intro duction of linear accelrator by asif rasheed

7/30/2019 Intro Duction of Linear Accelrator by Asif Rasheed

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Definition of instability:

Particles in the linac are driven by magnetic lenses and RF cavities; the correspondingforces are generally periodic, sometimes with adiabatic changing parameters. In this

way the accelerator designer determines a beam with the same periodicity, in order to

have a simple transport all over the structure.

In other words it is required as a nominal (or unperturbed) situation:

with number of particles in the elementary volume of phase space, since are the three

spatial coordinated

are the associated moments; tis time and Tis the period.

This is the result achieved for single particle dynamics in the first lessons, where the

periodic beta function defines the envelope equal to itself period after period. But if the

beam intensity increases the electromagnetic forces determined by the beam cannot be

neglected, the condition (1) has to be preserved in presence of external plus internal

beam forces. In the next section we shall say something more about internal forces.

Beam breakup:

A very well known instability affecting beam quality in electron linacs is the beam

breakup (BBU). When the bunch current is high it can happen that the transverse

oscillation of the head of the bunch causes a resonant growth of the transverse

oscillation of the tail, with consequent deformation of the bunch, emittance growth and

possibly beam losses. The emittance growth is particularly detrimental in the case of a

linear collider where the luminosity is affected.

Electromagnetic fields generated by the beam :

Each particle in the beam feels the effect of the electromagnetic field generated by the

other particles in the field boundaries due to the accelerator elements. The

mathematical problem corresponds to the solution of the Maxwell equations in the

presence of the beam charges in motion, with accelerator boundary conditions. In


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practice the problem can often be simplified, in consideration of beam energy and

acceleration frequency.

Linear Accelerator

View larger with caption

The Electron Gun

The electron gun, is where electron acceleration begins. The electrons start out

attached to the molecules in a plate of barium aluminate or other thermionic materials

such as thorium. This is the cathode of the electron gun. A cathode is a surface that

has a negative electrical charge. In linac electron guns this charge/ electrons are

emitted on heating the cathode. Barium aluminate is a "thermionic" material; this means

that it's electrons tend to break free of their atoms when heated. These electrons "boil"

near the surface of the cathode. The gate is like a switch. It consists of a copper screen,

or "grid," and is an anode. An anode is a surface with a positive electrical charge. Every

500 millionth of a second the gate is given a strong positive charge that causes

electrons to fly toward it from the cathode in tremendous numbers. As these electrons

reach the gate, they become attracted even more strongly by the main anode, and pass

through the gate. Because the gate is pulsing at a rate of 500 million times per second

(500 MHz), the electrons arrive at the anode in loose bunches, a 500 millionth of a

second apart. The anode is a torus (a doughnut) shaped to create an electromagnetic
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field that guides most of the electrons through the hole into the next part of the

accelerator, called the buncher. B. The Buncher The purpose of the buncher is to

accelerate the pulsing electrons as they come out of the electron gun and pack them

into bunches. To do this the buncher receives powerful microwave radiation from the

klystron. The microwaves accelerate the electrons in somewhat the same way that

ocean waves accelerate surfers on surfboards. Look at the following graph:

The yellow-orange disks are electrons in the buncher. The curve is the microwave

radiation in the buncher. The electrons receive more energy from the wave--more

acceleration--depending on how near they are to the crest of the wave, so the electrons

riding higher on the wave catch up with the slower ones riding lower. The right-hand

wave shows the same group of electrons a split second later. On the front of the wave,

the two faster electrons have almost caught up with the slower electron. They won't

pass it though, because they are now lower on the wave and therefore receive less

acceleration. 8

The higher electron on the back of the wave gets just enough acceleration to match the

speed of the wave, and is in the same position as it was on the left-hand wave. This

represents the last electron in the bunch. The lower electron on the back of the wave

gets too little energy to keep up with the bunch and ends up even lower on the right-

hand wave. Eventually it will fall back to the electron bunch forming one wave behind.

C. The Linac The linac itself is just an extension of the buncher. It receives additional

RF power to continue accelerating the electrons and compacting them into tighter

bunches. Electrons enter the linac from the buncher at a velocity of 0.6c--that's 60% of

the speed of light. By the time the electrons leave the linac, they are traveling very close

to the speed of light.

1. Gridded Electron Gun:-


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Controls dose rate rapidly and accurately. Permits precise beam control for dynamic

treatments, since gun can be gated. Removable for cost - effective replacement.

2. Energy Switch

Patented switch provides energies within the full therapeutic range at consistently high

stable dose rates, even with low energy x ray beams. Ensures optimum performance

and spectral purity at both energies.

3. Wave Guide

High efficiency, side coupled standing wave accelerator guide with demountable

electron gun and energy switch

4. Achromatic 3 field bending magnet

Unique design with fixed +/- 3 % energy slits ensures exact replication of the input beam

for every treatment. The 270 degree bending system, coupled with varians 3

dimensional servo system, provides for a 2mm circular focal spot size for optimal portal

imaging.

5. Real time beam control steering system

Radial and transverse steering coils and a real time feed back system ensures that

beam symmetry is within +/- 2%at all gantry angles.

6. Focal spot size

Even at maximum dose rate and gantry angle the circular focal pot remains less

than 2mm, held constant by a focus solenoid. Assures optimum image quality for portal

imaging

7. 10 port carousel

New electron scattering foils provide homogenous electron beams at therapeutic

depths. Extra ports allow for future development of specialized beams.


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8. Ion chamber

Dual sealed ion chambers with 8 sectors for rigorous beam control provide two

independent channels, impervious to changes in temperature and pressure. Beam

Dosimetry is monitored to be within +/- 2% for long term consistency and stability.

9. Asymmetric Jaws

Four independent collimators provide flexible beam definition of symmetric and

asymmetric fields. 10. Multi-Leaf Collimator Dynamic full field resolution 120 leaf MLC

with dual redundant safely read out for most accurate conformal beam shaping for IMRT

treatments.

WORKING AND DESCRIPTION MEDICAL LINEAR ACCELERATOR : - With the

advances in linear accelerator models and working condition, the disadvantages of

earlier cobalt machine based gamma radiation are overcome. The main advantage of

linac over Co 60 is skin sparing, means the radiation affects the skin surface. Most of x-

ray energy goes to tumour; secondly there is minimal scatter of x-ray energy outside the

beam. Sharply defined x-ray beam minimizes the side effects of treatment, so only a

small amount of radiation travel to other parts of the body. Linac allows sharpness of the

beam edge, allows a very precise treatment and adjacent tissues spared. And this linac

may be programmed to treat with electron rather than x rays for special situations.

Most linear Accelerators make use of isocentric mount, modern linacs use 100 cm

isocentre, i.e the source of x-ray is 100 cm from the axis of rotation, so the patient is

positioned with the center of tumour on the axis of rotation. Treatment delivered using

several angular positioned, without realigning the patient simply by rotating the unit. For

low energy medical linacs, having energy range of 4 6 MeV, the accelerating tube can

be made short enough to allow the arrangement. The electrons travel down the tube

and strike the target, produce x ray beam symmetrical about the line of source to center

of the machine. If electron beam required the target is moved to the side to allow the

electron beam emerge thro a thin window


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For high energy machines, the accelerating tube must be made longer. RF power from

klystron is brought to the wave guide through the axis of machine using a rotating

vacuum seal. The electron beam is bent thro an angle of 60 degree passes thro

quadruplet focusing magnets and then bent 90 degree in the head to hit a target or

emerge through a window for electron beam. In high energy medical linacs there are

some important components required to control and shape the beam from a Linac used

in both electron and photon modes. High energy electron from wave guide is focused at

the target which is thick enough to stop the electron. When we use electron mode of

operation in the linac, the tungsten target on the sliding mount moved towards right side

and allowing the electron to pass through an open window. Just below the target one

more sliding mount is located, which contains a variety of electron scattering foil for

different electron energy and a port for photon mode. These beams passes through a

conical hole in the primary collimator made of heavy metal. Below this primary

collimator a secondary slide containing the flattening filter of tungsten and a one section

for a electron beam. Ion chamber is positioned near the secondary slide to monitor the

dose and a quadrant is connected to a servo mechanism to control the beam optics. A

light localizer is placed below the ion chamber which is retraced from the beam after

patient position under the beam.

Types of External Beam Radiation Therapy Conventional external Beam radiation

therapy

The science of radiation oncology and medical physics has developed standard

approaches to dose delivery. In many cancer cases the treatment approach may be

very similar and allows for conventional treatment. For example, many tumors can be

treated with a single field from the front and a single field from the back or with two

fields from the opposite sides. These are examples of parallel opposed fields. The

combination of fields helps to uniformly deliver dose across the tumor. Sometimes 3 or

4 fields will be used. Occasionally, the gantry of the linear accelerator will rotate during

treatment using what is called arc therapy. 3-D Conformal Radiation Therapy

Through the advancement of imaging technology enhanced images of the body allow

for programming of treatment beams to conform better to the shape of a tumor. Hence


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treatment is more effective and side effects are reduced. By treating with large numbers

of beams each shaped with a multileaf collimator (MLC) or cerrobend block, radiation

dose is delivered uniformly and conformally to the tumorIntensity Modulated

Radiation Therapy (IMRT)

IMRT is one of the latest advancements in radiation therapy. This new approach to

treatment allows for dose sculpting and even distribution of delivery to avoid critical

structures while delivering precise uniform treatment. In this technique, the multileaf

collimator (MLC) moves and modulates the radiation as the linac treats the patient.


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With the improvement in computer field, diagnostic methods and other breakthroughs in

the modern electronics systems, made possible a variety of treatment technique and

methods to treat the patient with more accurate and less side effects or nil side effects,

sparing the critical organs in and around the tumour region. The electronic Portal

Imaging device (EPID) made possible to verify the treatment portal without delivering a

high dose and make correction where ever it is mandatory. Computer assisted

treatment delivery and record verify systems reduces the errors and assures the

accurate treatment delivery with reduced time frame, and with the less man power.

Newer treatment modalities like IGRT, DART and other high resolution treatment

modalities made possible the treatment of organs involved with motion due to breath

and circulations. Robotic arm based radiosurgery made the skull based radiosurgery

programme a grand success with high accuracy.

Technology-driven delivery methods

Technological developments in computers and accelerator designs, particularly inverse

treatment planning and dynamic multi-leaf collimation systems (dMLCs), have given the

radiotherapy community the ability to deliver conformal and intensity-modulated

radiation therapy (IMRT) treatments. With the implementation of image-guided radiation

therapy (IGRT), there is a potential to deliver inhomogeneous dose distributions that

increase the dose inside the target by 20-30% over the minimal peripheral dose, while

decreasing possible normal tissue complications by collapsing the planning target

volume (PTV) onto the clinical target volume (CTV), as is often done in Stereotactic

radiosurgery.

Image-guided intensity-modulated and adaptive helical TomoTherapy

Image-guided IMRT is redefining the practice of radiation oncology. Traditional methods

of implementing beam intensity modulation have included individually designed

compensators, static multi-leaf collimators (MLC), dynamic MLC and sequential (serial)

tomotherapy. Helical Tomotherapy provides added functionality to enhance the

application of IMRT. It facilitates adaptive radiotherapy and conformal avoidance. These

advances improve normal tissue sparing and permit dose reconstruction and

verification, thereby allowing significant biologically effective dose escalation and

reduced radiation toxicity. Recent radiobiological findings can be translated into altered


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fractionation schemes that aim to improve the local control and long-term survival. The

intrinsic capability of helical Tomotherapy for 14

megavoltage CT (MVCT) imaging for IMRT image-guidance is an added feature aiding

in further

What is this equipment used for?

How does the equipment work?

Who operates this equipment?

How is safety ensured?

What is this equipment used for?

A linear accelerator (LINAC) is the device most commonly used forexternal beam

radiation treatmentsfor patients with cancer. The linear accelerator is used to treat all

parts/organs of the body. It delivers high-energyx-raysto the region of the patient's

tumor. These x-ray treatments can be designed in such a way that they destroy the

cancer cells while sparing the surrounding normal tissue. The LINAC is used to treat all

body sites, using conventional techniques,Intensity-Modulated Radiation Therapy

(IMRT),Image Guided Radiation Therapy (IGRT),Stereotactic Radiosurgery (SRS) and

Stereotactic Body Radio Therapy (SBRT).

How does the equipment work?

The linear accelerator uses microwave technology (similar to that used for radar) to

accelerate electrons in a part of the accelerator called the "wave guide," then allows

these electrons to collide with a heavy metal target. As a result of the collisions, high-

energy x-rays are produced from the target. These high energy x-rays are shaped as

they exit the machine to conform to the shape of the patient's tumor and the customized

beam is directed to the patient's tumor. The beam may be shaped either by blocks that

are placed in the head of the machine or by a multileaf collimator that is incorporated

into the head of the machine. The patient lies on a moveable treatment couch and
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lasers are used to make sure the patient is in the proper position. The treatment couch

can move in many beam comes out of a part of the accelerator called a gantry, which

can be rotated around the patient. Radiation can be delivered to the tumor from any

angle by rotating the gantry and moving the treatment couch.directions including up,

down, right, left, in and out. The

Who operates this equipment?

View larger with caption

The patient'sradiation oncologistprescribes the appropriate treatment volume and

dosage. Themedical radiation physicistand thedosimetristdetermine how to deliver

the prescribed dose and calculate the amount of time it will take the accelerator to

deliver that dose.Radiation therapistsoperate the linear accelerator and give patients

their daily radiation treatme

How is safety ensured?

Patient safety is very important and is assured in several ways.

Before treatment is delivered to the patient, the treatment plan is developed and

approved by the radiation oncologist in collaboration with the radiation dosimetrist and

physicist. The plan is double-checked before treatment is given and quality-control

procedures ensure that the treatment delivered is the same as was planned.

Quality control of the linear accelerator is also very important. There are several

systems built into the accelerator so that it will not deliver a higher dose than the

radiation oncologist has prescribed. Each morning before any patients are treated, the
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radiation therapist performs checks on the machine using a piece of equipment called a

"tracker" to make sure that the radiation intensity is uniform across the beam and that it

is working properly. In addition, the radiation physicist conducts more detailed weekly

and monthly checks of the linear accelerator.

Modern linear accelerators also have internal checking systems to provide further safety

so that the machine will not turn on until all the treatment requirements prescribed by

your physician are perfect. When all the checks match and are perfect, the machine will

turn on to provide your treatment.

During treatment the radiation therapist continuously watches the patient through a

closed-circuit television monitor. There is also a microphone in the treatment room so

that the patient can speak to the therapist if needed. Port films (x-rays 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.

Safety of the staff operating the linear accelerator is also important. The linear

accelerator sits in a room with lead and concrete walls so that the high-energy x-rays

are shielded. The radiation therapist must turn on the accelerator from outside the

treatment room. Because the accelerator only gives off radiation when it is actuallyturned on, the risk of accidental exposure is extremely low. The treatment room is

shielded to such an extent that even pregnant women may safely operate linear

accelerators.

THEACCELERATING STRUCTURE AND THE BEAM

Linear accelerators consist of two basic elements: the accelerating structure and

the particle beam. The accelerating structure depends on the type of linac. The main

types of linacs are

:


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Electric field polarizes the drift tubes so that periodically the two ends are charged toopposite sign.

When charging currents flow to the right along the tubes, an equal current flows to theleft along the inside wall of the outer cylinder.The tubes are supported at their centers by radial rods along which no current flows.

DC linacs:

like Van de Graafs, in which the structure consists of some kind

of column of electrodes. These electrodes sustain a DC electric field which

Accelerates a continuous stream of particles. DC linacs are limited to a few tens

of MeV.

Induction linacs:

in which the accelerating electric fields are obtained, according

to Faradays law, from changing magnetic fluxes. These changing magnetic

fluxes are generated by large pulsed currents driven through linear arrays of

magnetic toroids. The beam path, along which the electric field develops, can

be considered as the single turn secondary of a transformer. Induction linacs

are generally used in medium-energy high-current pulsed applications.

RF linacs:

the type on which we will concentrate here, can be categorized in

a number of ways: low frequency (UHF), microwave frequency (L, S, C, or Xband),

laser frequency; CW or pulsed; traveling-wave or standing-wave; room

temperature or superconducting. In all these cases, the structure is a conducting


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Array of gaps, cavities or gratings along which rf waves with an electric field

Parallel to the beam can be supported and built up through some resonant process.

RF linacs are used for a wide spectrum of applications from injectors into

Circular accelerators, to entire high-energy accelerators such as SLAC, medical

Accelerators and many others.

While the accelerating structure can be considered the heart of each individual

Machine, it cannot work without its associated systems such~-as the power source, the

Vacuum, cooling, support and alignment, and instrumentation and control systems. It

is important to note that a technological breakthrough in any one of these associated

Systems can have profound effects on the main design of the accelerator.

The second basic element of the linac is, of course, the beam awn d the particles which

Compose_ it. The vast majority of linacs today are elect& (for positron) machines; they -

Number about 1400 accelerators of which a large percentage are commercial radiation

Therapy machines. The other linacs accelerate protons (H*) and in a few cases ions.

There are about 50 proton or ion linacs in existence in the world.

Very little will be said in these lectures about the sources of these particles, guns,

Ion sources, duoplasmatrons, polarized beams, strippers, positron radiators, etc. This

Is an encyclopedic subject by itself with many specialties and sub-specialties which

Can not profitably be summarized here. The reader should not conclude that because

The subject has been left out, it is not of crucial importance to the design and operation

Of a given linac. Not only does the source have an effect on how well the accelerator

Can perform its function, but in some cases it determines how a new concept can or

Cannot be approached. An example of this is the electron source for the linear collider

For which a conventional electron gun cannot create a beam with a sufficiently small

Remittance and must be followed by a damping ring to cool it down.

The fundamental problems in beam dynamics are:

1. Longitudinal bunching and stability,

2. Focusing and transverse stability, and

3. Steering and transport to a target or interaction area.


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Finally we note that instead of operating a SW-cavity in the -mode one can operate in

the 0-modewith good efficiency.

Again, forward and backward space harmonics add up in phase in every cell and both

contribute to the acceleration. Since the electromagnetic fields in adjacent cells are in

phase the separating walls can be left out without affecting the field distribution.

An additional analysis of the mode indicates that there are cell-to-cell phase errors due

to Q-losses, and moreover it is rather sensitive to dimensional or frequency errors. on

the other hand, is far less sensitive to these effects. For these reasons, for room

temperature lilacs one often operates in the /2 or 2/3 mode. However, we note there is

technique to mitigate against the sensitivity disadvantages of the mode. Namely, we

add coupling cavities to either side of the main accelerating cavities in a such a way that

the chain uses a /2 mode while the main-to-main cell phase shift is

Machine Specifications and Purchase Agreement

The recommendations of the equipment selection committee are followed by the

development of comprehensive machine specifications and a binding purchase

agreement. If the equipment is purchased through a bid process, then the machine

specifications are developed before the decision is made to select a manufacturer.

Otherwise, the final specifications may be developed in close collaboration with the

manufacturers representative. All manufacturers have developed product specifications


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for the functional performance of their equipment in response to requirements from

potential users and in commercial competition with other manufacturers.

These are available in the form of product data and specification sheets, which can

serve as a good starting point for the purchase agreement. Any special requirement can

then be added as an addendum. This saves lot of time and effort from being expended

in repetitive work. An example of an addendum to the purchase agreement is presented

in the Appendix, which illustrates how special requests are included in the agreement.

It is imperative that the facility physicist develop a comprehensive acceptance testing

document with a detailed test procedure to verify each term of the agreement and

machine specifications. This document should be shared with the manufacturers

representative before the installation begins so that all ambiguities are clarified in

advance. It is not prudent to depend on the manufacturer-supplied acceptance test

procedure exclusively. However, it should be reviewed thoroughly before acceptance

testing. IEC publication 976, entitled Medical Electron

Accelerators: Functional Performance Characteristics is an excellent resource to set

up test procedures.

It is essential that the physicist review the facility layout with the planning and

installation department of the accelerator manufacturer. They can provide very useful

information on work flow, equipment layout and special requirements. A joint meeting ofthe equipment planning coordinator, architect, contractor, and physicist in the earlier

stages of construction is very helpful and productive. This meeting can resolve all

potential problems regarding electrical power supply, conduit layout, air conditioning,

and chilled water requirements for the machine. The shielding design and its final

approval are solely the responsibility of the physicist, even if

generic vault design and shielding barrier thicknesses are available from other sources.

IV. Accelerator Installation

The physicist and facility engineer (if available) should work closely with the installation

engineer. A close collaboration during the installation can reduce the acceptance testing

time considerably. It is important that the facility personnel do not interfere in the work of

the installation engineer but observe the progress in the background. As soon as the


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accelerator is capable of producing a radiation beam, a series of tests should be

conducted to assure the safety

of all concerned. These include

Testing of door interlocks

Testing of proper operation of the emergency off switches

A preliminary calibration of the machine output in all modes

Modern-Day Linac Acceptance Testing and Commissioning - J.R. Palta, Ph.D.

A radiation survey in both controlled and uncontrolled areas around the treatment

vault at the highest available dose rate and under worst irradiation conditions (without

phantom)

A full radiation survey including the photon and neutron leakage measurements will still

have to be completed to comply with regulatory requirements after a full calibration. The

preliminary survey is done to assure the safety of individuals during the acceptance

testing and commissioning.

V. Acceptance Testing

The installation is followed by acceptance testing by the physicist to ensure that the

machine meets the product specifications and the purchase agreement. These tests are

conducted

according to the acceptance testing procedure agreed on between the manufacturers

representative and the facility physicist. Each facility should have the necessary

equipment for acceptance testing. This includes a 3-D water phantom scanner with

computer interface, ion

chambers, and electrometer X ray films; film laser scanner; and precision level. It is

important to know that each machine comes with the functional performance test values

performed in test cells in the factory. These are helpful for comparison during

acceptance testing. IEC Report 977

provides suggested values of functional performance that all manufacturers voluntarily

comply with. A summary of the suggested values of functional performance is given in

the Table. Some of these values are required to be more stringent for special

application of the linac. For example, it is not unusual to require a radiation isocenter

tolerance within 1 mm diameter of the


19/19

linac scheduled to be used for high precision radiation therapy and radiosurgery.

intro duction of linear accelrator by asif rasheed

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