Describe A Linear Accelerator Biology Essay

Presents patient with malignant neoplastic disease are treated by radiation, surgery, chemotherapy or with a combination of these options. The radiation therapy intervention unit used to present radiation to cancerous cells and tissues is the additive gas pedal, besides known as linear accelerator.

The additive gas pedal has been defined by Khan F. M. ( 2003 ) as a device that uses high-frequency electromagnetic moving ridges to speed up negatrons, to high energies through a additive tubing. The negatron beam itself can be used for handling superficial toumors, or it can strike a mark to bring forth X raies for handling deep-rooted toumors. The energy used for the radiation therapy intervention of deep located tissues varies from 6-15 MV ( photons ) and the intervention of superficial toumors ( less than 5cm deep ) is between 6-20MeV. ( Khan, 2003 )

The intent of this essay is to depict a additive gas pedal, analyze its constituents in the base and the gauntry of the linear accelerator, and explicate the rules of operation and so discourse why it is best situated to the undertaking for which it was designed. Some advantages and disadvantages of the linear accelerator will besides be included in the treatment portion of the essay.

Main organic structure:

Figure 1: hypertext transfer protocol: //

As you can see from the conventional image above, the major constituents of a linear accelerator are:

Klystron: beginning of microwave power

Electron gun: beginning of negatrons.

Waveguide ( provender and speed uping wave guide ) : microwaves travel through the provender wave guide and so to the speed uping wave guide, where negatrons are accelerated from the negatron gun.

Circulator: a device that prevents microwaves of being reflected back from the gas pedal.

Cooling H2O system: cools the constituents of the linear accelerator.

Bending magnet: “ A bending magnet is used to alter the way of the accelerated negatron beam from horizontal to perpendicular. ” ( Hendee et al, 2005 )

X-ray mark: negatrons hit the mark and bring forth X raies.

Flattening filter: even out the strength of the beam.

Ionization Chamberss: they control the dosage go forthing the caput of the linear accelerator.

Beam collimation: form the radiation beam to a certain size


There are two types of microwave power. The klystron and the magnetron. Magnetrons are used for lower energy linear accelerator. In the high energy additive gas pedal klystron is used. All modern linear accelerators have klystrons. Both klystron and magnetron are particular types of evacuated tubings that are used to bring forth microwave power to speed up negatrons. ( Karzmark and Morton, 1998 ) . “ The tubing requires a low-power radiofrequency oscillator to provide radiofrequency power to the first pit called the buncher. ” ( Hendee et al, 2005 ) In the bunching pit, negatrons produced from the negatron gun, are bunched together to modulate their velocity.

The microwave frequence is 1000s times higher than ordinary wireless moving ridge frequence. For a linear accelerator to work, the microwave frequence needed is 3 billion rhythms per second. ( 3000MHz ) ( Karzmark and Morton, 1998 )

Electron gun:

The negatron gun is portion of the klystron. Here, negatrons are produced and so accelerated to radiofrequency pits. The beginning of negatrons is a straight heated fibril made from wolfram, which will let go of negatrons by thermionic emanation. ( Bomford, 2003 ) Tungsten is used because it is a good thermionic emitter with high atomic figure, supplying a good beginning of negatrons. Klystrons normally have 3-5 pits, used to bunch negatrons together and increase microwave power elaboration.


There are two different types of wave guides used in linear accelerator. The first is the provender wave guide and the 2nd is the speed uping wave guide. The first one connects the klystron to the chief portion of the linear accelerator. Sulphur hexafluoride ( SF6 ) is used in the provender wave guide, to halt the arcing of negatrons, caused by the microwaves that create strong electric Fieldss.

A circulator is placed in the wave guide system, to forestall microwaves being reflected back.

Microwaves travel so to the speed uping wave guide. “ The gas pedal usher of a linear accelerator requires a high vacuity to forestall power loss and electrical arcing, caused by interactions of negatrons with gas molecules. “ ( Cook, 1998 ) The acceleration of negatrons takes topographic point here. The gas pedal wave guide clump and speed up the negatrons with the microwaves. Electrons travel with a high speed to about the velocity of visible radiation. ( 98 % of velocity of visible radiation ) Microwaves travel to the velocity of visible radiation, so flags are used to decelerate them down, so that negatrons can maintain up with the microwaves and be accelerated.

There are two types of gas pedal wave guide: the traveling and the standing wave guide. The difference between the standing and the traveling moving ridge gas pedals is the design of the gas pedal wave guide. In the traveling moving ridge gas pedal, negatrons travel towards the machine and microwaves are absorbed, but in the standing moving ridge gas pedal microwaves are reflected back upon themselves. The standing moving ridge gas pedal is the chief type used in medical additive gas pedals.

Bending magnet:

The negatron beam go forthing from the gas pedal wave guide continues through the bending magnet. This is used to alter the way of the negatron beam, to go out through the intervention caput. The bending magnet deflects the beam in a cringle of 270o, or 90o. The most common grade of flexing magnet used in linear accelerator is the 270o neutral magnet. The of import belongings of this magnet is that the negatrons are brought together despite the difference in energies. They are brought back together to the same place, angle, and beam cross subdivision at the mark, as they were when they left the gas pedal wave guide.

X-rays mark:

The mark is made of tungsten because of its high atomic figure. When negatrons, with their high velocity, hit the mark, made up from a high atomic figure stuff, they undergo rapid slowing. This sudden loss of energy consequences in the formation of X raies and photons. “ To maximize the X-ray beam strength, the transmittal mark will be thick plenty to halt all the negatrons pelting it but thin plenty to understate the ego soaking up. “ ( Bomford, 2003 )

In order to exchange from photon to electron therapy, the mark is removed to let the negatron watercourse to go on into the caput of the machine.

Flattening filter:

In order to do the beam strength unvarying across the field a flattening filter is used. It is normally made of lead, although wolfram, U, steel, aluminum, or a combination has besides been used or suggested. The flattening filter absorbs more photons from the Centre of the beam and fewer from the fringe of the beam.

Ionization Chamberss:

Ionization champers measure the sum of radiation go forthing the machine, quantified in units – Monitor units. Every linear accelerator has two ionization Chamberss for safety grounds. The ionization is a unit of ammunition, level construction, filled with gas, divided into a figure of sections, where each section contains electrodes. When radiation passes through the gas, it is so ionized making a high charged denseness that is controlled by the electrodes. The intervention terminates when the readings from the electrodes have reached a pre-set M.U value.


A primary collimator limits the maximal field size for x-ray therapy ( 40 x 40cm ) . It ensures that x-rays go forthing the mark leave in a forward way in order to understate radiation escape through the caput.

The intervention field size is defined by the secondary collimator. This collimator reduces the transmittal penumbra, since radiation must go through the full collimator thickness. It consists of four midst metal blocks, called jaws. There are two braces of jaws, upper and lower jaws. With the usage of asymmetric jaws, by traveling each jaw separately, asymmetric field sizes can be produced. Half beam barricading can besides be enabling. Different strength forms can be produced, from the standard level beam profile, by traveling during intervention. Multi-leaf collimators are “ finger like projections ” , 1cm midst. These fingers like projections move independently in order to organize the field form more closely to the form of the planning mark volume. By utilizing MLC ‘s, less radiation is given to normal tissues.


From the debut of this essay, the definition of the linear accelerator was given. A linear accelerator is a high electromotive force machine, used for the intervention of cancerous cells and tissues. With the construction of a linear accelerator this is achieved. By radiating cancerous tissues, with day-to-day radiation intervention, cancerous tissues can be destroyed and so replaced by normal tissues.

Every constituent in the linear accelerator is carefully selected for the map for which it is designed. First of wholly, the klystron is used to bring forth microwaves, because it is better than a magnetron that is used for lower energy additive gas pedals. Because additive gas pedals have higher energy beam, klystron are used for production of X raies.

Continuing to the negatron gun, tungsten wire or fibril is used, because of its high thaw point, high atomic figure, and it is malleable. With this characteristics tungsten is a good thermionic emitter, is a good beginning of negatrons and can be easy shaped into coiling, in order to make a larger surface country for the negatrons to be emitted.

In the provender wave guide Sulphur hexafluoride ( SF6 ) is placed with the purpose of halting the arcing of negatrons. At the terminal of the wave guide a circulator is placed so as microwaves can non be reflected back.

As we move on to the speed uping wave guide, and the standing moving ridge gas pedal used in linear accelerator, we can see why the standing moving ridge gas pedal is used. “ The backward going moving ridge interferes with the forward going moving ridge, instead constructively and destructively. The ensuing standing moving ridge has a magnitude of about dual that of the traveling moving ridge, and the peak strength travels along the wave guide at the stage speed of the traveling moving ridge. “ ( Knapp et al, 1968 )

Following the wave guide is the bending magnet. Here we have the neutral magnet where its chief undertaking is to alter the way of the negatrons, but more of import to convey the negatrons together despite the difference in energies.

A flattening filter is used to do the beam even from the cardinal axis to its peripheral borders, to hold homogeneous distribution of the dosage.

Ionization Chamberss are indispensable in a linear accelerator. They monitor the dosage go forthing the intervention caput, so that the additive gas pedal knows when to stop the intervention.

Collimation in a additive gas pedal is necessary. Without the primary and secondary radiation a linear accelerator would n’t be as suited for the undertaking for which it was designed. The field size and form is critical in order to radiate merely the cancerous tissues and non normal tissues. Presents with the progresss of engineering and the usage non merely of MLC ‘s, but besides IMRT and IGRT, survival rates of malignant neoplastic disease have increased.

Last but non least, a additive gas pedal can handle a patient with different energy modes. By taking the mark, the negatron watercourse can go on into the caput of the machine and so be used for the intervention of superficial toumors. By go forthing the mark, photons are produced to handle deep-situated toumors.

Presents most linear accelerators have practical cuneuss, compared to some decennaries ago, where there where merely manual cuneuss. Now radiographers with the usage of practical cuneuss do n’t hold to concern about manual handling, as they do n’t hold to make anything.

I believe that additive gas pedals are non perfect. Linacs are highly expensive to purchase, so hapless states do n’t hold the chance to handle their patients from malignant neoplastic disease. It is difficult to maintain up with the progresss of engineering, as everything is really expensive to purchase, and merely affluent states can purchase the latest equipment. A disadvantage of exchanging from photon to electron modes is that appliers and blocks are used to direct the negatrons and determine the beam. Applicators are really heavy. Blocks are made of lead which may do lead poisoning if they are non handled with attention. The lone disadvantage with MLC ‘s is that when conforming the beam form to the PTV, some radiation will be leaking, even when utilizing the lingua and groove consequence. Last but non least the linear accelerator to work expeditiously they need day-to-day quality confidence trials and care from physicists.


Linear gas pedal is the chief intervention unit used for the intervention of unnatural tissues. With its precise place of the beam, shaped otherwise for every patient separately, it surely is the best machine for the intervention of malignant neoplastic disease.

A linac uses microwaves to speed up negatrons and so hit the mark where X raies are produced. This x-rays are collected and so organize the form of the beam. Nowadays with the rapid progresss of engineering, linear accelerator in a few old ages clip will be even more efficient than today.

Decidedly additive gas pedals are best suited for the undertaking for which they were designed. All the constituents of a additive gas pedal are carefully selected for its demands. From the smallest to the bigger parts of the linear accelerator, are designed for the best result.


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