Analysis OF ANY Linear Accelerator

Nowadays patient with tumors are cared for by radiation, surgery, chemotherapy or with a combination of these options. The radiotherapy treatment unit used to provide radiation to cancerous cells and cells is the linear accelerator, also known as linac.

The linear accelerator has been defined by Khan F. M. (2003) as a tool that uses high-frequency electromagnetic waves to accelerate electrons, to high energies via a linear tube. The electron beam itself can be utilized for treating superficial toumors, or it can punch a target to create x-rays for dealing with deep-seated toumors. The vitality used for the radiotherapy treatment of profound situated tissues varies from 6-15 MV (photons) and the treating superficial toumors (significantly less than 5cm profound) is between 6-20MeV. (Khan, 2003)

The purpose of this article is to spell it out a linear accelerator, analyse its components in the stand and the gantry of the linac, and make clear the guidelines of operation and then discuss why it's best situated to the task for which it was designed. Some benefits and drawbacks of the linac may also be included in the discussion area of the essay.

Main body:

Figure 1:http://www. cerebromente. org. br/

As you can see from the schematic picture above, the major the different parts of a linac are:

Klystron: way to obtain microwave power

Electron gun: source of electrons.

Waveguide (give food to and accelerating waveguide): microwaves travel through the give food to waveguide and then to the accelerating waveguide, where electrons are accelerated from the electron gun.

Circulator: a tool that inhibits microwaves of being reflected back again from the accelerator.

Cooling water system: cools the the different parts of the linac.

Bending magnet: "A bending magnet is employed to change the path of the accelerated electron beam from horizontal to vertical. " (Hendee et al, 2005)

X-ray concentrate on: electrons struck the prospective and produce x-rays.

Flattening filtration system: even out the depth of the beam.

Ionisation chambers: they control the dose leaving the top of the linac.

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 linacs. Inside the high energy linear accelerator klystron can be used. All modern linacs have klystrons. Both klystron and magnetron are special types of evacuated pipes that are being used to produce microwave power to speed up electrons. (Karzmark and Morton, 1998). "The pipe takes a low-power radiofrequency oscillator to supply radiofrequency capacity to the first cavity called the buncher. " (Hendee et al, 2005) Inside the bunching cavity, electrons created from the electron gun, are bunched mutually to regulate their velocity.

The microwave occurrence is thousands times greater than ordinary radio influx frequency. For your linac to work, the microwave frequency needed is 3 billion cycles per second. (3000MHz) (Karzmark and Morton, 1998)

Electron gun:

The electron weapon is part of the klystron. Here, electrons are produced and then accelerated to radiofrequency cavities. The foundation of electrons is a immediately heated filament created from tungsten, that may release electrons by thermionic emission. (Bomford, 2003) Tungsten is utilized because it is an excellent thermionic emitter with high atomic number, providing a good way to obtain electrons. Klystrons will often have 3-5 cavities, used to bunch electrons collectively and increase microwave power amplification.


There are two different kinds of waveguides found in linacs. The foremost is the feed waveguide and the second reason is the accelerating waveguide. The first one connects the klystron to the primary area of the linac. Sulphur hexafluoride (SF6) can be used in the feed waveguide, to avoid the arcing of electrons, triggered by the microwaves that induce strong electric areas.

A circulator is positioned in the waveguide system, to avoid microwaves being shown back.

Microwaves travel then to the accelerating waveguide. "The accelerator guide of a linac requires a high vacuum to avoid power loss and electrical power arcing, induced by interactions of electrons with gas substances. "(Cook, 1998)The acceleration of electrons takes place here. The accelerator waveguide number and accelerate the electrons with the microwaves. Electrons travel with a higher velocity to almost the acceleration of light. (98% of acceleration of light) Microwaves happen to be the velocity of light, so irises are being used to slow them down, so that electrons will keep up with the microwaves and be accelerated.

There are two types of accelerator waveguide: the visiting and the status waveguide. The difference between your ranking and the traveling wave accelerators is the look of the accelerator waveguide. Inside the travelling wave accelerator, electrons travel towards the device and microwaves are consumed, however in the standing influx accelerator microwaves are reflected back upon themselves. The located influx accelerator is the primary type found in medical linear accelerators.

Bending magnet:

The electron beam giving from the accelerator waveguide goes on through the twisting magnet. This can be used to change the direction of the electron beam, to leave through the treatment head. The bending magnet deflects the beam in a loop of 270o, or 90o. The most common degree of twisting magnet used in linacs is the 270o achromatic magnet. The top property of this magnet is that the electrons are brought together despite the difference in energies. They are simply brought back jointly to the same position, angle, and beam cross section at the target, as these were when they kept the accelerator waveguide.

X-rays aim for:

The target is constructed of tungsten because of its high atomic amount. When electrons, using their high speed, hit the target, made from a high atomic number material, they undergo quick deceleration. This rapid lack of energy ends in the formation of x-rays and photons. "To increase the X-ray beam intensity, the transmission aim for will be solid enough to avoid all the electrons bombarding it but skinny enough to minimise the self applied absorption. "( Bomford, 2003)

In order to change from photon to electron therapy, the target is removed to allow the electron stream to continue into the head of the device.

Flattening filter:

In order to help make the beam intensity standard over the field a flattening filtration system is used. It is usually manufactured from business lead, although tungsten, uranium, metal, aluminium, or a combination in addition has been used or recommended. The flattening filtration system absorbs more photons from the centre of the beam and fewer from the periphery of the beam.

Ionisation chambers:

Ionisation champers measure the amount of radiation going out of the machine, quantified in items - Monitor units. Every linac has two ionisation chambers for protection reasons. The ionisation is a round, flat structure, filled with gas, divided into a number of sections, where each segment includes electrodes. When radiation moves through the gas, it is then ionised creating a high charged density that is manipulated by the electrodes. The procedure terminates when the readings from the electrodes have come to a pre-set M. U value.


A key collimator limits the utmost field size for x-ray therapy (40 x 40cm). It means that x-rays leaving the prospective leave in a forwards direction in order to minimise rays leakage through the head.

The treatment field size is described by the extra collimator. This collimator reduces the transmission penumbra, since rays must travel through the entire collimator width. It contains four thick metal blocks, called jaws. You can find two pairs of jaws, higher and lower jaws. By using asymmetric jaws, by moving each jaw independently, asymmetric field sizes can be produced. Half beam blocking can be enabling. Different level habits can be produced, from the standard flat beam account, by moving during treatment. Multi-leaf collimators are "finger like projections", 1cm thick. These hands like projections move separately in order to create the field shape more directly to the shape of the planning target volume. By using MLC's, less rays is directed at normal tissue.


From the launch of this article, this is of the linac was given. A linac is a higher voltage machine, used for the treating cancerous cells and tissues. With the structure of your linac this is achieved. By radiating cancerous tissues, with daily radiation treatment, cancerous tissue can be ruined and then substituted by normal tissues.

Every element in the linac is carefully selected for the function for which it was created. To begin with, the klystron is used to create microwaves, since it is better than a magnetron that is used for lower energy linear accelerators. Because linear accelerators have higher energy beam, klystron are used for production of x-rays.

Continuing to the electron weapon, tungsten line or filament can be used, because of its high melting point, high atomic quantity, and it is ductile. With this features tungsten is a good thermionic emitter, is an excellent source of electrons and can be easily designed into spiral, in order to make a larger surface area for the electrons to be emitted.

In the supply waveguide Sulphur hexafluoride (SF6) is put with the intent of stopping the arcing of electrons. By the end of the waveguide a circulator is put so as microwaves cannot be reflected back.

As we move to the accelerating waveguide, and the standing wave accelerator used in linacs, we can easily see why the position wave accelerator can be used. "The backward going wave interferes with the forward exploring wave, otherwise constructively and destructively. The producing standing influx has a magnitude of around two times that of the visiting wave, and the peak intensity travels over the waveguide at the phase speed of the traveling influx. "(Knapp et al, 1968)

Following the waveguide is the twisting magnet. Here we have the achromatic magnet where its main activity is to change the direction of the electrons, but more important to bring the electrons jointly regardless of the difference in energies.

A flattening filtration system is used to make the beam even from the central axis to its peripheral edges, to own homogenous syndication of the dose.

Ionisation chambers are crucial in a linac. They screen the dose leaving the treatment mind, so that the linear accelerator is aware when to get rid of the treatment.

Collimation in a linear accelerator is essential. Without the primary and secondary rays a linac wouldn't be as ideal for the task that it was designed. The field size and form is vital in order to radiate only the cancerous cells rather than normal tissues. Nowadays with the advances of technology and the utilization not only of MLC's, but also IMRT and IGRT, survival rates of cancer tumor have increased.

Last but not least, a linear accelerator can treat an individual with different energy modalities. By detatching the prospective, the electron stream can continue in to the head of the device and then be utilized for the treatment of superficial toumors. By going out of the mark, photons are produced to treat deep-situated toumors.

Nowadays most linacs have exclusive wedges, in comparison to some decades ago, where there where only manual wedges. Now radiographers by using virtual wedges need not concern about manual handling, as they don't really want to do anything.

I think that linear accelerators are not perfect. Linacs are extremely expensive to buy, so poor countries don't have the chance to treat their patients from cancer tumor. It is hard to keep up with the innovations of technology, as everything is very expensive to buy, in support of wealthy countries can purchase the latest equipment. A disadvantage of turning from photon to electron modalities is that applicators and blocks are being used to guide the electrons and condition the beam. Applicators are very heavy. Blocks are constructed of lead which may cause business lead poisoning if they are not handled with care. The only disadvantage with MLC's is that when conforming the beam form to the PTV, some rays will be seeping, even though using the tongue and groove impact. Finally the linacs to work successfully they want daily quality assurance assessments and maintenance from physicists.


Linear accelerator is the primary treatment unit used for the treating abnormal tissues. Using its exact position of the beam, designed differently for each patient individually, it definitely is the best machine for the treatment of cancer.

A linac uses microwaves to speed up electrons and then hit the target where x-rays are produced. This x-rays are accumulated and then form the form of the beam. Nowadays with the fast developments of technology, linacs in a few years time will be even more efficient than today.

Definitely linear accelerators are suitable for the duty for which these were designed. All of the the different parts of a linear accelerator are carefully selected for its needs. From the tiniest to the bigger elements of the linac, are designed for the best final result.


BOMFORD, C. K. , 2003. Megavoltage beam generators. In: C. K BOMFORD and I. H KUNKLER, ed. Walter and Miller's Textbook of Radiotherapy. London: Churchill Livingstone, Internet pages 162-183.

COOK, M. , 1998. X-Ray Creation. In: A. DUXBURY and P. CHERRY, ed. Useful Radiotherapy Physics and Equipment. London: GMM, Pages 21-26.

HENDEE W. R. , IBBOTT G. S. and HENDEE E. G. , 2005. Radiation Remedy Physics. 3rd ed. Hoboken, New Jersey: Wiley-Liss.

KARZMARK, C. J and MORTON, R. , 1998. A Primer on Theory and Procedure of Linear Accelerators in Radiotherapy. 2nd ed. Madison, Wisconsin: Medical Physics Publishing.

KHAN M. F. , 2003. The Physics of Rays Therapy. 3rd ed. Philadelphia: Lippincott Williams and Wilkins.

KNAPP, E. A. , KNAPP, B. C. and POTTER I. M. , 1968. Position Influx High Energy Linear Accelerator Set ups. In: HENDEE W. R. , IBBOTT G. S. and HENDEE E. G. , 2005. Rays Therapy Physics. 3rd ed. Hoboken, NJ: Wiley-Liss.

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