before the ions go through that tube entirely, they will be accelerated again after the exit of the first before entering the second tube if the alternator can make a direction change, and this goes on for the following tubes. That being said, since the velocities of the particles increase as it is accelerated, the phase between the tubes may not be in sync if the drift tubes have the same length. As such, the drift tube length increases as it goes along the path in order to allow the one radio-frequency (RF) alternator to accelerate the particles all the way till the end (Das & Ferbel, 2004).
Figure 1: A linear ion accelerator (Das & Ferbel, 2004)
Employing the same concept, a LINAC is now widely used for external beam radiation therapy treatments. It gives a uniform dose of high energy x-rays to a targeted region of patient's tumour. It is a large machine that is very heavy and uses very sophisticated technologies driven by brilliantly crafted software. However, for certain applications like intraoperative radiation therapy, there is a portable LINAC that can be brought in for that purpose (MedWOW, 2011).
In the case of our LINAC for medical use, they produce high energy electrons or photon beams. For an electron beam, electrons are injected into one of the structure's end and accelerated pass several components before travelling to the other end, leaving the machine. If the treatment desires a photon beam, the electrons are instead made to head towards a target material of high density. This material stops the electrons and causes photons to be produced. The photons also will go through several other components to change them into a therapeutic beam, which leaves the LINAC and is then aimed at the target area of a patient (MedWOW, 2011).
Figure 2 : Typical LINAC components (ERCI, 2011)
LINAC consist of four major components (ERCI, 2011) -
Amplification of the AC power supply is done by this component. The modulator then, rectifies it to DC power, and produces high-voltage DC pulses that are used to power the electron gun and RF power source. High-voltage cables electrically connect the electron gun and RF power source to the modulator, which is located in the gantry, or the gantry supporting stand, or a separate cabinet.
An electron gun
The electron gun injects electrons into the accelerator guide in pulses of the appropriate duration, velocity, and position to capitalize on the acceleration. The electron gun can be attached to the accelerator guide by a removable vacuum flange, which allows easy replacement of the gun. In designs with a permanently attached electron gun, the entire accelerator must be replaced when the gun's filament burns out.
A radio-frequency (RF) power source An accelerator guide
The RF power source (magnetron or a klystron) supplies high-frequency electromagnetic waves (3,000 MHz), which accelerate the electrons injected from the electron gun down the accelerator guide.
An accelerator guide
The electron beam produced by a LINAC can be utilized for treatment usage or produces x-rays by directing it toward a metallic target.
The LINAC has become the standard method of creating photons and electrons needed, generating energy for radiation using external beams to treat cancer. The LINAC will cause electrons to have an increase in velocity as they go through a wave-guide and the particles are then shot at a heavy metal target, creating x-rays that have high energy. These lines of accelerated x-ray radiation are formed and shaped to target a specific tumor or surgery site. It aims to kill the cancer cells but at the same time, tries to limit the exposure to radiation for the healthy tissues surrounding it (MedWOW, 2011).
However, high doses of radiation are very dangerous to a patient's health and could possibly cause more damage than relief. As such, LINAC is always used as a multi-session treatment plan so that we can avoid damage to the healthy surrounding tissues. One of its greatest achievements is being able to target larger brain and body cancers but at the same time cause smaller amounts of damage to its healthy surroundings. Aside from its application in cancer therapy, the LINAC has other medical uses. Radiation from the LINAC can actually be used to restrain the immune systems of patients that have to undergo blood and marrow transplant. It also helps in the suppression of organ transplant rejection and corrects some cardiovascular and neurological disorders (MedWOW, 2011).
Figure 2: Linear Accelerator for Radiation Therapy (MedWOW, 2011)
Figure 2 here shows the non-portable LINAC used in our medical institutions. A collimator shapes the x-rays and they are inserted into the head of the machine. The beam leaves the accelerator by the head which is connected to a vertical arm that is able to move according to target. This setup is known as a gantry that is able to rotate around the patient (MedWOW, 2011). This is so that the target area will receive the radiation through different points of the skin at different times. The patient just needs to lie there, and everything else can be mechanically controlled. Amazing, no?
However, it's clear that such a complicated machine has a lot of safety programs and features to consider. And thus, that is what we will be taking a look at, next.
The Safety Operation Concern of Linear Accelerator
To ensure that the LINAC is operating safely, the radiation therapist has to watch the patient continuously through a closed-circuit television monitor during treatment and a microphone has to be put inside the treatment room so that the patient can speak to the therapist if needed and to allow the radiation therapist to assist the patient during the entire treatment process in the plant room. In addition to that, port films where x-rays are taken with the treatment beam or other imaging tools are checked regularly to make sure that the beam position doesn't vary from the original plan. The radiation therapist must too, turn on the accelerator from outside the treatment room. This is because the accelerator only gives off radiation when it is turned on so as to lower the risk of accidental exposure.(RadiologyInfo.org, 2010). Modern radiation machines have internal checking systems to provide further safety so that the machine will not turn on until all the treatment requirements prescribed by the physician are perfect. When all the checks match and everything is in place, the machine will turn on to provide treatment
Quality control of the LINAC also plays a very important role. There are several systems built into the accelerator so that it won't deliver a higher dose than the radiation oncologist prescribed. Each morning before any patients are treated, the radiation therapist performs checks on the machine to ensure that it is working properly using a piece of equipment called a "tracker" to make sure that the radiation intensity is uniform across the beam. In addition, the radiation physicist makes more detailed weekly and monthly checks of the linear accelerator. All LINAC have a dosimetry system in the treatment head that terminates the radiation at the preset dose. This system incorporates a compartmented, dual-system ionization chamber, which should be sealed against temperature and pressure fluctuations; the performance of this chamber must be checked frequently. Most dosimetry systems can detect asymmetries in the treatment beam and terminate irradiation if the asymmetry exceeds a preset value. Meanwhile, certain LINAC can be repositioned to the beam after an asymmetry is detected. Some systems also have beam-steering circuitry to automatically compensate for changes in the angle or position of the beam caused by gantry or collimator rotation (ERCI, 2011).
All units of LINAC hardware should have fault-detection systems that minimize the probability of an equipment-induced treatment error and able to locate the fault in the system quickly. All active elements in the system are also duplicated to insure that beam may be turned off in the event any component failures. The redundant circuits are designed to enable the computer to make operational tests to verify that they function properly (Buren V. & Dale T., 1969). To mitigate the effects of hardware and software errors on treatment quality, a set of comprehensive safety strategies can be developed. Combination of "watchdog processes and timers" can be used to monitor the application for crash failures. The watchdog processes also control the hardware relays that turn the radiation beam on and off (Sharp G. & Kandasamy N., 2006 ).
In most LINAC, online monitors continuously check the imaging system and the pattern recognition algorithms for tumor-tracking errors that can potentially affect treatment quality and patient safety. In addition, these monitors provide feedback to the human operator, who can decide to manually interrupt the treatment (Sharp & Kandasamy, 2006) In terms of ground isolation, all inputs to the system are via relay contacts, solid state or conventional, rendering complete ground isolation from module to module (Buren V. & Dale T., 1969).
The Safety Storage Concern of Linear Accelerator
There are a few concerns that we need to consider when it comes to the safety of a LINAC. Not only that its safety revolved about the operation of practices, the safety of this machine too, includes the storage safety of the LINAC when it is not in use. This is a critical issue as improper storage might cause faults and damage of the device which will then cause serious consequence such as deviations of desired outcome and even fatalities in worst case scenario. Hence, it is common that the engineering support personnel especially the biomedical engineers available in radiation oncology department often collaborate with radiation oncology, surgery and anesthesiology expert. (AAPM TG72, 2006)
In order to preserve the linear accelerator in well perform condition there are a lot of issues which have to be evaluated. One good example is the criteria of the storage room. The selection criteria of the storage room have to be carefully done as wrong decision of the room design might lead to severe safety concern to both the operation personnel and the patient. The utmost basic criteria of the LINAC storage room is the room structural strength as strong structure will be able to provide the support for the accommodation of LINAC with its entire auxiliary components. Let's assume that the structural strength of the room is insufficient to support the weight of the device; in cases like this, the structure will collapse and the machine would be damaged, leading to injuries and even fatalities on the patient.
Besides the structural strength of the room, room with larger size will be preferable where this gives the advantages to house in all auxiliary components and the entire the machine. This will also provide the full range motion convenient of the gantry. Not only that larger room can provide the convenient of motion but also the radiation safety where larger room will able to reduce the dose of radiation to adjacent areas.
When the discussion goes further to the static LINAC, the mobility of this equipment will required a huge sum of cost budget if we were to transport it to another place. Hence, the storage room for this type of LINAC will be also the operation room where the machine will be operated and stored in the same room. Hence, the location of the room is preferable to be located at the area which is seldom occupied for other purposes in order to reduce the radiation exposure to the surrounding environment.
Meanwhile, the storage standard of LINAC also includes other criteria such as the supply of electricity which provide power to the machine itself and its other electrical devices attached. This is also essential to maintain the vacuum condition in the accelerator when it is not in use. (AAPM TG72, 2006) For static LINAC, the storage room which is also the operation room must also have other logistical item such as appropriate forms, proper location of accelerator instruction manual, proper location of emergency instruction, proper location of fire extinguishers, proper emergency lighting, proper voice and visual communication system between the patient and the radiation operation personnel. This precaution will be able to ensure the safety of patient and operation personnel and protect them in case of emergencies. Moreover, the application of search button might also be applied in the room. When the door is opened, the search button will not allow the linear accelerator to operate and vice versa to the condition of when the door is closed. (Nath R. et. al., 1994)
In order to enhance the safety precaution of storage room, the shielding design should also be put at high importance whereby proper selection of material will be able to shield the leaked radioactive particle that scatter around the environment and stop them from bringing any adverse effect to the personnel close to the room. Hence, the selection of material and structure of the storage room for linear accelerator must always be considered in a thoughtfully way.
There is an example of "maze" structure which is suggested by Nath et. al. in 1994 where LINAC is placed in a room which is connected to a simple "maze" and this will provide the advantage of light weight door to be used which will reduced the opening time to access the patient instead of using the heavy doors which are added with several type of material to block off the radioactive particle.
Next, the design consideration comes to the selection of building material which requires high performance in terms of blockage of the entire leaked radioactive particle from LINAC. As commonly used, lead will be applied as the material for door. However, LINAC is a radiotherapy machine which dealt with high energy neutrons and lead will only possess minor shielding effect on high energy neutrons. Hence, the solution was brought out by using the addition of borated polyethylene in the door material construction to compensate with lead. Secondly, it would be the use of concrete in building the shielding structure where it has high water content and fits well in the economic (Nath R. et.al.,1994)
The Safety Maintenance Concern of a Linear Accelerator
Plenty of international recommendations and protocols involving quality assurance and quality control of radiotherapy equipment exist. (i.e. IEC of 1989, APM of 1994 and IPEM of 1999) (Thwaites D.I. et. al., 2005) It is of utmost importance that the performance of a LINAC machine to remain consistent within accepted tolerances throughout its clinical life. This is because the patient treatments on the machine will be planned and performed based on the performance measurements at acceptance and commissioning. Thus, a continuing quality control programme must be conducted on the machine to ensure its safety and standard of performance.
Often, a planned preventive maintenance (PPM) programme is conducted in accordance with the manufacturer's recommendations. To assist on the values of tolerance and to ensure that standards of the safety programme are consistent, a few institute and associations have recommended a set of requirements for the programme. The three major publications are IEC 977 (International Electrochemical Commission, Report 977), IPEM 81 (Institute of Physics and Engineering on Medicine, Report 81) and AAPM TG 40 (American Association of Physicists in Medicine, Task Group 40)
As compared to AAPM TG 40, IPEM 81 includes tests on some parameters not listed in the AAPM protocols and provides a simpler field size for daily check, a larger tolerance level on daily output constancy but a weekly maintenance with a smaller tolerance level. In addition, it also has a frequency structure of daily, weekly, twice weekly, monthly, half annually and annually. (Thwaites D.I. et.al., 2005)
Stated below would be the criteria needed for the safety maintenance of LINAC based on IPEM 81.
Failure on safety interlocks may cause disastrous consequences on both patients and hospital personnel. Check of the interlocks is conducted on a regular basis to ensure the safety of the machine. Often, a formal written analysis is carried out of the entire possible fault conditions. This eases the person in-charged to decide on which interlocks need to be checked explicitly and to ensure that appropriate records are made of any implicit checks. During normal use, interlocks will trigger from time to time. It is hence, important that procedures must be in place for control of the resetting of such interlocks, so that fault conditions are brought to the attention of the personnel in charged. To perform the test to check on the interlock, the equipment is set into appropriate state to challenge an interlock situation.
"Deadman's switch' is one of the most important movement interlock for a standard linear accelerator. This interlock should be checked at least weekly to ensure that when the switch released one or more of the machine movements is disabled. Meanwhile, a systematic monthly check should also be carried out to ensure that none of the machine movements will move if the appropriate switch is not being pressed. Touch guard is installed in any modern linear accelerator to ensure that activation of any patient collision causes the machine movement to be stop. As well as ensuring that contact with the touch guard stops machine movement, it must be checked that the touch guard operation can be cleared to re-enable machine movement. Touch guard operation should be checked weekly while operation of all other limit switches, applicator and lead tray interlocks are required to check on a monthly basis.
The fundamental safety requirement for the protection of the room is to ensure that the interlocks on the maze entrance are operating perfectly. While these operations are checked daily, secondary access doors such as those designed for the plant room, and emergency-off switches are checked monthly. The test for these may be conducted by testing them as a series circuit by pushing all emergency-off switches in the circuit (which include switches within the room, at the control area and in plant rooms) to their latch-open position and then unlatching each in turn while checking the power at the linear accelerator.
It is essential to check that the accelerator will switch off with the second dosimetry channel. It is necessary to change the calibration of one of the dosimetry channels during this test. For computer controlled accelerators, an impermanent adjustment in the calibration can be made to allow more regular checks of the second channel. Accelerators should also have a high dose rate interlock. This test can be done at least annually as it provides the ultimate protection against serious dosimetry faults.
Couch deflection under load
The IEC standard (1989 a,b) has stated that this should be conducted by engaging a 30 kg weight on the couch at the isocentre and distributed over 1 m with the couch fully retracted over its support. The height of the couch is noted prior to the test. The couch should then be fully extended and a 135 kg load spread out symmetrically on either side of the isocentre to represent a patient (i.e. over a 2 m length). The difference in the height of the couch at the isocentre under these two conditions should be less than 5 mm. The angle of roll of the table top as it is moved laterally from the center to one side should also be measured. The value obtained is to be less than 0.5Â°.
A number of high voltage circuits contained within the linear accelerator and they are likely to include charge storage devices. Hence, tests on electrical safety must be carried out at a regular basis to ensure the performance standard of the machine. Specific emphasis should be placed on the integrity of the earth circuits and of the establishment for discharge of high voltages.
Radiation and Electron Check
The purpose of the test is to check the integrity of the shielding system. The test is performed through dose measurement using film around the gantry head. The leakage dose is specified at a location of 1m from the beam focus. The tolerance for this test is 0.5% in the reference point.
Output calibration check
Maintenance on this aspect should be done weekly to verify that for each available electron energy, a known dose will be delivered to a reference depth in water (normally the depth of maximum dose, dmax) through a standard applicator which is known as phantom. This should be done at a gantry angle of 0Â°, and, typically, a 10 cm x 10 cm applicator would be used as the standard of the energy. It is usual for the linear accelerator to be calibrated to deliver 1.00 Gy at dmax in water phantom. According to IPEMB (1996) protocol, the measured dose should be within 2 per cent of the calibrated dose.
Water-equivalent material is preferred for the measurement phantom although other materials such as polystyrene or polymethylmethacrylate (PMMA) are also acceptable if the measurements can be related to those which would have been made in water. Polystyrene or PMMA phantoms should consist of slabs no thicker than 12 mm in order to minimize charge storage effects.
Based the recommendations from National Radiological Protection Board's Guidance Notes in year1988, electron-beam apparatus should be calibrated at least twice weekly, with a tolerance level of Â±5%. Output constancy checks should be performed at each energy which may be used during the day. Not only that this would check the output, it could also indirectly check if the collimator settings for each applicator are correctly interlocked.
Table 1: Summary of a number of checks for electron beams of linear accelerator (Adapted from IPEM, Report 81)
Tolerance level or functionality
Output constancy check
Output calibration check for each energy
Output constancy at gantry angles of 0Â°,
90Â° and 270Â°
Max/min output £ 1.02