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Nowadays, there are rather a few medical devices that are used in the intervention of malignant neoplastic disease. One of these is a machine called additive gas pedal, or LINAC, for short. Now, it has a fancy name but what does a additive gas pedal truly make? Accelerate something linearly? Well, technically, yes that is what it does. Before looking at its application in medical technology, allow ‘s take a expression at its basic rules.

As seen by its name, a LINAC accelerates atoms along additive waies alternatively of round orbits. The LINAC is based on the rule of resonance. It is set up in the signifier of a series of metal tubings ( they are named drift tubings ) and placed in a vacuity vas. The tubings are arranged one-by-one and connected to jump terminuss of a wireless frequence oscillator. This can be seen in Figure 1. Looking at our figure with the Fieldss present, the electric field will speed up the positive ions from the beginning towards the first impetus tubing. Before the ions go through that tubing wholly, they will be accelerated once more after the issue of the first before come ining the 2nd tubing if the alternator can do a way alteration, and this goes on for the undermentioned tubing. That being said, since the speeds of the atoms addition as it is accelerated, the stage between the tubings may non be in sync if the impetus tubings have the same length. As such, the impetus tubing length increases as it goes along the way in order to let the one radio-frequency ( RF ) alternator to speed up the atoms all the manner till the terminal ( Das & A ; Ferbel, 2004 ) .

Figure 1: A additive ion gas pedal ( Das & A ; Ferbel, 2004 )

Using the same construct, a LINAC is now widely used for external beam radiation therapy interventions. It gives a unvarying dosage of high energy X raies to a targeted part of patient ‘s tumor. It is a big machine that is really heavy and uses really sophisticated engineerings driven by brightly crafted package. However, for certain applications like intraoperative radiation therapy, there is a portable Linear accelerator that can be brought in for that intent ( MedWOW, 2011 ) .

In the instance of our LINAC for medical usage, they produce high energy negatrons or photon beams. For an negatron beam, negatrons are injected into one of the construction ‘s terminal and accelerated base on balls several constituents before going to the other terminal, go forthing the machine. If the intervention desires a photon beam, the negatrons are alternatively made to head towards a mark stuff of high denseness. This material stops the negatrons and causes photons to be produced. The photons besides will travel through several other constituents to alter them into a curative beam, which leaves the LINAC and is so aimed at the mark country of a patient ( MedWOW, 2011 ) .

Figure 2: Typical LINAC constituents ( ERCI, 2011 )

LINAC consist of four major constituents ( ERCI, 2011 ) –

A modulator

Amplification of the AC power supply is done by this constituent. The modulator so, rectifies it to DC power, and produces high-voltage DC pulses that are used to power the negatron gun and RF power beginning. High-voltage overseas telegrams electrically connect the negatron gun and RF power beginning to the modulator, which is located in the gauntry, or the gauntry back uping base, or a separate cabinet.

An negatron gun

The negatron gun injects negatrons into the gas pedal usher in pulsations of the appropriate continuance, speed, and place to capitalise on the acceleration. The negatron gun can be attached to the gas pedal usher by a removable vacuity rim, which allows easy replacing of the gun. In designs with a for good attached negatron gun, the full gas pedal must be replaced when the gun ‘s fibril burns out.

A radio-frequency ( RF ) power beginning An gas pedal usher

The RF power beginning ( magnetron or a klystron ) supplies high-frequency electromagnetic moving ridges ( 3,000 MHz ) , which accelerate the negatrons injected from the negatron gun down the gas pedal usher.

An gas pedal usher

The negatron beam produced by a LINAC can be utilized for intervention use or produces X raies by directing it toward a metallic mark.

The LINAC has become the standard method of making photons and negatrons needed, bring forthing energy for radiation utilizing external beams to handle malignant neoplastic disease. The LINAC will do negatrons to hold an addition in speed as they go through a wave-guide and the atoms are so shot at a heavy metal mark, making X raies that have high energy. These lines of accelerated x-ray radiation are formed and shaped to aim a specific tumour or surgery site. It aims to kill the malignant neoplastic disease cells but at the same clip, attempts to restrict the exposure to radiation for the healthy tissues environing it ( MedWOW, 2011 ) .

However, high doses of radiation are really unsafe to a patient ‘s wellness and could perchance do more harm than alleviation. As such, LINAC is ever used as a multi-session intervention program so that we can avoid harm to the healthy environing tissues. One of its greatest accomplishments is being able to aim larger encephalon and organic structure malignant neoplastic diseases but at the same clip cause smaller sums of harm to its healthy milieus. Aside from its application in malignant neoplastic disease therapy, the LINAC has other medical utilizations. Radiation from the LINAC can really be used to keep the immune systems of patients that have to undergo blood and marrow graft. It besides helps in the suppression of organ transplant rejection and corrects some cardiovascular and neurological upsets ( MedWOW, 2011 ) .

Figure 2: Linear Accelerator for Radiation Therapy ( MedWOW, 2011 )

Figure 2 here shows the non-portable LINAC used in our medical establishments. A collimator shapes the X raies and they are inserted into the caput of the machine. The beam leaves the gas pedal by the caput which is connected to a perpendicular arm that is able to travel harmonizing to mark. This apparatus is known as a gauntry that is able to revolve around the patient ( MedWOW, 2011 ) . This is so that the mark country will have the radiation through different points of the tegument at different times. The patient merely needs to lie at that place, and everything else can be automatically controlled. Amazing, no?

However, it ‘s clear that such a complicated machine has a batch of safety plans and characteristics to see. And therefore, that is what we will be taking a expression at, following.

The Safety Operation Concern of Linear Accelerator

To guarantee that the LINAC is runing safely, the radiation healer has to watch the patient continuously through a closed-circuit telecasting proctor during intervention and a mike has to be put inside the intervention room so that the patient can talk to the healer if needed and to let the radiation healer to help the patient during the full intervention procedure in the works room. In add-on to that, port movies where X raies are taken with the intervention beam or other imagination tools are checked on a regular basis to do certain that the beam place does n’t change from the original program. The radiation healer must excessively, turn on the gas pedal from outside the intervention room. This is because the gas pedal merely gives off radiation when it is turned on so as to take down the hazard of inadvertent exposure. (, 2010 ) . Modern radiation machines have internal look intoing systems to supply farther safety so that the machine will non turn on until all the intervention demands prescribed by the doctor are perfect. When all the cheques lucifer and everything is in topographic point, the machine will turn on to supply intervention

Quality control of the LINAC besides plays a really of import function. There are several systems built into the gas pedal so that it wo n’t present a higher dosage than the radiation oncologist prescribed. Each forenoon before any patients are treated, the radiation healer performs cheques on the machine to guarantee that it is working decently utilizing a piece of equipment called a “ tracker ” to do certain that the radiation strength is unvarying across the beam. In add-on, the radiation physicist makes more elaborate hebdomadal and monthly cheques of the additive gas pedal. All LINAC have a dosimetry system in the intervention caput that terminates the radiation at the preset dosage. This system incorporates a compartmented, dual-system ionisation chamber, which should be sealed against temperature and force per unit area fluctuations ; the public presentation of this chamber must be checked often. Most dosimetry systems can observe dissymmetries in the intervention beam and end irradiation if the dissymmetry exceeds a preset value. Meanwhile, certain LINAC can be repositioned to the beam after an dissymmetry is detected. Some systems besides have beam-steering circuitry to automatically counterbalance for alterations in the angle or place of the beam caused by gauntry or collimator rotary motion ( ERCI, 2011 ) .

All units of LINAC hardware should hold fault-detection systems that minimize the chance of an equipment-induced intervention mistake and able to turn up the mistake in the system rapidly. All active elements in the system are besides duplicated to see that beam may be turned off in the event any constituent failures. The excess circuits are designed to enable the computing machine to do operational trials to verify that they function decently ( Buren V. & A ; Dale T. , 1969 ) . To extenuate the effects of hardware and package mistakes on intervention quality, a set of comprehensive safety schemes can be developed. Combination of “ watchdog procedures and timers ” can be used to supervise the application for clang failures. The watchdog processes besides control the hardware relays that turn the radiation beam on and off ( Sharp G. & A ; Kandasamy N. , 2006 ) .

In most LINAC, on-line proctors continuously check the imagination system and the form acknowledgment algorithms for tumor-tracking mistakes that can potentially impact intervention quality and patient safety. In add-on, these proctors provide feedback to the homo operator, who can make up one’s mind to manually disrupt the intervention ( Sharp & A ; Kandasamy, 2006 ) In footings of land isolation, all inputs to the system are via relay contacts, solid province or conventional, rendering complete land isolation from faculty to module ( Buren V. & A ; Dale T. , 1969 ) .

The Safety Storage Concern of Linear Accelerator

There are a few concerns that we need to see when it comes to the safety of a LINAC. Not merely that its safety revolved about the operation of patterns, the safety of this machine excessively, includes the storage safety of the LINAC when it is non in usage. This is a critical issue as improper storage might do mistakes and harm of the device which will so do serious effect such as divergences of coveted result and even human deaths in worst instance scenario. Hence, it is common that the technology support forces particularly the biomedical applied scientists available in radiation oncology section frequently collaborate with radiation oncology, surgery and anesthesiology expert. ( AAPM TG72, 2006 )

In order to continue the additive gas pedal in good perform status there are a batch of issues which have to be evaluated. One good illustration is the standards of the storage room. The choice standards of the storage room have to be carefully done as incorrect determination of the room design might take to severe safety concern to both the operation forces and the patient. The extreme basic standards of the LINAC storage room is the room structural strength as strong construction will be able to supply the support for the adjustment of LINAC with its full subsidiary constituents. Let ‘s presume that the structural strength of the room is deficient to back up the weight of the device ; in instances like this, the construction will fall in and the machine would be damaged, taking to hurts and even human deaths on the patient.

Besides the structural strength of the room, room with larger size will be preferred where this gives the advantages to house in all subsidiary constituents and the full the machine. This will besides supply the full scope gesture convenient of the gauntry. Not merely that larger room can supply the convenient of gesture but besides the radiation safety where larger room will able to cut down the dosage of radiation to next countries.

When the treatment goes farther to the inactive LINAC, the mobility of this equipment will necessitate a immense amount of cost budget if we were to transport it to another topographic point. Hence, the storage room for this type of LINAC will be besides the operation room where the machine will be operated and stored in the same room. Hence, the location of the room is preferred to be located at the country which is rarely occupied for other intents in order to cut down the radiation exposure to the environing environment.

Meanwhile, the storage criterion of LINAC besides includes other standards such as the supply of electricity which provide power to the machine itself and its other electrical devices attached. This is besides indispensable to keep the vacuity status in the gas pedal when it is non in usage. ( AAPM TG72, 2006 ) For inactive LINAC, the storage room which is besides the operation room must besides hold other logistical point such as appropriate signifiers, proper location of gas pedal direction manual, proper location of exigency direction, proper location of fire asphyxiators, proper exigency lighting, proper voice and ocular communicating system between the patient and the radiation operation forces. This safeguard will be able to guarantee the safety of patient and operation forces and protect them in instance of exigencies. Furthermore, the application of hunt button might besides be applied in the room. When the door is opened, the hunt button will non let the additive gas pedal to run and frailty versa to the status of when the door is closed. ( Nath R. et. al. , 1994 )

In order to heighten the safety safeguard of storage room, the screening design should besides be put at high importance whereby proper choice of stuff will be able to screen the leaked radioactive atom that spread around the environment and halt them from conveying any inauspicious consequence to the forces near to the room. Hence, the choice of stuff and construction of the storage room for additive gas pedal must ever be considered in a thoughtfully manner.

There is an illustration of “ maze ” construction which is suggested by Nath et. Al. in 1994 where LINAC is placed in a room which is connected to a simple “ labyrinth ” and this will supply the advantage of light weight door to be used which will cut down the gap clip to entree the patient alternatively of utilizing the heavy doors which are added with several type of stuff to barricade off the radioactive atom.

Next, the design consideration comes to the choice of constructing stuff which requires high public presentation in footings of obstruction of the full leaked radioactive atom from LINAC. As normally used, take will be applied as the stuff for door. However, LINAC is a radiation therapy machine which dealt with high energy neutrons and lead will merely possess minor screening consequence on high energy neutrons. Hence, the solution was brought out by utilizing the add-on of borated polythene in the door stuff building to counterbalance with lead. Second, it would be the usage of concrete in constructing the shielding construction where it has high H2O content and tantrums good in the economic ( Nath R.,1994 )

The Safety Maintenance Concern of a Linear Accelerator

Plenty of international recommendations and protocols affecting quality confidence 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 extreme importance that the public presentation of a LINAC machine to stay consistent within recognized tolerances throughout its clinical life. This is because the patient interventions on the machine will be planned and performed based on the public presentation measurings at credence and commissioning. Therefore, a go oning quality control programme must be conducted on the machine to guarantee its safety and criterion of public presentation.

Frequently, a planned preventative care ( PPM ) programme is conducted in conformity with the maker ‘s recommendations. To help on the values of tolerance and to guarantee that criterions of the safety programme are consistent, a few institute and associations have recommended a set of demands 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 trials on some parametric quantities non listed in the AAPM protocols and provides a simpler field size for day-to-day cheque, a larger tolerance degree on day-to-day end product stability but a hebdomadal care with a smaller tolerance degree. In add-on, it besides has a frequence construction of day-to-day, hebdomadal, twice hebdomadally, monthly, half yearly and yearly. ( Thwaites D.I. , 2005 )

Stated below would be the standards needed for the safety care of LINAC based on IPEM 81.

Safety Interlocks

Motion Interlocks

Failure on safety interlocks may do black effects on both patients and hospital forces. Check of the interlocks is conducted on a regular footing to guarantee the safety of the machine. Often, a formal written analysis is carried out of the full possible mistake conditions. This eases the individual in-charged to make up one’s mind on which interlocks need to be checked explicitly and to guarantee that appropriate records are made of any inexplicit cheques. During normal usage, interlocks will trip from clip to clip. It is therefore, of import that processs must be in topographic point for control of the resetting of such interlocks, so that mistake conditions are brought to the attending of the forces in charged. To execute the trial to look into on the interlock, the equipment is set into appropriate province to dispute an interlock state of affairs.

“ Deadman ‘s switch ‘ is one of the most of import motion interlock for a standard additive gas pedal. This interlock should be checked at least hebdomadal to guarantee that when the switch released one or more of the machine motions is handicapped. Meanwhile, a systematic monthly cheque should besides be carried out to guarantee that none of the machine motions will travel if the appropriate switch is non being pressed. Touch guard is installed in any modern additive gas pedal to guarantee that activation of any patient hit causes the machine motion to be stop. Equally good as guaranting that contact with the touch guard stops machine motion, it must be checked that the touch guard operation can be cleared to re-enable machine motion. Touch guard operation should be checked hebdomadal while operation of all other bound switches, applier and lead tray interlocks are required to look into on a monthly footing.

Room Protection

The cardinal safety demand for the protection of the room is to guarantee that the interlocks on the labyrinth entryway are runing absolutely. While these operations are checked daily, secondary entree doors such as those designed for the works room, and emergency-off switches are checked monthly. The trial for these may be conducted by proving them as a series circuit by forcing all emergency-off switches in the circuit ( which include switches within the room, at the control country and in works suites ) to their latch-open place and so unlatching each in bend while look intoing the power at the additive gas pedal.

Dosimetry interlocks

It is indispensable to look into that the gas pedal will exchange off with the 2nd dosimetry channel. It is necessary to alter the standardization of one of the dosimetry channels during this trial. For computing machine controlled gas pedals, an temporary accommodation in the standardization can be made to let more regular cheques of the 2nd channel. Accelerators should besides hold a high dosage rate interlock. This trial can be done at least yearly as it provides the ultimate protection against serious dosimetry mistakes.

Mechanical Integrity

Couch warp under burden

The IEC criterion ( 1989 a, B ) has stated that this should be conducted by prosecuting a 30 kilogram weight on the sofa at the isocentre and distributed over 1 m with the couch to the full retracted over its support. The tallness of the sofa is noted prior to the trial. The sofa should so be to the full extended and a 135 kilogram burden spread out symmetrically on either side of the isocentre to stand for a patient ( i.e. over a 2 m length ) . The difference in the tallness of the sofa at the isocentre under these two conditions should be less than 5 millimeter. The angle of axial rotation of the table top as it is moved laterally from the centre to one side should besides be measured. The value obtained is to be less than 0.5A° .

Electrical Safety

A figure of high electromotive force circuits contained within the additive gas pedal and they are likely to include charge storage devices. Hence, trials on electrical safety must be carried out at a regular footing to guarantee the public presentation criterion of the machine. Specific accent should be placed on the unity of the Earth circuits and of the constitution for discharge of high electromotive forces.

Radiation and Electron Check

Radiation Escape

The intent of the trial is to look into the unity of the shielding system. The trial is performed through dose measuring utilizing movie around the gauntry caput. The escape dosage is specified at a location of 1m from the beam focal point. The tolerance for this trial is 0.5 % in the mention point.

Output standardization cheque

Care on this facet should be done hebdomadal to verify that for each available negatron energy, a known dosage will be delivered to a mention deepness in H2O ( usually the deepness of upper limit dosage, dmax ) through a criterion applier which is known as apparition. This should be done at a gauntry angle of 0A° , and, typically, a 10 centimeter x 10 cm applier would be used as the criterion of the energy. It is usual for the additive gas pedal to be calibrated to present 1.00 Gy at dmax in H2O apparition. Harmonizing to IPEMB ( 1996 ) protocol, the measured dosage should be within 2 per cent of the calibrated dosage.

Water-equivalent stuff is preferred for the measuring apparition although other stuffs such as polystyrene or polymethylmethacrylate ( PMMA ) are besides acceptable if the measurings can be related to those which would hold been made in H2O. Polystyrene or PMMA apparitions should dwell of slabs no thicker than 12 millimeter in order to minimise charge storage effects.

Constancy cheque

Based the recommendations from National Radiological Protection Board ‘s Guidance Notes in year1988, electron-beam setup should be calibrated at least twice hebdomadal, with a tolerance degree of A±5 % . Output stability cheques should be performed at each energy which may be used during the twenty-four hours. Not merely that this would look into the end product, it could besides indirectly look into if the collimator scenes for each applier are right interlocked.

Table 1: Summary of a figure of cheques for electron beams of additive gas pedal ( Adapted from IPEM, Report 81 )



Tolerance degree or functionality

Output stability cheque


A±5 %

Output standardization cheque for each energy


A±2 %

Applicator interlocks



Output stability at gauntry angles of 0A° ,

90A° and 270A°


Max/min end product ? 1.02

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