implementation of the iaea-aapm code of practice for the …€¦ · · 2018-05-07implementation...

M. Saiful Huq, PhD, FAAPM, FInstP

Dept. of Radiation Oncology, University of Pittsburgh Cancer Institute and UPMC Hillman Cancer Center, Pittsburgh, Pennsylvania, USA

Implementation of the IAEA-AAPM Code of Practice for the dosimetry of small

static fields used in external beam radiotherapy

Collaborators

• Yongqian Zhang, Ph.D.

• Min-sig Huang, Ph.D.

• Troy Teo, Ph.D.

• Kevin Fallon, M.S.

• Cihat Ozhasoglu, M.S.

• Ron Lalonde, Ph.D.

Contributors to IAEA TRS-483

Missing: Ahmed Meghzifene and Stan Vatnitsky

• The Code of Practice addresses the reference and

relative dosimetry of small static fields used for

external beam photon radiotherapy of energies with

nominal accelerating potential up to 10 MV. It does

not address other radiotherapy modalities such as

electron, proton and orthovoltage beams

TRS-483 CoP

• It provides a CoP for machine specific reference (msr)

dosimetry in a clinical high energy photon beams. It is

based on the use of a ionization chamber that has been

calibrated in terms of absorbed dose to water ND,w,Qo or

ND,w,Qmsr in a standard’s laboratory’s reference beam of

quality Qo or Qmsr.

• It also provides guidance for measurements of field

output factors and lateral beam profiles at the

measurement depth

TRS-483 CoP

• Radiation generators: 10 cm x 10 cm field can be set

• Follow TRS-398 CoP or AAPM TG-51 or equivalent protocol

• Radiation generators: 10 cm x 10 cm ref field cannot be

set

• Define machine specific reference (msr) field, fmsr

• Dimension of fmsr field

• Should be as close as possible to the conventional reference

field

Reference dosimetry: msr field

Machine type msr field CyberKnife 6 cm diameter fixed collimator

TomoTherapy 5 cm x 10 cm field

GammaKnife 1.6 cm or 1.8 cm diameter collimator helmet, all sources simultaneously out

BrainLab microMLC add on For example 9.8 cm x 9.8 cm or 9.6 cm x 10.4 cm

SRS cone add-ons The closest to a 10 cm x 10 cm equivalent square msr field achievable

• fmsr should extend at least a distance rLCPE beyond the

outer boundaries of the reference ionization chamber

FWHM ≥ 2rLCPE + d

msr fields for common radiotherapy machines

• How were the field sizes for the msr dosimetry

arrived at?

• Ask: What is the size restriction on an ionization

chamber for msr dosimetry?

msr fields: selection of chambers

• CPE conditions exist when one of the edges of the field extends at least a distance rLCPE beyond the outer boundaries of the ionization chamber. If the size of the detector is d, the FWHM of the field has to fulfil the condition:

FWHM ≥ 2 rLCPE + d

rLCPE (in cm) = 0.07797•%dd(10,10)x – 4.112

rLCPE (in cm) = 8.369•TPR20,10(10) – 4.382 d

rLCPE

CPE condition for field size

• Consider a 6 MV beam. It’s TPR20,10(10) = 0.677 • rLCPE = 8.369 0.677 – 4.382 = 12.8 mm • PTW 30013 Farmer type chamber:

• cavity length l = 23mm • cavity radius r = 3.1 mm • wall thickness twall= 0.057 g/cm2 • With ρ (PMMA) = 1.19 g/cm3, twall= 0.48mm • In the longitudinal direction, the chamber outer size will be dl = l + twall =

23.48 mm (say 23.5mm) • In the radial direction dr = 2( r + twall) = 7.2mm • As dl > dr, the largest detector size is dl

• Eq. for FWHM yields a FWHM = 2 x 12.8 + 23.5 = 49.1 mm

For a PTW 30013 chamber: FWHM ≥ 4.9 cm

Example

• Chambers must meet specifications for reference class

ionizations chambers. Table 3 in the CoP

• Refers to chamber settling time, polarity effect, leakage,

recombination correction, chamber stability, chamber material

• fmsr ≥ 6 cm x 6 cm

• Chambers listed in Table 4 meet this criteria


• Farmer type chambers: • WFF beams: Farmer type chambers listed in Table 4 meets this

criteria

• FFF beams use a chamber with a length shorter than the length of Farmer type chambers

• If you have to use a Farmer type chamber a correction for the non-uniformity of the beam profile should be used. For 6 MV beam this can be about 1.5%


• For field sizes smaller than 6 cm x 6 cm similar

analysis led to the chambers listed in Table 5

(including Gamma Knife)

• These are chambers with volumes smaller than 0.3

cc (chamber length 7 mm)


• For reference dosimetry in msr fields, you will need to determine “equivalent square msr field” sizes. For non-square field sizes, the corresponds to the field for which the phantom scatter is the same.

• Tables 15-17 tells you how to do this. This is needed to calculate TPR20,10(10) or %dd(10,10)x using Palman’s equation.

Equivalent square msr field sizes

15

Table 15 (Tables 16 & 17 are for FFF beams)

• Choice of an appropriate detector for small field dosimetry measurements depends on the parameter to be measured.

• Note: NO ideal detector exists for measurements in small fields

• Use two or three different types of suitable detectors so that redundancy in results can provide more assurance that no significant errors in dosimetry are made

Relative dosimetry: Detectors

• Assume that detectors used for large field dosimetry will not perform well in small fields

• Ion chambers: major issues are volume averaging and substantial perturbations in the absence of LCPE, signal to background ratio for small volume ionization chambers

• Below certain field sizes, volume averaging effects become unacceptably large. Below these field sizes only liquid ion chamber and solid state detectors are suitable for dosimetry, but even those exhibit substantial perturbations for the smallest field sizes

Relative dosimetry: Detectors

• Output correction factors are given as a function of

the size of the square fields. For non-square fields,

one determines a “equivalent square small field” for

which output corrections are the same

Output correction factor

• Field output correction factors are given as a

function of Collimator setting for CyberKnife and

Gamma Knife (Tables 23 and 25) and as a function of

“equivalent square” for Tomotherapy, MLC and SRS

cones for 6 and 10 MV beams in Tables 24, 26 and 27

Output correction factor

Table 23: Output correction factors for CyberKnife

Table 26: Output correction factors for 6MV in WFF and FFF beams

• For relative dosimetry ensure placement of the

detector in the center of the radiation field

Practical considerations

• Correct and incorrect orientations of detectors

for measurements of beam profile

Practical considerations

• The absorbed dose to water at the reference depth zref in water

for the fmsr field in a beam of quality as Qmsr and in the absence

of the ionization chamber is given by:

• is the chamber reading corrected for influence quantities

• is the absorbed dose to water calibration coefficient of the chamber at beam quality Qmsr

msr

msr

msr

msr

msr

msr

fQwD

fQ

fQw NMD ,,, ⋅=

Chamber calibrated specifically for the msr field

Formalism: Preferred option

• The absorbed dose to water at the reference depth zref in water for the fmsr field

in a beam of quality as Qmsr and in the absence of the ionization chamber is given

by:

• is the chamber reading corrected for influence quantities • is the absorbed dose to water calibration coefficient of the chamber at

beam quality Q0 in the ref field fref = 10x10 cm2

• is a correction factor that accounts for the differences between the

response of an ionization chamber in the field fref and beam quality Qo and the field fmsr and beam quality Qmsr

Chamber calibrated in a conventional reference field and generic values of beam quality corrections factors available

Formalism: Option b

Table 12: vs %dd(10,10)x and TPR20,10(10) for WFF beams

Table 13: vs% dd(10,10)x and TPR20,10(10) for FFF beams

Table 14: for GammaKnife

Beam quality

TPR20,10(S) %dd(10,S)x

• Equations for beam quality in non-standard reference fields

(Palmans 2012 Med Phys 39:5513)

0.55

0.60

0.65

0.70

0.75

0.80

0.85

2 4 6 8 10 12

s / cm

TP

R20

,10(

s)

(b)4 MV

10 MV8 MV

6 MV

5 MV

25 MV21 MV18 MV15 MV12 MV

Beam quality

C=(16.15 ± 0.12) x 10 -3

• Field output factor relative to reference field (ref stands here for

a conventional reference or msr field)

where is the so-called output correction factor, which

can be determined as a directly measured value, an

experimentally generic value or a Monte Carlo calculated

generic value

Field output factor

• Method 1: Field output factor relative to msr field is given by

• Method 2: Field output factor relative to msr field using

intermediate field or ‘daisy chaining’ method

where

Assumed to be unity

Field output factor

Field output correction factor

Presenter

Presentation Notes

Data from Francesco. Just an illustration.

34

Field output correction factor

• Rectangular small fields with uneven in-plane and cross-plane FWHM, the equivalent square field size is given by

Sclin = √ A.B 0.7 < A/B < 1.4 • For circular small fields with FWHM radius r Sclin = r√ π = 1.77r

Equivalent square field size

Application of TRS-483

What did we measure?

• TPR20,10(10) and %dd(10)x in msr fields for a 6MV

beam based on measurements in different field sizes

in a TrueBeam STx linac

• Polarity effect in a GammaKnife Perfexion machine

• Reference and relative dosimetry (Field output

factors) in GammaKnife Perfexion, CyberKnife M6

with Incise MLC and TrueBeam STx machines

Measured …..

msr Field Size

(S)(cm2)

Measured TPR20,10 (S)

Calculated TPR20,10 (10)*

3x3 0.631 0.669 4x4 0.634 0.666 6x6 0.645 0.666 8x8 0.656 0.667

10x10 0.667 0.667

* Measured using CC13 chamber. Calculated using Palman’s equation

0.6

0.62

0.64

0.66

0.68

0.7

3x3 4x4 6x6 8x8 10x10

TPR

20,1

0

Field Size (cm2)

Calculated TPR20,10 (10)Measured TPR20,10 (S)

TPR20,10(10) for 6 MV beam

* Measured using CC13 chamber. Calculated using Palman’s equation

%dd(10,10)x for 6 MV beam

msr Field Size (cm2)

Measured %dd (S)

Calculated %dd (10,10)X *

3x3 60.48 65.79

4x4 61.71 66.15

6x6 63.80 66.65

8x8 65.34 66.76

10x10 66.54 66.54

60

62

64

66

68

70

3x3 4x4 6x6 8x8 10x10

% d

d

Field Size (cm2)

Calculated %dd (10,10)X

Measured %dd (S)

• Palman’s equation accurately calculates TPR20,10(10) or

%dd(10,10)x from measured values of of TPR20,10(s) and

%dd(10,s) for various field sizes

Conclusion on Beam quality index measurements

Polarity effect

Co-60 beam 16 mm collimator

3 Exradin A16 chamber, 1 Exradin A14 chamber, 1 PTW 31016 chamber, 2 Capintec PR05P chamber

V (volts)

Pola

rity

fact

or

0.97

0.975

0.98

0.985

0.99

0.995

1

1.005

1.01

0 100 200 300 400 500 600 700 800

A16SN100075

A16SN040907

A16SN031113

PTW31016

A14

PR05P9546

PR05P7837

Polarity effect

V (volts)

Reference dosimetry

GammaKnife® - IconTM

Chamber Serial number

Dose rate TRS-483

Dose rate TRS-398

Difference (%)

Exradin A16 040907 3.235 3.177 1.8

Exradin A16 092725 3.230 3.171 1.8

Gamma Knife (Icon)

CyberKnife M6TM InciseTM MLC

Chambers Dw(zmax)/MU TRS-483 (cGy/MU)

Dw(zmax)/MU TRS-398 (cGy/MU)

Ratio TRS-398TRS-483

Exradin A12

1.016 1.009 0.993

PTW 30013 Sl. No. 1551

1.014

1.010

0.996

PTW 30013 Sl. No. 0262

1.018

1.013

0.995

PTW 30013 Sl. No. 0343

1.017

1.012

0.995

PTW 30013 Sl. No. 0905

1.024

1.019

0.995

Avg ± sd 1.018 ± 0.4% 1.012 ± 0.4%

CyberKnife : 6X FFF

TrueBeamTM STx

Chamber-Serial #

Mcorrected (nC)

ND,w

109(Gy/C) %dd(10,10)x

Dw/MU at zmax

TPR20,10(10)

Dw/MU at zmax

Dw (%dd)x/Dw(TPR)

PTW 30013-1551

14.46

5.397

66.38

0.991

1.007

0.668

0.993

1.009

0.998

PTW 30013-0262

14.57

5.343

66.38

0.991

1.004

0.668

0.993

1.006

0.998

PTW 30013-0905

14.54

5.37

66.38

0.991

1.008

0.668

0.993

1.010

0.998

EXR A12 -020581

15.75 4.915 66.38 0.995 1.002 0.668 0.995 1.003 0.999

TRS-483 : 6X

Chamber-Serial #

Mcorrected (nC)

ND,w

109(Gy/C) %dd(10,10)x

Dw/MU at zmax

TPR20,10(10)

Dw/MU at zmax

Dw (%dd)x/Dw(TPR)

PTW 30013-1551

13.77

5.397

63.89

0.995

1.007

0.632

0.995

1.007

1.000

PTW 30013-0262

13.88

5.343

63.89

0.995

1.005

0.632

0.995

1.005

1.000

PTW 30013-0905

13.85

5.37

63.89

0.995

1.008

0.632

0.995

1.008

1.000

EXR A12 -020581

14.98 4.915 63.89 0.998 1.001 0.632 0.998 1.001 1.000

TRS-483 : 6X FFF

Chamber-Serial #

Mcorrected (-nC)

ND,w

109(Gy/C) %dd(10,10)x

Dw/MU at zmax

TPR20,10(10)

Dw/MU at zmax

Dw (%dd)x/Dw(TPR)

PTW 30013-1551

15.97

5.397

73.15

0.979

1.004

0.740

0.980

1.004

0.999

PTW 30013-0262

16.13

5.343

73.15

0.979

1.004

0.740

0.980

1.004

0.999

PTW 30013-0905

16.05

5.37

73.15

0.979

1.004

0.740

0.980

1.004

0.999

EXR A12 -020581

17.33 4.915 73.15 0.985 0.998 0.740 0.986 0.999 0.999

TRS-483 : 10X

Chamber-Serial #

Mcorrected (-nC)

ND,w

109(Gy/C) %dd(10,10)x

Dw/MU at zmax

TPR20,10(10)

Dw/MU at zmax

Dw (%dd)x/Dw(TPR)

PTW 30013-1551

15.33

5.397

71.39

0.985

1.005

0.707

0.987

1.007

0.998

PTW 30013-0262

15.48

5.343

71.39

0.985

1.005

0.707

0.987

1.007

0.998

PTW 30013-0905

15.38

5.37

71.39

0.985

1.004

0.707

0.987

1.005

0.998

EXR A12 -020581

16.60 4.915 71.39 0.991 0.997 0.707 0.992 0.998 0.999

TRS-483 : 10X FFF

Chamber-Serial #

TPR20,10(10) Dw/MU at zmax

Dw(TRS398)/ Dw(TRS483)

PTW 30013-1551

0.668

0.992

0.993

1.008

1.009

0.999

PTW 30013-0262

0.668

0.992

0.993

1.005

1.006

0.999

PTW 30013-0905

0.668

0.992

0.993

1.008

1.010

0.999

EXR A12 -020581

0.668 0.995 0.995 1.003

1.003 1.000

TRS-483 vs TRS-398 : 6X

Chamber-Serial #



PTW 30013-1551

0.632

0.996

0.995

1.008

1.007

1.001

PTW 30013-0262

0.632

0.996

0.995

1.006

1.005

1.001

PTW 30013-0905

0.632

0.996

0.995

1.009

1.008

1.001

EXR A12 -020581

0.632 0.998 0.998 1.001

1.001 1.000

TRS-483 vs TRS-398 : 6X FFF

Chamber-Serial #



PTW 30013-1551

0.740

0.980

0.980

1.004

1.004

1.000

PTW 30013-0262

0.740

0.980

0.980

1.004

1.004

1.000

PTW 30013-0905

0.740

0.980

0.980

1.004

1.004

1.000

EXR A12 -020581

0.740 0.986 0.986 0.999 0.999 1.000

TRS-483 vs TRS-398 : 10X

Chamber-Serial #



PTW 30013-1551

0.707

0.987

0.987

1.006

1.007

0.999

PTW 30013-0262

0.707

0.987

0.987

1.006

1.007

0.999

PTW 30013-0905

0.707

0.980

0.980

1.005

1.005

0.999

EXR A12 -020581

0.707 0.98 0.987 0.997 0.998 0.999

TRS-483 vs TRS-398 : 10X FFF

Relative dosimetry

Field output factor

CyberKnife

TrueBeam STx

61

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors

Equivalent square field size, Sclin (cm)

Sun Nuclear EdgePTW 60017

6X

mean = 0.713 ∆uncorr = - 2%

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs



6X

mean = 0.728

TrueBeam STx : 6X

62

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



6X (IMF)

mean = 0.715 ∆uncorr = - 2%

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs



6X

mean = 0.728

TrueBeam STx : 6X

63

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors


UncorrectedCorrectedCorrected (IFM)

6X, PTW60017

0.5

0.55

0.6

0.65

0.7

0.75

0 0.5 1 1.5 2

Fiel

d ou

tput

fact

ors


UncorrectedCorrectedCorrected (IF)

6X, PTW60017

mean = 0.591 ∆uncorr = - 3.6%

mean = 0.714 ∆uncorr = - 0.9%

TrueBeam STx : 6X

64

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs



6X FFF

mean = 0.743

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



6X FFF

mean = 0.728 ∆uncorr = - 2%

TrueBeam STx : 6XFFF

65

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs



6X FFF

mean = 0.743

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



6X FFF (IFM)

mean = 0.732 ∆uncorr = - 2%


66

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



6XFFF, PTW60017

0.5

0.55

0.6

0.65

0.7

0.75

0 0.5 1 1.5 2

Fiel

d ou

tput

fact

ors



6XFFF, PTW60017

mean = 0.607 ∆uncorr = - 3.5%

mean = 0.730 ∆uncorr = - 0.6%


67

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs


Sun Nuclear Edge

PTW 60017

10X

mean = 0.692

0.5

0.6

0.7

0.8

0.9

1.0

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



10X

mean = 0.677 ∆uncorr = - 2%

TrueBeam STx : 10X

68

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs


Sun Nuclear Edge

PTW 60017

10X

mean = 0.692

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



10X (IFM)

mean = 0.678 ∆uncorr = - 2%

TrueBeam STx : 10X

69

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



10X, PTW60017

0.5

0.55

0.6

0.65

0.7

0.75

0 0.5 1 1.5 2

Fiel

d ou

tput

fact

ors



10X, PTW60017

mean = 0.506 ∆uncorr = - 3.9%

mean = 0.675 ∆uncorr = - 1.5 %

TrueBeam STx : 10X

70

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs


PTW 60017

10XFFF

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors


PTW 60017

10X FFF

TrueBeam STx : 10X FFF

71

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Unc

orre

cted

ratio

of r

eadi

ngs


PTW 60017

10XFF

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors


PTW 60017

10X FFF (IFM)


72

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10

Fiel

d ou

tput

fact

ors



10XFFF, PTW60017

0.5

0.55

0.6

0.65

0.7

0.75

0 0.5 1 1.5

Fiel

d ou

tput

fact

ors



10XFFF, PTW60017

mean = 0.506 ∆uncorr = - 3.9%

mean = 0.675 ∆uncorr = - 1.5 %


• For the GammaKnife msr beam, differences in references dosimetry using TRS-483 and TRS-398 can be up to 2% assuming depth scaling is taken into consideration

• For linac WFF and FFF beams, the values of Dw/MU following TRS-483 are consistent within better than 1% with those obtained using TRS-398

• The small field dosimetry of certain msr (reference) and most relative (using field output factor) beams can be significantly improved when the correction factors or different detectors included in TRS-483 are appropriately incorporated into their dosimetry

Summary

implementation of the iaea-aapm code of practice for the …€¦ · · 2018-05-07implementation...

Documents