Radiotherapy makes usage of assorted equipments to transport out its function in curative medicine of patients with tumors or other disease. One of these equipments is the additive gas pedal.

Wideroe showed in the twelvemonth 1928 that it is possible to speed up negatrons utilizing a tubing through application of electromotive force to assorted sections of the tubing. As a consequence, the negatrons go throughing the hollow tubing created an electric field which was accelerated in the class of transition. The additive gas pedal was simply based on this thought, bring forthing an drawn-out additive agreement of cells accelerated by wireless frequence ( Nave, 2005 ) .

This paper aims to depict the design and operation of a typical additive gas pedal, every bit good as its methods of x-ray coevals and beam definition. It besides aims to associate the equipment ‘s design to the natural philosophies of its operation, to discourse its safe and effectual bringing of intervention and imagination ; and explicate its suitableness for its map.


During the class of intervention, the additive gas pedal ‘s gauntry is rotated around the patient ; this means that the beam can be distributed from any angle to the tumor. This process is besides assisted by the usage of a movable intervention bed or couch. Use of optical masers is besides employed to guarantee proper place of the patient as the intervention is being delivered ( Radiologyinfo, 2005 ) .

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Electron Gun

The first constituent of the Linear Accelerator it the Electron Gun. This is a tungsten fibril, which is heated and hence emits negatrons by Thermoinic emanation. An electrical field accelerates the negatrons to the beginning of the gas pedal construction. ( 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 negatron gun which produces a flow of negatrons, Bunching cavities that modulate the velocity of negatrons so that they arrive in Bunches at the end product pits, and a Vacuum Chamber. The bunching negatrons excite microwaves in the end product pit ; the microwaves flow into the wave guide and acquire transported to the gas pedal. ( Saldin et al ) . The negatrons are absorbed by an negatron backstop e.g. , chilling H2O which dissipates the energy ( Bomford and Kunkler 2003 )

The Waveguide

The Waveguide is another of import constituent of the Linear Accelerator. The microwaves travel up this hollow Cu pipe. Inside the wave guide is a vacuity maintained by a vacuity pump this is to let negatrons to be propagated without hits with gas molecules. The wave guide besides contains sulphur hexafluoride to forestall arching of negatrons. ( Cherry and Duxbury 1998 ) . There are 2 subdivisions, the bunching pit which allow the negatrons to bunch together and the accelerating pit which spreads the negatrons apart utilizing metal home bases known as Iriss and hence accelerates the negatrons.

The Bending Magnet

The Bending Magnet which can either be electromagnetic or lasting, is to alter the way of the negatrons before they strike the mark. It bends the negatron watercourse so it hits the mark at 90 & A ; deg ; or 270 & A ; deg ; & A ; deg ; ( Weideman, 2002 ) . The lower energy negatrons will go through the bending magnet at an discharge of smaller radius whereas higher energy negatrons travel at an discharge of larger radius. However, all negatrons will all make the mark at the same clip due to the form of the bending magnet, and hence ensuing in homogeneous energies. ( Cherry and Duxbury 1998 ) .

The Target

The 5th constituent is the Target. A mark made up of wolfram is chosen when X raies are to be produced through the acceleration of the negatrons, as discussed by Chin and Regine ( 2008 ) . Fast traveling negatrons hit a high atomic figure stuff ( Tungsten ) , and hence loose enery quikly which consequences in the formation of xrays. To avoid presenting a significantly higher radiation dosage to the Centre of the beam a flattening filter is used ( Cherry and Duxbury 2008 ) .

Beam Flattening Filter

The Beam Flattening filter is conelike shaped where the thickest part is in line with the cardinal axis of the beam. Therefore the filter absrobs the photons in the cardinal axis ensuing in a more unvarying dosage ( Cherry and Duxbury 1998 ) .

Ionisation Chamber

The ionization Chamberss monitors the sum of radiation go forthing the additive gas pedal in the signifier of Monitor Units. ( Cherry and Duxbury 1998 ) . The chamber is round, level and is made up of a figure of sections which besides determine the two-dimensionality and symmetricalness profile of the beam

Beam Collimator System

The 8th constituent is the Beam collimation. This includes the primary and secondary collimation. The primary collimator ensures that the bulk of the xray that leaves the mark moves in a forward way ( Cherry and Duxbury 1998 ) . Whereas the secondary collimator includes the Jaws and Multi foliage collimators ( MLC ‘s ) which define the form and size of the beam. The jaw system includes a upper and lower jaw where the upper jaw is closest to the mark. An assymetric field size can be achived utilizing Assymetric jaws where one jaw moves but non the other. This technique is usually used when half beam blocking is required. MLC ‘s are big Numberss of finger like sub units known as foliage which can be controled independently and hence, permit the field size to be shaped more closley to the PTV ( Cherry and Duxbury 1998 ) .

Many Linear Accelerators include Electrol Portal imagination devices which are devices for takin images with high energy radiation beams. Electronic portal imagination devices is going a more important tool for the confirmation of the patient apparatus ( Vetterli. D 2004 ) .


The gauntry and Collimator can revolve through an angle of 360 & A ; deg ; so that many different angles of the patient can be reached alternatively of the patient altering places taking to inaccurate angles and bring oning patient emphasis.

The negatron gun produces negatrons by a current passing through the fibril and doing it to heat. The negatrons gain heat and move from their orbital shells. A tungsten wire is used because it had a high atomic figure but has a high opposition to electricity and this causes it to heat when a current base on ballss through it. Tungsten is ductile so it can be easy shaped into a spiral, which creates a larger surface country and hence potentially more negatrons can be emitted. Besides the negatron gun is little and efficient at bring forthing negatrons.

The Magnetron and Klystron produce microwaves to transport and speed up negatrons. The bunching pits in the klystron bunch the negatrons together for higher power, the negatrons so travel and cause the end product pit in the klystron to vibrate and bring forth microwaves ( Bomford and Kunkler 2003 ) . A vacuity cylinder is present so that the negatrons do non interact with atoms in the air and hence be deflected cut downing the strength of the microwave ( Saldin et al, 2002 ) . Klystron is the most common beginning of microwave power on Linear gas pedals due to its higher energy microwaves speed uping the negatrons at a higher velocity so a Magnetron. This speeds up the procedure and creates less cluttering within the machine.

The wave guide is a tract for microwaves to go through so the negatrons can make the mark. The microwaves carry bunched negatrons at the entryway of the wave guide and so begins to speed up, transporting the negatrons to the mark ( Cherry and Duxbury 1998 ) . A vacuity in the wave guide is maintained so that negatrons do non interact with gases, this would do no or really small x-ray production and barely any negatrons making the mark. Along the wave guide are 2 types of spirals, concentrating spiral where negatrons are stopped from diverging and, Centring spiral, which insures negatrons are non floating from the cardinal axis. Further more this insures the negatron watercourse reaches the bending magnet in one axis and can be easy dead set ( Wiedemanm 1999 ) Due to the spirals the negatrons do non pelting the wall of the wave guide damaging and have oning out the machine.

The additive gas pedal with a horizontally mounted gas pedal structures requires altering the way of the negatrons before they hit the mark. This is achieved by the rule that any charged atom traveling in a magnetic field will see a force at right angles to its way of travel, this is known as the Bending Magnet. ( Cherry and Duxbury 1998 ) . Interactions between negatrons and mark atoms causes negatrons to be produced on the other side of the mark in a forward way. Electromagnets are most normally used because they can exchange on and off at any needed clip, moreover there power can besides be altered to suit the energy of the negatrons. A tungsten mark is used because it has a high thaw point guaranting the mark habit melt due to the high temperatures and alteration form. Besides because it has a high atomic figure which would give a bigger opportunity of interactions happening. Furthermore Tungsten is a good music director of heat and hence corsets hot increasing the productiveness of the X raies.

Jaws are projections that move in and out of the beam determining the beam of X raies. These jaws block parts of the beam that need to be blocked by traveling the Y axis merely. MLC ‘s farther collimate the beam by traveling independently on the X-axis. Most MLC systems consist of 40 foliages curtailing the transmittal through the foliages to about 1 % of the unfastened field dosage and hence, enlightening less tissue outside the Target Volume. ( Cherry and Duxbury 1998 ) .

The x-rays base on balls through the ionization chamber before they reach the MLC ‘s, the x-rays ionise the gas doing a flow of charge which can be detected and converted into radiation strength. Pressure inside the chamber can alter harmonizing to outside environment.

On intervention images are taken utilizing Electron Portal Imaging devices. These are taken to be compared with bony landmarks and mention images normally taken at simulator, this checks that the patient is in the right place and that the right country is being treated ( Langmack KA 2000 ) .

The usage of additive gas pedals in radiation therapy is considered safe and effectual. In footings of safety, the healer monitors the patient through a telecasting. A mike is besides present in the room where the patient is located to give manner for communicating between the patient and the healer. The movies are besides checked on a regular footing to guarantee that the place or angle of the beam is in conformity with the original readying particular for the patient ( TRIUMF, 2008 ) .

The machine is placed in a room design with concrete and lead walls to incorporate the X raies so that they would non get away. The equipment is turned on utilizing a switch located outside the intervention room to avoid unneeded exposure of the healer to the radiation. This makes the hazard of unwanted exposure peculiarly low, since the additive gas pedal emits radiation merely when turned on ( Nave, 2005 ) .

More modern designs of additive gas pedals are equipped with a constitutional checking plan which aims to add to safety steps. This system ensures foremost that the all the presciptions of the phycisian are considered. When the checking and matching are assured of, this is the lone clip that the machine will turn on for intervention ( ROS, 2008 ) .


All the constituents discussed work together to organize a single x-ray beam for patients single mark volumes to destruct the tumor. The form and size of the tumor is achieved by MLC ‘s and the angle distinction the machine can accomplish. The way of the beam should be considered because normal tissue should non be irradiated and hence the location of the tumor effects the intervention. Overall Due to its design and mechanism of map, the additive gas pedal is normally used in infirmaries for radiation intervention. Its design is dependent on the nature of the accelerated atom and specific map of the machine. The equipment is safe to utilize due to its internal checking systems and plans.

The hereafter of radiation therapy is looking at Intensity Modulated Radiotherapy and Image Guided Radiotherapy as machine has excelled in many ways presenting high quality does distribution to the mark Volume and therefore irradiation less normal tissue.

Refernce List

Bromford C.K. , I.H. Kunkler and B.W. Hancock. ( 2003 ) Walter and Miller ‘s Textbook of Radiotherapy Radiation Physics, Therapy and Oncology ( 6th Edn ) . Churchill Livingstone.

Cherry.P, Duxbury. A 1998. Practical Radiotherapy Physics and Equipment. Oxford University Press.

Chin, L. and Regine, W. ( 2008 ) Principles and Practice of Stereotactic Radiosurgery. USA.

Langmack K.A. 2001 Portal Imaging. The British Journal of Radiography 74 ( 885 ) 789-804. Available at: hypertext transfer protocol: //

Nave, R. ( 2005 ) . Particles. [ on-line ] . Available from: & A ; lt ; hypertext transfer protocol: // [ June 4, 2008 ] .

Radiologyinfo ( 2005 ) . Linac. [ on-line ] . Available from & A ; lt ; hypertext transfer protocol: // & A ; gt ; . [ June 4, 2008 ] .

ROS ( 2008 ) . Linear Accelerator. [ on-line ] . Available from & A ; lt ; hypertext transfer protocol: //www. & A ; gt ; . [ June 4, 2008 ] .

Saldin. E.L. Schneidmiller. E.A. Yurkov. M.V. 2002. klystron instability of a Relativistic Electron Beam in a Bunch Compressor. Nuclear Instruments and Methods in Physics Research Section Angstrom: Accelerator, spectrometers, detectos and Associated Equipment. 490 ( 1-2 ) pp1-8

TRIUMF ( 2008 ) . LINAC. [ on-line ] . Available from & A ; lt ; hypertext transfer protocol: // & A ; gt ; . [ June 4, 2008 ] .

Vetterli D, Riem H, Aebersold D.M, Greiner R.H, Manser P, Cossmann P, Kemmerling L, Born E.J, Mini R. 2004 Introduction of a novel dose salvaging acquisition manner for the Portal Vision aS500 EPID to ease online patient apparatus confirmation. Medical Physicss 31 ( 4 ) 828-31.

Available at: hypertext transfer protocol: //

Wiedemanm. H, 1999. Particle Accelerator Physics 1: Basic Principles and Linear Beam Dynamics. 2nd edition. New York. Springer.


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