Chapter 7 WL Chapter 9 SS Linear Accelerators

8/9/2019 Chapter 7 WL Chapter 9 SS Linear Accelerators

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Linear Accelerators

Chapter 7 W/L

Chapter 9 S/S


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Radiation Therapy Equipment

• Lowener!y machines" #ses $rays !enerated at %olta!es up to&''()p * Grenz rays

• +'+,()p• Treatment o- in-lammatory disorders .Lan!erhans cells01 2owens disease1

patchysta!e mycosis -un!oides1 herpes simple$

* Contact therapy• Super-icial s(in lesions• Endoca%itary treatments -or curati%e intent .rectal0• heman!iomas

* Superficial equipment• ,'+,'()p

• S(in cancer and tumors no deeper than '3, cm * Orthovoltage machines

• +,',''()p• S(in1 mouth1 and cer%ical carcinoma• E$perience limitation in the treatment o- lesions deeper than 4 to & cm3


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Limitations o-

Low Ener!y 5achines

• Can not reach deepseated tumors with anadequate dosa!e o- radiation3

• 6o not spare s(in and normal tissues3


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Central A$is 6epth 6ose

• Central a$is depth dose and physical penumra arerelated to eam quality3

• The central a$is depth dose distriution -or a speci-iceam depends on the ener!y3

• 8sodose cur%e" a line representin! %arious points o-similar %alue in a eam alon! the central a$is andelsewhere * The depth o- an isodose cur%e increases with eam quality

*The asored dose in the medium outside the primary eam is!reater -or lowener!y eams than -or those o- a hi!her ener!y3

* Limited scatter outside the -ield -or me!a%olta!e eams occursecause o- predominantly -orward scatterin! o- the eam3


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Linear Accelerator

• Treatment machine that uses hi!h-requencyelectroma!netic wa%es to accelerate char!ed particlessuch as electrons to hi!h ener!ies %ia a linear tue3

• Char!ed particles tra%el in strai!ht lines as they !ain

ener!y -rom an alternatin! electroma!netic -ield3• i!her ener!y eams can e !enerated with !reater s(in

sparin!1 -ield ed!es are more sharply de-ined with lesspenumra and computer technolo!y shapes thetreatment eam and personnel recei%e less e$posure toradiation lea(a!e3


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istory

• +9:;" A wor(in! +5) linear accelerator was installed at the


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Accelerator >enerations

• Early Accelerators .+9,&+9=+0" * E$tremely lar!e and ul(y * Limited !antry motion

• Second Generations .+9=4+9;40" * &=' de!ree rotational

* Allow treatment to a patent -rom any !antry an!le * 8mpro%ement in accuracy and dose deli%ery

• Third generation accelerators" * 8mpro%ed accelerator !uide * 5a!net systems * 2eammodi-yin! systems to pro%ide wide ran!es o- eam ener!y1 dose

rate1 -ield si?e * @peratin! modes with impro%ed eam characteristics * i!hly reliale * Compact desi!n * 5ay include" dual photon ener!ies1 multilea- collimation1 se%eral

electron ener!ies electronic portal %eri-ication systems


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5odulator Cainet

• Modulator cabinet contains components that distriuteand monitor primary electrical power and hi!h%olta!epulses to the ma!netron or (lystron

• Located in the treatment room

• Three maor components" * The -an control" automatically turns the -ans o-- and on as the

need arises -or coolin! the power distriution

* Au$iliary powerdistriution system" contains the emer!ency o--utton that shuts o-- the power to the treatment unit3

* Drimary powerdistriution system


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Console

• Console electronic cabinet pro%ides acentral location -or monitorin! andcontrollin! the linac

* Ta(e the -orm o- a di!ital display1 push uttonpanel or %ideo display terminal .)6T0

* All interloc(s must e satis-ied -or themachine to allow the eam to e started

* Dro%ides a di!ital display -or prescrie dose.monitor units01 mechanical eam parameterssuch as collimator settin! or !antry an!le


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!rive Stand

• !rive Stand a stand containin! the apparatusthat dri%es the linear accelerator * @pen on oth sides with swin!in! doors -or easy

access to !au!es1 %al%es1 tan(s1 and uttons

* "lystron#Magnetron power source used to !enerateelectroma!netic wa%es -or the accelerator !uides

* $aveguide hollow tueli(e structure that !uide theelectroma!netic wa%es -rom the ma!netron to theacceleratin! !uide where electrons are accelerated

* Circulator directs the R< ener!y into the wa%e!uideand pre%ents any re-lected microwa%es -rom returnin!to the (lystron

* $ater%cooling system allows many components inthe !antry and dri%e stand to operate at a constanttemperature


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Blystron

• "lystron A linear eam microwa%eampli-ier requirin! an e$ternal oscillator orradio-requency .R


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5a!netron

• Magnetron de%ice that pro%ides hi!h-requency

microwa%e power that is used to accelerate the

electrons in the acceleratin! wa%e!uide3

• Electrons are emitted -rom the cathode andspiral in the perpendicular ma!netic -ield3 The

interaction o- the spiralin! electrons with the

ca%ities in the anode creates the hi!h-requency

E5 wa%es3

• oscillator and ampli-ier used in lowener!y


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>antry

• Gantry responsile -or directin! the

photon .$ray0 ener!y or electron eam at

a patients tumor3

* Electron gun produce electrons and inects

them into the accelerator structure

* Accelerator structure a special type o-

wa%e !uide in which electrons areaccelerated3

* Treatment head components desi!ned to

shape and monitor the treatment eam


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Accelerator Structure

• 5icrowa%e power .produced in the (lyston0 is transported to theaccelerator stricture in which corru!ations are used to slow up thewa%es synchronous with the -lowin! electrons3 A-ter the -lowin!electrons lea%e the accelerator structure1 they are directed towardthe tar!et .-or photon production0 or scatterin! -oil .-or electronproduction0 located in the treatment head3

• Ampli-ication that occurs in the accelerator structure is in theclosed ended1 precision cra-ted copper ca%ities where theelectrical power pro%ides momentum to the lowle%el electronstream mi$ed with the microwa%es3 Alternatin! positi%e andne!ati%e electric char!e accelerates the electrons toward thetreatment head1 the ne!ati%e %olta!e repels electrons whilethe positi%e %olta!e attracts then1 therey pushin! and pullin!the electrons alon!3

• Char!ed particles e$perience the equi%alent o- a small%olta!e multiple times1 endin! up with a lar!e amount o-(inetic ener!y


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Accelerator Structure

• Len!th %aries dependin! on the eam ener!y o- the linac1 as moreca%ities are used1 hi!her ener!y is deri%ed

• Tra%elin! wa%e" an electroma!netic wa%e tra%els to the ri!ht alon!with the electron1 the electron is continuously accelerated as itmo%es

* Limitation" the electron and the electric -ield must mo%e at the same%elocity * 8rises" washer shaped metal discs that pro%ide resistance to the tra%el o-

the electroma!netic wa%es3• As the electron increases in ener!y and %elocity1 the need -or irises is

reduced1 so irises are increasin!ly -ar apart and ha%e increasin!ly wideropenin!s3

• Standin! wa%e" microwa%e power is oined into the structure ysidecouplin! ca%ities1 rather than throu!h the eam aperture1pro%ides a shorter acceleratin! tue * 5a(es use o- the concept i- inter-erence * 5ore e--icient1 more costly


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Treatment head

* Treatment head components desi!ned to shape andmonitor the treatment eam

• 2endin! ma!net" direct the electrons %ertically toward thepatient

• ray tar!et"• Drimary collimator" desi!ned to limit the ma$imum -ield si?e

• 2eam -lattenin! -ilter" shaped the $ray eam in its crosssectional dimension

• 8on chamer" monitors the eam -or its symmetry in the ri!ht

le-t and in-eriorsuperior direction• Secondary collimators" upper and lower collimator aws

•


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2endin! 5a!net

• 2endin! ma!net" ends the electron eam throu!h ari!ht an!le1 so it ends up pointed at the patient * 9' de!ree ma!nets .chromatic0 ha%e the property that any

ener!y spread results in spatial dispersion o- the eam3

• Electrons are ent in proportion with their ener!y1 the lower ener!yelectrons are ent more1 the hi!her ener!y electrons less

• Results in a eam that is spread -rom side to side accordin! toener!y

• Ener!y sensiti%e1 act as ener!y di--erentiators

* 47' de!ree ma!nets .achromatic0 desi!ned to eliminate spatial

dispersion• So not si!ni-icantly disperse the di--erent electron ener!ies in the

eam3


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Scatterin! -oil

• Scatterin! -oil" thin metal sheets pro%ide

electrons with which they can scatter1

e$pandin! the use-ul si?e o- the eam

• @ther accessories


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5onitor Chamers

• Electron and photon eams must -irst e

measured or monitored in order to allow

deli%ery o- the prescried amount o-

radiation3


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Treatment Couch

• Treatment couch mounted on a rotational a$isaround the isocenter * Also called patient support assemly .DSA0

* 5o%e mechanically in a hori?ontal and len!thwisedirection must e smooth and accurate allowin! -orprecise and e$act positionin! o- the isocenter durin!treatment positionin!

* Support up to :,' ls

* Ran!e in width -rom :,,' cm * Rac(etli(e -rame should e periodically ti!htened to

pro%ide more patient support and reduce the amounto- sa! durin! treatment positionin!3


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5ultimodality Treatment

• 5ultimodality treatment units o--er se%eral

ad%anta!es"

* 6ual photon ener!ies they can pro%ide

ac(up -or other treatment units that maye$perience downtime

* Datients can e treated with multiple eam

ener!ies on the same treatment unit


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Hew Technolo!ies

• Three%dimensional conformal therapy &'!%C(T) the-ield shape and eam an!le chan!e as the !antry mo%esaround the patient * 8ma!es -rom CT scanners are trans-erred to treatment plannin!

computers1 where normal tissues and tumor %olumes are de-ined

* I-orward plannin!J process" eam arran!ement are tested y trialand error

• *ntensity Modulated radiation therapy &*M(T) ene-icial in escalatin! the dose to the tumor %olume andreducin! the dose to normal tissue

* Iin%erse treatment plannin!J" the radiation oncolo!ist selectsdose parameters -or normal tissues and the tar!et %olume andthe computer ac( calculates the desired dose distriution andeam arran!ements

* Adusts the intensity o- radiation eam across the -ield with theaid o- 5LC mo%in! in and out o- the eam portal under precisecomputer !uidance3


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Additional Ad%ances

• 8ndependent collimators .dual asymmetrical aws0" pro%ide increased -le$iility in treatmentplannin!

• 5LCs allow an increased numer o- treatment-ields without the use o- hea%y Cerroendloc(in!

• 6ynamic wed!e" computeri?ed shapin! o- thetreatment -ield

• Electronic portal ima!in!" pro%ides -eedac( onsin!lee%ent setup accuracy or oser%ation o-treatment in near realtime


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Additional Ad%ances

• )eri-ication and record de%ices" * Allow incorrect setup parameters to e corrected

e-ore the machine is turned on

* Dro%ide data in computer assisted setup

* Recordin! o- patient data

* Allowin! -or data trans-er -rom the simulator ortreatment plannin! computer

* Assistin! with quality control

• Stereotactic radiation therapy" in%ol%es theaimin! and deli%ery o- a well de-ined narroweam to e$tremely hard to reach places


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8nterloc(s

• 6esi!ned to protect the patient1 sta-- memersand equipment -rom ha?ards

• Datient protection interloc(s1 includin! eam

ener!y1 eam symmetry1 dose1 and doseratemonitorin!1 pre%ent radiation and mechanicalha?ards to the patient

• Emer!ency o-- uttons terminate irradiation and

machine -unctions require a complete warmupprocedure e-ore the treatment machine canproduce an electron or photon eam


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Chapter 7 WL Chapter 9 SS Linear Accelerators

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