A_Dosimetry Calibration Reference Dosimetry

IAEA/AFRA Training Course on Commissioning of Linear Accelerators used in Radiotherapy

Algiers, Algeria 24-28 November 2007

Mehenna ARIB / SSDL OfficerIAEACRNA

Workshop on Commissioning of Linear accelerators used in Radiotherapy

TABLE OF CONTENTS

IntroductionIonization chamber based dosimetryDetermination of dose using calibrated chambersBeam quality specificationCalibration of MV photon and electron beamsError and uncertainty analysis for ionization chambers

IAEACRNAWorkshop on Commissioning of Linear accelerators used in Radiotherapy

Modern radiotherapy relies on accurate dose delivery to the prescribed target volume.

ICRU recommends an overall accuracy in tumour dose delivery of 5%, based on: An analysis of dose response data. An evaluation of errors in dose delivery in a clinical setting.

Considering all uncertainties involved in the dose delivery to the patient, the 5% accuracy is by no means easy to attain.

INTRODUCTION


Accurate dose delivery to the target with external photon or electron beams is governed by a chain consisting of the following main links: Basic output calibration of the beam Procedures for measuring the relative dose data. Equipment commissioning and quality assurance. Treatment planning Patient set-up on the treatment machine.

INTRODUCTION


The basic output for a clinical beam is usually stated as: Dose rate for a point P in

G/min or Gy/MU.

At a reference depth zref(often the depth of dose maximum zmax).

In a water phantom for a nominal source to surface distance (SSD) or source to axis distance (SAD).

At a reference field size on the phantom surface or the isocentre (usually 10x10 cm2).

INTRODUCTION


Machine basic output is usually given in:

Gy/min for kilovoltage x-ray generators and teletherapy units.

Gy/MU for clinical linear accelerators.

This should be determined At a given distance from the source

and For a given nominal collimator or applicator setting.

INTRODUCTION


The basic output calibration for photon and electron beams is carried out with: Radiation dosimeters Special dosimetry techniques.

Radiation dosimetry refers to a determination by measure-ment and/or calculation of Absorbed dose

at a given point in the medium.

INTRODUCTION


Basic output calibration of a clinical radiation beam, by direct determination of dose or dose rate in water under specific reference conditions, is referred to as reference dosimetry.

Three types of reference dosimetry technique are known: Calorimetry Fricke (chemical, ferrous sulfate) dosimetry Ionization chamber dosimetry

INTRODUCTION


Calorimetric dosimetry is the most fundamental of all reference dosimetry techniques, since it relies on basic definition of either electrical energy or temperature. In principle, calorimetric dosimetry (calorimetry) is simple. In practice, calorimetric dosimetry is very complex because of

the need for measuring very small temperature differences.

This complexity relegates the calorimetry to sophisticated standards laboratories.

CalorimetryINTRODUCTION


Main characteristics of calorimetic dosimetry: Energy imparted to matter by radiation produces an increase in

temperature which is measured with thermocouples or thermistors.

T



The following simple relationship holds:

is the average dose in the sensitive volume is the thermal capacity of the sensitive volume is the thermal defect is the temperature increase

Note:

D = dE

dm=

CpT1

D

Cp T

T(water, 1 Gy) = 2.4 104 K



Ionizing radiation absorbed in certain media produces a chemical change in the media and the amount of this chemical change in the absorbing medium may be used as a measure of absorbed dose.

The best known chemical radiation dosimeter is the Fricke dosimeter which relies on oxidation of ferrous ions into ferric ions in an irradiated ferrous sulfate FeSO4 solution.

(Fe2+ ) (Fe3+ )

Fricke (chemical) dosimetryINTRODUCTION


Average absorbed dose in a Fricke solution is given as:

is the change in molar concentration of is the density of the Fricke solution. is the increase in optical density after irradiation. is the extinction coefficient. is the thickness of the solution. is the chemical yield of in mole/J.

D = M

G(Fe3+ )=

(O.D.) l G(Fe3+ )

= 278(O.D.)

M

(O.D.)

l+3(Fe )G Fe3+

Fe3+ .



Recommended G-values in molecule/100 eV Photon beams (ICRU 14)

Cs-137 15.32 MV 15.4Co-60 15.54 MV 15.55 MV to 10 MV 15.611 MV to 30 MV 15.7

Electron beams (ICRU 35)1 MeV to 30 MeV 15.7



Ionization chamber is the most practical and most widely used type of dosimeter for accurate measurement of machine output in radiotherapy.

It may be used as an absolute or relative dosimeter.

Its sensitive volume is usually filled with ambient air and: The dose related measured quantity is charge Q, The dose rate related measured quantity is current I,

produced by radiation in the chamber sensitive volume.

Ionization chamber dosimetryINTRODUCTION


Measured charge Q and sensitive air mass mair are related to absorbed dose in air Dair by:

is the mean energy required to produce an ion pair in air per unit charge e.

Currently, the value of for dry air is 33.97 eV/ion pair or 33.97 J/C.

airair

air

Q WDm e

=

air /W e

air /W e

Ionization chamber dosimetryINTRODUCTION


The subsequent conversion of the air cavity dose Dair to dose to medium (usually water) Dw is based on: Bragg-Gray cavity theory Spencer-Attix cavity theory

INTRODUCTIONIonization chamber dosimetry

Dmed = Dair smed,air


Reference dosimetry with ionization chambers

Clinical photon and electron beams are most commonly calibrated with ionization chambers that: Are used as relative dosimeters. Have calibration coefficients determined either in air or in water

and are traceable to a national primary standards dosimetry laboratory (PSDL).


Traceability of chamber calibration coefficient to a national PSDL implies that:1. Either the chamber was calibrated directly at the PSDL in terms of:

Air kerma in air Absorbed dose in water

2. Or the chamber was calibrated directly at an accredited dosimetry calibration laboratory (ADCL) or at secondary standards dosimetry laboratory (SSDL) that traces its calibration to a PSDL.

4. Or the chamber calibration coefficient was obtained through a cross-calibration with another ionization chamber, the calibration coefficient of which was measured directly at a PSDL, an ADCL oran SSDL.



Dosimetry protocols or codes of practice state the procedures to be followed when calibrating a clinical photon or electron beam.

Choice of which protocol to use is left to individual radiotherapy departments or jurisdictions.

Dosimetry protocols are generally issued by national, regional, or international organizations.




19871987

NK - based IAEA TRS 277

update ofTRS-277

IAEATRS-381

19971997ND,w - based IAEA

TRS 398

2000200019971997


IONIZATION CHAMBER BASED DOSIMETRY SYSTEMS Ionization chambers

Examples of typical ionization chambers:(a) Cylindrical chambers

used for relative dosimetry.(b) Pinpoint mini-chamber

and Co-60 buildup cap.(c) Farmer type cylindrical

chamber and cobalt-60buildup cap.

(d) Parallel-plate Roos typeelectron beam chamber.


USE OF CALIBRATED IONIZATION CHAMBERS Air kerma based protocols

Air kerma based protocols use the air kerma in air cali-bration coefficient NK,Co obtained for a local reference ionization chamber in a cobalt-60 beam at a standards laboratory.

Two steps are involved in air kerma based protocols for calibration of megavoltage photon and electron beams.

The cavity air calibration coefficient ND,air is determined from the air kerma in air calibration coefficient NK,Co.

Absorbed dose to water is determined using the Bragg-Gray relationship in conjunction with the chamber signal MQ and the cavity air calibration coefficient ND,air.



Calibration in a cobalt-60 beam at standards laboratory:Absorbed dose to air in the cavity Dair,Co is determined from the total air kerma in air (Kair)air as follows:

is the radiative fraction, i.e., the fraction of the total transferred energy expended in radiation interactions on slowing down of the secondary electrons in air.

km corrects for the non-air equivalence of the chamber wall and buildup cap needed for an air kerma in air measurement.

katt corrects for attenuation and scatter in the chamber wall. kcel corrects for non-air equivalence of chamber central electrode.

air,Co air air m att cel( ) (1 ) D K g k k k=

g



Under these special conditions, according to the B-G cavity theory, the dose to the medium Dmed is related to the dose to the cavity Dcav as:

is the ratio of the average unrestricted mass collision stopping powers medium to cavity.

The Spencer-Attix (S-A) cavity theory is more general and accounts for creation of secondary (delta) electrons. The dose to medium is given as:

is the ratio of the average restricted mass collision stopping powers medium to cavity.

Dmed = Dcav (smed.cav )

(smed.cav )

Dmed = Dcav (S / )med,cav

med,cav( / )S



With a known value of the cavity air calibration coefficient ND,air for a specific chamber, the chamber signal corrected for influence quantities MQ at a point in phantom allows determination of absorbed dose to water Dw,Q:

is the ratio of average restricted collision stopping powers of water to air for a radiation beam of quality Q.

pQ is the perturbation correction factor which corrects for effects that cause deviations from Bragg-Gray behaviour:

w,Q air,Q w,air Q Q Q D,air w,air Q Q( ) ( ) D D s p M N s p= =

w,air Q( )s

w,Q Q K m att cel w,air Q dis wall cel cav Q (1 ) ( ) ( )D M N g k k k s p p p p=


USE OF CALIBRATED IONIZATION CHAMBERS Absorbed dose to water based protocols

Calibration in a cobalt-60 beam at standards laboratory:

Recent developments have provided support for a change in the quantity used to calibrate ionization chambers and provide calibration coefficients in terms of absorbed dose to water at beam quality Qo .

At standards laboratory , absorbed dose to water at the reference depth zref in water for a reference beam Qo (usually cobalt-60) is known and used to determine the water dose calibration coefficient .

Dw,Qo

ND,w,Qo

ND,w,Qo


DETERMINATION OFABSORBED DOSE TO WATER

Using ND,w (TRS 298)

at any other user quality Q:

at the calibration quality Qo:

Dw,Qo =MQo ND,w,Qo

Dw,Q=MQ ND,w,Qo kQ,Qo

beamqualityfactor


Q,QoQw,D,QQw,p)airw,airD,QQw,

p)air,wQ,airQw,

Qair,D,Qair,D,

celmatt(Q,K

Q

Q,airQ,air,D

Q

Q,wQ,w,D

Q

Qo,airQ,K

Q,

kNM=D(sNM=D

s(D=D( )5N=N( )4

kkk)g-1N=

MD

=N( )3

MD

=NM

K=N( )2

oQQ

QQ

o

o

o

o

o

o

o

o

o

o

( )1 Q,wD oairK o

6

Steps in the determination of Dw at the reference point using the NK -ND,air formalism and the ND,w - formalism


kkQ,QoQ,Qo is given as a function of a quality index is given as a function of a quality index TPR20,10

Need to experimentally determine Need to experimentally determine TPR20,10

Beam Beam qquality uality sspecificationpecificationPhoton beamsPhoton beams


Beam Beam qquality uality sspecificationpecificationWhy TPR20,10 ?


Beam Beam qquality factoruality factor

photon beam quality (TPR20,10)0.55 0.60 0.65 0.70 0.75 0.80 0.85

k Q

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

LPRI (Delaunay)Palm NPL (Sharpe)NRC soft (Seuntjens)(BEL) Palmans ENEA (Guerra)PTB (Shortt) Andreo et al (IAEA CoP 2000)NRC soft (Shortt) NRC soft (Ross)fit to all experimental dataNRC hard (Ross)

NE 2571

Co-60

Andreo Phys Med Biol 45 (2000)




k Q

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

Palm NRC soft (Seuntjens)Vatnitsky Andreo et al (IAEA CoP 2000)fit to all experimental data excl NRC high-E soft

PTW 30001





k Q

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

1.01

PTB (Short) NRC soft (Ross)Palm NRC soft (Seuntjens) Vanitsky NRC soft (Shortt)ENEA (Guerra)Andreo et al (IAEA CoP 2000)NRC hard (Ross)fit to all experimental data

PR-06C



Beam Beam qquality uality sspecificationpecification

photon beams (High energy X-rays)TPR20,10 is given by a ratio of dose (ionization) at two

depths, for constant SCD, (independent of electron contamination)

Attenuation property


Beam Quality: determination of TPR20,10

10 g/cm2

20 g/cm2

SCD100 cm

10M

10

20

MM

10

201020 D

DTPR =,

20M

10 cm x 10 cm


SSD100 cm

Beam Quality: determination of TPR20,10

10M

10

20

MM

10

201020 D

DPDD =,

20M

20 g/cm2

10 g/cm2

20TPR

= 1.2661,10

1020PDD - 0.0595,

10 cm x 10 cm


M

MNN field

refrefD,w

fieldwD =,

Cross-calibration of field ionization chambers


The beam quality index is the half-value depth in water R50. This is the depth in water (in g/cm2) at which the absorbed dose is 50% of its value at the absorbed dose maximum.

SSD = 100 cm Field size at the phantom surface of at least 10 cm 10 cm for

R50 7 g/cm2 and at least 20 cm 20 cm for R50 > 7 g/cm2

The choice of R50 as the beam quality index is a change from the current practice of specifying beam quality in terms of the meanenergy at the phantom surface Eo.

As Eo = f(R50), this change in beam quality index is merely a simplification which avoids the need for a conversion to energy.

Beam Beam qquality uality sspecificationpecificationElectron beamsElectron beams



R50 = 1.029 R50,ion - 0.06 g/cm2 (R50,ion 10 g/cm2)

R50 = 1.059 R50,ion - 0.37 g/cm2 (R50,ion 10 g/cm2)



This depth has been shown to reduce significantly the influence of spectral differences between different accelerators as well as that of electron and photon contamination in clinical electron beams.


Electron beamsElectron beams

Plane parallel chamber cylindrical chamber



Cross Calibration of Ionisation Chambers

Problem: the water absorbed dose has to be determined in a beam with R50 4 g/cm2.

the Co-60 calibration factor of the plane-parallel chamber is not sufficiently reliable

Solution: Cross-calibration of the user plane-parallel chamber by direct comparison against a cylindrical reference chamber calibrated in a Co-60 gamma radiation


Plane parallel chamber

Zref

cylindrical chamber

@ Zref

refQQ

xQwDx

Q

refQx

QwD crosscross

cross

crosskN

MM

N00 ,,,,,

=

The highest energy electron beam available should be used; R50 > 7 g/cm2 (Eo 16 MeV) is recommended



xQQ

xQwD

xQQw crosscross

kNMD ,,,, =

kQ,Qcross(R50) are determined from Table 19 (TRS398)

xQQ

xQQx

QQ

cross

cross kk

kint

int

,

,, =



Experimental determination DExperimental determination Dw w (TRS 398)(TRS 398)Photon beamsPhoton beams


Experimental determination DExperimental determination Dw w (TRS 398)(TRS 398)Photon beamsPhoton beams


Influence quantity Reference value or reference characteristics

REFERENCE CONDITIONS FOR DE-TERMINATION OF DW IN HIGH-ENERGY PHOTONS

Phantom material water

Chamber type cylindrical

Measurement depth zref for TPR20,10 < 0.7, 10 g/cm2 (or 5 g/cm2)

for TPR20,10 0.7, 10 g/cm2

Reference point of chamber on central axis at centre of the cavity volume

Position of referencepoint of chamber

at the measurement depth zref

SSD / SCD 100 cm

Field size 10 cm x 10 cm


Reference conditions are the same as with the determination of R50

Positioning the chamber at reference depth

zref = 0.6 R50 0.1 g/cm2 (R50 in g/cm2)

Plane parallel chamber

Zref

cylindrical chamber

Note: zref may be deeper than zmax, cavity perturbation for cyl chamber may then be larger

REFERENCE CONDITIONS FOR DETERMINATION OF DW IN HIGH-ENERGY ELECTRONS


Determination of absorbed dose to water

Absorbed dose at zmaxD(zmax) determined from PDD or TMR distribution

max )(w refD D PDD Z= max )(w refD D TMR Z=

SSD SAD

Dw,Q (Zref) =MQ ND,w,Qo kQ,Qo

( ), , , ,cross crossx x x

w Q ref Q D w Q Q QD Z M N k=

PHOTONS

ELECTRONS


Corrections to the dosimeter reading, MQ

( )( )273.2273.2

oTP

o

T PkT P+

= +

pol

M Mk

2M+ +=

2

2

12

2

11

+

+=

MMa

MMaap os

Experimental determination DExperimental determination Dw w (TRS 398)(TRS 398)

Temperature and pressure

Polarity effect

Recombination (not all charge produced is collected)

0

0

20 101.325

T CP kPa= =

M1 V1 , M2 V2IAEACRNA


PRACTICAL CONSIDERATIONS (1)

0.994

0.995

0.996

0.997

0.998

0.999

1.000

1.001

0 5 10 15 20 25

Time (min)

Q/Q

max

WDIC70 # 141 WDIC70 # 039 PTW 30001 # 1245 NACP02 # 33.11 NE 2611 # 149

IAEA TECDOC in press

Stabilization time for ionization chambers



Stabilization time for a chamber and electrometer

0.995

0.997

0.999

1.001

1.003

1.005

0 10 20 30 40 50 60

Time (min)

Q/Q

max

IAEA TECDOC 1455



Temperature measurements in air and in water

In water ?

In cavity chamber ? In s

leeve

?

Free in

air ?



Temperature measurements in air and in water

IAEA TECDOC 1455



Phantom window bowing

IAEA CUBIC PHANTOM

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

-2 0 2 4 6 8 10 12 14Distance to the center of the window (cm)

Def

lect

ion

mm

filling removing

0

0.5

1

1.5

2

2.5

3

-14 -9 -4 1 6Distance to the center of the window (cm)

defle

ctio

n (m

m)

filling removing

TG51 PHANTOM



Effect of waterproofing sleeve thickness

Use the same sleeve used for chamber calibration


Use of IAEA Worksheet (HE x-rays)








These worksheets are available at:http://www-naweb.iaea.org/nahu/dmrp/codeofpractice.shtm

Use of IAEA Worksheet (HE electrons)


Estimated uncertainty in Dw (HE X-rays) (Q0 = 60Co)Physical quantity or procedure Rel. std uncertainty (%)

Step 1: Standards Laboratory ND,w calibration of secondary standard at PSDL 0.5Long term stability of secondary standard 0.1ND,w calibration of the user dosimeter at the standard laboratory 0.4Combined uncertainty of Step 1 0.6

Step 2: User high-energy photon beamLong-term stability of user dosimeter 0.3Establishment of reference conditions 0.4Dosimeter reading MQ relative to beam monitor 0.6Correction for influence quantities ki 0.4Beam quality correction MQ (calculated values) 1.0Combined uncertainty of Step 2 1.4

Combined standard uncertainty of Dw,Q (steps 1+2) 1.5


Estimated uncertainty in Dw (HE electrons) (Q0 = 60Co)

Physical quantity or procedure User chamber type :

Relative stand. uncertainty (%) Cyl PP

Step 1: Standards laboratory

ND,w calibration of secondary standard at PSDL 0.5 0.5

Long term stability of secondary standard 0.1 0.1 ND,w calibration of user dosimeter at SSDL 0.4 0.4

Combined uncertainty of step 1b 0.6 0.6

Step 2: User electron beamLong term stability of user dosimeter 0.3 0.4 Establishment of reference conditions 0.4 0.6

Dosimeter reading MQ relative to beam monitor 0.6 0.6

Correction for influence quantities ki 0.4 0.5 Beam quality correction kQ (calculated values) 1.2 1.7

Combined uncertainty of step 2 1.5 2.0

Combined standard uncertainty of Dw,Q (steps 1+2) 1.6 2.1


Estimated uncertainty in Dw (HE electrons) (Q0 = electrons)

Relative standard uncertainty (%)Physical quantity or procedure User chamber type: Cyl PP Step 1: PSDL

ND,w calibration of user dosimeter at PSDL 0.7 0.7

Combined uncertainty in step 1 0.7 0.7

Step 2: User electron beam

Long term stability of user dosimeter 0.3 0.4

Establishment of reference conditions 0.4 0.6

Dosimeter reading MQ relative to beam monitor 0.6 0.6

Correction for influence quantities ki 0.4 0.5

Beam quality correction kQ,Qo (calculated values) 0.9 0.6

Combined uncertainty in step 2 1.3 1.2

Combined standard uncertainty of Dw,Q (steps 1+2) 1.4 1.4

IAEACRNA

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A_Dosimetry Calibration Reference Dosimetry