RT02 Phys2 Equip WEB

8/9/2019 RT02 Phys2 Equip WEB

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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 2

RationaleRationale

Radiation dose delivered to the target

and surrounding tissues is one of the

major predictors of radiotherapytreatment outcome (compare part 3 of

the course) !t is generally assumed that

the dose must "e accurately delivered#ithin $%&' of the prescri"ed dose to

ensure the treatment aims are met


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"jectives"jectives

• *o understand the relevance of radiation dose*o understand the relevance of radiation dose

and dosimetry for radiotherapyand dosimetry for radiotherapy

• *o "e a"le to e+plain the difference "et#een*o "e a"le to e+plain the difference "et#een

a"solute and relative dosimetrya"solute and relative dosimetry• *o "e a"le to discuss the features of the most*o "e a"le to discuss the features of the most

common dosimeters in radiotherapy:common dosimeters in radiotherapy:

ioniation cham"ers, semiconductors,ioniation cham"ers, semiconductors,thermoluminescence dosimeters (*-D) andthermoluminescence dosimeters (*-D) and

filmfilm


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment .

/ontents of lecture 2/ontents of lecture 2

0 1"solute and relative dosimetry0 1"solute and relative dosimetry

2 *he dosimetric environment: phantoms2 *he dosimetric environment: phantoms

3 Dosimetric techniques3 Dosimetric techniques physical "acgroundphysical "acground

practical realiationpractical realiation


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment '

0 1"solute and relative dosimetry0 1"solute and relative dosimetry

Absolute dosimetry is a technique that yieldsinformation directly on a"sor"ed dose in y *his

a"solute dosimetric measurement is also referred to

as cali"ration 1ll further measurements are then

compared to this no#n dose under referenceconditions *his means 4

relative dosimetry is performed !n general no

conversion coefficients or correction factors are

required in relative dosimetry since it is only thecomparison of t#o dosimeter readings, one of them

"eing in reference conditions


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment

1"solute dosimetry 1"solute dosimetry

Required for every radiation qualityRequired for every radiation quality

onceonce

Determination of a"solute dose (in y)Determination of a"solute dose (in y)at one reference point in a phantomat one reference point in a phantom

6ell defined geometry: ;g #ater6ell defined geometry: ;g #ater

phantom, 077cm 89D, 07+07cmphantom, 077cm 89D, 07+07cm22

fieldfieldsie, depth 07cmsie, depth 07cm

8ollo#s protocols (compare part 07)8ollo#s protocols (compare part 07)

O f t re me ndo us

i m po r ta nce:

I f t he a bso l u te dos i me t r y

is i nco r rec t E V E R Y T H I N

G

w i l l be w ro n


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment >

1ns#er 1ns#er FALSE – 1Gy = 1J/kg. Delivering this amount

of energy oul! raise the tem"erature of

tissue #y less than $.$$1o%. Even for a 1$$kg

"erson it is mu&h less than the energy

&onsume! ith a #ol of muesli – "lease note

the amount of energy in foo! is often liste! onthe "a&kage.


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;+amples for relative dosimetry;+amples for relative dosimetry

/haracteriation of a radiation "eam/haracteriation of a radiation "eam percentage depth dose, tissue ma+imumpercentage depth dose, tissue ma+imum

ratios or similar ratios or similar

profilesprofiles

Determination of factors affecting outputDetermination of factors affecting output field sie factors, applicator factorsfield sie factors, applicator factors

filter factors, #edge factorsfilter factors, #edge factors patient specific factors (patient specific factors (e.g.e.g. electron cut&electron cut&

out)out)


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Percentage depth dosePercentage depth dose

measurementmeasurement ?ariation of dose?ariation of dose

in a mediumin a medium

(typically #ater)(typically #ater)#ith depth#ith depth

!ncludes!ncludes

attenuation andattenuation andinverse squareinverse square

la# componentsla# components


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Percentage depth dosePercentage depth dose

Relates dose

at different

depths in water

!or the patient"

to the dose at the

depth of dose

ma#imum $ note

that the y a#is is

relati%e&&&


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 0.

*1R, *@R, *PR*1R, *@R, *PR

Relative dosimetry for isocentric treatmentRelative dosimetry for isocentric treatment

set&up (compare part ')set&up (compare part ')

1ll can "e converted into percentage depth 1ll can "e converted into percentage depth

dosedose *1R A ratio of dose in phantom #ith + cm*1R A ratio of dose in phantom #ith + cm

overlaying tissue to dose at the same point in air overlaying tissue to dose at the same point in air

*@R A ratio of dose #ith + cm overlaying tissue to*@R A ratio of dose #ith + cm overlaying tissue to

dose at dose ma+imum (detector position fi+ed)dose at dose ma+imum (detector position fi+ed)

*PR as *@R "ut as a ratio to dose at a reference*PR as *@R "ut as a ratio to dose at a reference

point (point (e.g.e.g. 07cm overlaying tissue)07cm overlaying tissue)


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 0'

*@R, *PR*@R, *PR

@imics isocentric@imics isocentric

conditionsconditions

*@R is a special*@R is a special

case of *PRcase of *PR

#here the#here the

referencereference

phantom depth isphantom depth isdepth ofdepth of

ma+imum dosema+imum dose


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utput factorsutput factors

/ompare dose #ith dose under/ompare dose #ith dose under

reference conditionsreference conditions

different field siesdifferent field sies #edge factor #edge factor

tray factor tray factor

applicator factor applicator factor electron cutout factor electron cutout factor


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 0B

;+ample: #edge factor ;+ample: #edge factor

+ose under

reference

conditions

,ould also in%ol%e different field si-es and.or

different depths of the detector in the phantom


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1ns#er 1ns#er

Relative dosimetry: #e relate the dose #ith different aluminium

or copper filters in the "eam to the dose

#ithout the filters to determine #hich filterthicness attenuates the "eam to half itsoriginal intensity

the result is independent of the actual dose

given & #e could measure for 07s or 27s or57s each time, as long as #e ensure theirradiation is identical for all measurements


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2 *he dosimetric environment2 *he dosimetric environment

PhantomsPhantoms 1 phantom represents the radiation properties of 1 phantom represents the radiation properties of

the patient and allo#s the introduction of athe patient and allo#s the introduction of a

radiation detector into this environment, a tas thatradiation detector into this environment, a tas that#ould "e difficult in a real patient#ould "e difficult in a real patient

1 very important e+ample is the scanning #ater 1 very important e+ample is the scanning #ater

phantomphantom

1lternatively, the phantom can "e made of sla"s of 1lternatively, the phantom can "e made of sla"s oftissue mimicing material or even shaped as atissue mimicing material or even shaped as a

human "ody (anthropomorphic)human "ody (anthropomorphic)


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9canning #ater9canning #ater

phantomphantom


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*issue equivalent materials*issue equivalent materials

@any specifically manufactured materials@any specifically manufactured materials

such as solid #ater (previous slide), #hitesuch as solid #ater (previous slide), #hite

#ater, plastic #ater, 4#ater, plastic #ater, 4 Polystyrene (good for megavoltage "eams,Polystyrene (good for megavoltage "eams,

not ideal for lo# energy photons)not ideal for lo# energy photons)

Perspe+ (other names: P@@1, Ple+iglas) &Perspe+ (other names: P@@1, Ple+iglas) &

tissue equivalent composition, "ut #ith highertissue equivalent composition, "ut #ith higherphysical density & correction is necessaryphysical density & correction is necessary


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1nthropomorphic phantom 1nthropomorphic phantom

)hole body

phantom: /RT


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1llo#s placement of radiation detectors in 1llo#s placement of radiation detectors in

the phantom (sho#n here are *-Ds)the phantom (sho#n here are *-Ds)

Includes

inhomoeneities


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R1EDR1ED

phantomphantom

torso

,T slice

throuh lun

Head with

T(+ holes


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Pediatric phantomPediatric phantom


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 2>

9ome remars on phantoms9ome remars on phantoms

!t is essential that they are tested prior to use!t is essential that they are tested prior to use physical measurements & #eight, dimensionsphysical measurements & #eight, dimensions

radiation measurements & /* scan, attenuationradiation measurements & /* scan, attenuation

checschecs

/heaper alternatives can also "e used/heaper alternatives can also "e used #a+ for shaping of humanoid phantoms#a+ for shaping of humanoid phantoms

cor as lung equivalentcor as lung equivalent

1s long as their properties and limitations are 1s long as their properties and limitations areno#n & they are usefulno#n & they are useful


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Principles of radiation detectionPrinciples of radiation detection

!oniation cham"er !oniation cham"er

eiger @ueller /ounter eiger @ueller /ounter *hermoluminescence dosimetry*hermoluminescence dosimetry

8ilm8ilm

9emiconductors9emiconductors


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Detection of !oniation in 1ir Detection of !oniation in 1ir

/dapted

from ,ollins

4556

Ion chamber


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Detection of !oniation in 1ir Detection of !oniation in 1ir

/dapted

from

2etcalfe

6778


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/ross section through a 8armer type/ross section through a 8armer type

cham"er (from @etcalfe 0>>5)cham"er (from @etcalfe 0>>5)


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!oniation /ham"ers!oniation /ham"ers

8armer 75 cc8armer 75 cc

cham"er andcham"er and

electrometer electrometer

@ost important@ost important

cham"er incham"er in

radiotherapyradiotherapy

dosimetrydosimetry


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;lectrometer ;lectrometer

1rom the chamber


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!oniation cham"ers!oniation cham"ers

Relatively large volume for small signalRelatively large volume for small signal

(0y produces appro+imately 35n/ in(0y produces appro+imately 35n/ in

0cc of air)0cc of air) *o improve spatial resolution at least in*o improve spatial resolution at least in

one dimension parallel plate typeone dimension parallel plate type

cham"ers are usedcham"ers are used


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment .7

Parallel plate cham"ersParallel plate cham"ers

1rom 2etcalfe et al 6779


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Parallel Plate !oniationParallel Plate !oniation

/ham"ers & e+amples/ham"ers & e+amples @arus cham"er @arus cham"er

smallsmall

designed fordesigned forelectronselectrons

Colt cham"er Colt cham"er

ro"ustro"ust

em"edded inem"edded inpolystyrene sla"polystyrene sla"


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6ell type ioniation cham"er 6ell type ioniation cham"er

8or cali"ration of8or cali"ration of

"rachytherapy"rachytherapy

sourcessources

3rachytherapy

source


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment ..

!oniation cham"er type survey!oniation cham"er type survey

metersmeters not as sensitive as &@ devices "ut not affected "ynot as sensitive as &@ devices "ut not affected "y

pulsed "eams such as occur #ith acceleratorspulsed "eams such as occur #ith accelerators

"ecause of the a"ove,"ecause of the a"ove,

this is the preferredthis is the preferred

device around highdevice around highenergy radiotherapyenergy radiotherapy

acceleratorsaccelerators


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment .'

eiger&@ueller /ounter eiger&@ueller /ounter

Eot a dosimeter & just aEot a dosimeter & just a

counter of radiation eventscounter of radiation events

?ery sensitive?ery sensitive -ight #eight and-ight #eight and

convenient to useconvenient to use

9uita"le for miniaturiation9uita"le for miniaturiation


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eiger&@ueller (&@) Deviceseiger&@ueller (&@) Devices

Gseful forGseful for area monitoringarea monitoring

room monitoringroom monitoring

personnelpersonnel monitoringmonitoring

/are required in regions of high dose rate/are required in regions of high dose rate

or pulsed "eams as reading may "eor pulsed "eams as reading may "e

inaccurateinaccurate


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment .

*hermoluminescence*hermoluminescence

dosimetry (*-D)dosimetry (*-D) 9mall crystals9mall crystals

@any different materials@any different materials

Passive dosimeter & no ca"les requiredPassive dosimeter & no ca"les required

6ide dosimetric range (6ide dosimetric range (µµy to 077s ofy to 077s of

y)y)

@any different applications@any different applications


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment .B

?arious *-D types?arious *-D types


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment .>

9implified scheme of the *-D9implified scheme of the *-D

processprocess


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment '7

*-D glo# curves*-D glo# curves


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lo# curveslo# curves

1llo# research 1llo# research

1re po#erful


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*he role of different dopants*he role of different dopants


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Importance o thermalImportance o thermal

treatmenttreatment Determines the arrangement ofDetermines the arrangement of

impuritiesimpurities

sensitivitysensitivity fadingfading

response to different radiation qualitiesresponse to different radiation qualities

@aintain thermal treatment constant@aintain thermal treatment constant


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment '.

DoseDose

response ofresponse of-i8:@g,*i:-i8:@g,*i:

#ide dosimetric#ide dosimetricrangerange

#atch#atchsupralinearitysupralinearity

f


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment ''

?ariation of *-D response #ith?ariation of *-D response #ith

radiation qualityradiation quality


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@aterials: oh #hat a choice@aterials: oh #hat a choice

-i8:@g,*i (the JgoldJ standard)-i8:@g,*i (the JgoldJ standard)

/a8/a822 (all natural, or #ith @n, Dy or *m)(all natural, or #ith @n, Dy or *m)

/a9/a9..

KeKe

1l 1l2233 :/ (record sensitivity:/ (record sensitivity ≤≤ 0uy)0uy)

-i8:@g,/u,P (the ne# star=)-i8:@g,/u,P (the ne# star=)


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment '

*-D reader *-D reader

photomultiplier "asedphotomultiplier "ased planchet and hot Eplanchet and hot E22 gas heatinggas heating

availa"leavaila"le


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment 'B

!hat can one e"pect###!hat can one e"pect###

Reproduci"ility: single chipReproduci"ility: single chip ±± 2 (70y,2 (70y,09D)09D)

1ccuracy (. chips standard, 2 chips 1ccuracy (. chips standard, 2 chips

measurement)measurement) ±± 3 (70y, >'3 (70y, >'confidence)confidence)

a"out 37 minutes per measurementa"out 37 minutes per measurement


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Radiation Protection in Radiotherapy Part 2, lecture 2: Dosimetry and equipment '>

Radiographic filmRadiographic film

Reduction of silver halide to silver Reduction of silver halide to silver

Requires processing &&&F pro"lems #ithRequires processing &&&F pro"lems #ith

reproduci"ilityreproduci"ility *#o dimensional dosimeter *#o dimensional dosimeter

Cigh spatial resolutionCigh spatial resolution

Cigh atomic num"er &&&F variations ofCigh atomic num"er &&&F variations ofresponse #ith radiation qualityresponse #ith radiation quality


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Radiographic filmRadiographic film

,ross section

Often prepac*edfor ease of use


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8ilm: dose response8ilm: dose response

;valuation of film via;valuation of film via

optical densityoptical density

D A log (!D A log (!77 % !)% !) Densitometers areDensitometers are

commerciallycommercially

availa"leavaila"le


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Radiographic film dosimetry inRadiographic film dosimetry in

practicepractice Depends on e+cellentDepends on e+cellent

processor


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9emiconductor Devices9emiconductor Devices

DiodesDiodes

@98;* detectors@98;* detectors

+iodes for water phantom

measurements


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DiodesDiodes

1rom 2etcalfe et al. 6779

2ostly used li*ea photocell eneratin

a %oltae proportional

to the dose recei%ed


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@etal +ide 9emiconductor @etal +ide 9emiconductor

8ield ;ffect *ransistor 8ield ;ffect *ransistor

1rom 2etcalfe et al. 6779

2O'1ETs ; e#tremely

small sensiti%e %olume


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0 irradiation

2 /harge

carriers trapped

in 9i su"strate

3 /urrent"et#een source

and drain altered


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Diodes and other 9olid 9tateDiodes and other 9olid 9tate

DevicesDevices 1dvantages 1dvantages

direct readingdirect reading

sensitivesensitive

small siesmall sie

#aterproofing#aterproofing

possi"lepossi"le

DisadvantagesDisadvantages temperaturetemperature

sensitivesensitive

sensitivity maysensitivity may

change &&F re&change &&F re&

cali"ration necessarycali"ration necessary

regular


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9ummary of lecture 29ummary of lecture 2

Ion chambers 'emiconductors T(+s 1ilm

/d%antaes )ell understood<

accurate< %ariety of

forms a%ailable

'mall< robust 'mall< no cables

re0uired

Two dimensional<

ease of use

+isad%antaes (are< hih %oltae

re0uired

Temperature

dependence

+elayed readout<

comple# handlin

Not tissue

e0ui%alent< not

%ery reproducible

,ommon use Reference

dosimetry< beam

scannin

3eam scannin< in

%i%o dosimetry

+ose %erification<

in %i%o dosimetry

=/< assessment of

dose distributions

,omment 2ost common and

important

dosimetric

techni0ue

New de%elopments

!2O'1ETs" may

increase utility

/lso used for

dosimetric

intercomparisons

!audits"

New de%elopments

!radiochromic

film" may increase

utility


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eneral 9ummary: Physicseneral 9ummary: Physics !n radiotherapy,!n radiotherapy, photonsphotons ( Rays and gamma rays)( Rays and gamma rays)

andand electronselectrons are the most important radiation typesare the most important radiation types 1ccuracy of dose delivery is essential for good 1ccuracy of dose delivery is essential for good

practice in radiotherapypractice in radiotherapy

1"solute dosimetry 1"solute dosimetry determines the a"sor"ed dose indetermines the a"sor"ed dose in

ray at a #ell&defined reference pointray at a #ell&defined reference point RelativeRelativedosimetrydosimetry relates then the dose in all other points orrelates then the dose in all other points orthe dose under different irradiation conditions to thisthe dose under different irradiation conditions to thisa"solute measurementa"solute measurement

*here are*here are many different techniquesmany different techniques availa"le foravaila"le fordosimetry & none is perfect and it requires training anddosimetry & none is perfect and it requires training ande+perience to choose the most appropriate techniquee+perience to choose the most appropriate techniquefor a particular purpose and interpret the resultsfor a particular purpose and interpret the results


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6here to et @ore6here to et @ore

!nformation!nformation @edical physicists@edical physicists

*e+t"oos:*e+t"oos:

Ihan 8 *he physics of radiation therapy 0>>. @etcalfe PL Iron *L Co"an P *he physics of

radiotherapy &rays from linear accelerators 0>>

/em"er C !ntroduction to health physics 0>B3

6illiams ML *h#aites D Radiotherapy Physics 0>>3


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1ny questions= 1ny questions=


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Question:Question:

6hich radiation detectors could "e useful6hich radiation detectors could "e useful

for in vivo dosimetry and #hy=for in vivo dosimetry and #hy=


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