beam quality correction factors for plane-parallel chambers in

November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium

Ralf-Peter Kapsch1, Igor Gomola2

1 Physikalisch-Technische Bundesanstalt, Braunschweig, Germany2 IBA Dosimetry, Schwarzenbruck, Germany

[email protected]

Beam quality correction factors forplane-parallel chambers in photon beams

Beam quality correction factors forplane-parallel chambers in photon beams


Introduction

, ,w Q Q D w QD M N k=


The beam quality correction factor kQ

• Cylindrical chambers:– Chamber type specific values

IAEA TRS-398

AAPM TG-51


The beam quality correction factor kQ

• Plane-parallel chambers:– No tabulated values

• Reason:

“Note that plane-parallel chambers are not included becausethere is insufficient information about wall correction factorsin photon beams other than 60Co.”

TG-51 (1999), Sec. IX B

“…, the lack of data for the wall correction factor pwall forplane-parallel chambers in high energy photon beams …makes these chambers inappropriate.“

TRS-398 (2000), Footnote 23

“For plane-parallel chamber types, pwall is problematic and variationsof up to 3% between chambers of the same type have been reported …“

TRS-398 (2000), II.2.3.3.

“A major problem with the use of … pwall for commercial plane parallelchambers is the finding, reported by several groups, that chambersof the same design can yield significantly different results …“

TRS-381 (1997), C.2.1.


Experimental determination of kQ

IBA PPC05 IBA PPC40

# 83, 111, 156, 157, 158, 159, 160, 161, 474, 475 # 342, 344, 507, 509, 510, 512, 808, 809, 810, 811

Reference:NE2561 #244 pp p ref repp f

,fp re

,QQ D w D w QQkM M N kN =

ref ref

pref,p

p pp

p,

QQ

Q

D w

DQ

w

M Nk

M Nk =

Method:


Step 1: Calibration in 60Co

refrefpp re

p,p,

fpp

QQ

D wQ

DQ w

Mk k

MNN

=

83 111

156

157

158

159

160

161

474

475

0.54

0.55

0.56

0.57

0.58

0.59

0.60

0.61

0.62 Calibration in 2009 Calibration in 2005

serial number of chamber

calib

ratio

n fa

ctor

ND

,w

Gy/nC

-6%

-4%

-2%

0%

2%

4%

6%

rela

tive

chan

ge o

f cal

ibra

tion

fact

or

83 111

156

157

158

159

160

161

474

475

0.54

0.55

0.56

0.57

0.58

0.59

0.60

0.61



calib

ratio

n fa

ctor

ND

,w

Gy/nC

-6%

-4%

-2%

0%

2%

4%

6%

rela

tive

chan

ge o

f cal

ibra

tion

fact

orPPC05


Step 2: Calibration in high-energy photon beams

4 MV8 MV

25 MV

SSD: 100 cmFS: 10 cm ×

10 cm

z: 10 cm

refpp ref,

f

,

r

pp p

e

pD w

QQ

QD

Q

w

MM

Nk k

N=

0 5 10 15 20 25 30 35 40 45 50 55-2.0 %

-1.5 %

-1.0 %

-0.5 %

0.0 %

0.5 %

rela

tive

varia

tion

of I

PP

C05

/ I m

onito

r

Time since beginning of measurement

U = +100 V U = -100 V

min

PPC05-83, 25 MV

0 5 10 15 20 25 30 35 40 45 50 55-2.0 %

-1.5 %

-1.0 %

-0.5 %

0.0 %

0.5 %

rela

tive

varia

tion

of I

PP

C05

/ I m

onito

r

Time since beginning of measurement

U = +100 V U = -100 V

min

PPC05-83, 25 MV


Step 3: Calculation of kQ

ref refpp ,

pref

p pp,

Q D wQ Q

Q D w

M Nk

M Nk=


Results

83 111 156 157 158 159 160 161 474 4750.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

25 MV

8 MV

kQ

Serial number of chamber

4 MV

PPC05

4 MV 8 MV 25 MV

Relative span 0.51 % 0.64 % 0.65 %

Relative std. dev. 0.19 % 0.22 % 0.18 %


342 344 507 509 510 512 808 809 810 8110.93

0.94

0.95

0.96

0.97

0.98

0.99

1.00

25 MV

8 MV

Serial number of chamber

kQ

4 MV

Results

PPC40

4 MV 8 MV 25 MV

Relative span 0.59 % 0.43 % 0.70 %

Relative std. dev. 0.17 % 0.16 % 0.25 %

M.R. McEwen, S. Duane, R.A.S. Thomas,IAEA-Symposium 2002:

• 3 Roos chambers, 6 MV … 19 MV ΔkQ ≈

0.8 %


Estimation of uncertaintyref ref

,pp refpp pp

,

Q D wQ Q

Q D w

M Nk k

M N=

Quantityref,pp,

D w

D w

NN

ref

ppQ

Q

MM

refQk

Std. uncertainty

0.21 %

0.14 %

1.0 %(IAEA TRS-398)

0.26 %

1.04 %


Summary and Conclusion

Here:

variations of less than 0.7 %Here:

variations of less than 0.7 %


Thank You!


Calibration factors of the plane-parallel chambers

PPC05

PPC40

83 111

156

157

158

159

160

161

474

475

0.54

0.55

0.56

0.57

0.58

0.59

0.60

0.61



calib

ratio

n fa

ctor

ND

,w

Gy/nC

-6%

-4%

-2%

0%

2%

4%

6%

rela

tive

chan

ge o

f cal

ibra

tion

fact

or

83 111

156

157

158

159

160

161

474

475

0.54

0.55

0.56

0.57

0.58

0.59

0.60

0.61



calib

ratio

n fa

ctor

ND

,w

Gy/nC

-6%

-4%

-2%

0%

2%

4%

6%

rela

tive

chan

ge o

f cal

ibra

tion

fact

or

342

344

507

509

510

512

808

809

810

811

80

82

84

86

88

90

92


calib

ratio

n fa

ctor

ND

,w

Gy/µC

-6%

-4%

-2%

0%

2%

4%

6%

rela

tive

chan

ge o

f cal

ibra

tion

fact

or

342

344

507

509

510

512

808

809

810

811

80

82

84

86

88

90

92


calib

ratio

n fa

ctor

ND

,w

Gy/µC

-6%

-4%

-2%

0%

2%

4%

6%

rela

tive

chan

ge o

f cal

ibra

tion

fact

or


Polarity effect of the plane-parallel chambers

PPC05

PPC40

83 111

156

157

158

159

160

161

474

475

0.994

0.995

0.996

0.997

0.998

0.999

1.000

1.001

1.002

1.003

60Co 25 MV

k pol


83 111

156

157

158

159

160

161

474

475

0.994

0.995

0.996

0.997

0.998

0.999

1.000

1.001

1.002

1.003

60Co 25 MV

k pol


342

344

507

509

510

512

808

809

810

811

0.9995

1.0000

1.0005

1.0010

1.0015

60Co 8 MV

k pol


342

344

507

509

510

512

808

809

810

811

0.9995

1.0000

1.0005

1.0010

1.0015

60Co 8 MV

k pol



External monitor chamber


Performance of the external monitor chamber

0 10 20 30 40 50 60 70 80 90 100 110

0.0 %

0.2 %

0.4 %

0.6 %

0.8 %re

lativ

e de

viat

ion

of th

e ra

tio o

f det

ecto

r sig

nals

time since beginning of measurementmin

FC65-G / internal monitor of linac

FC65-G / improved monitor

Kapsch, R.-P., Krauss, A.,World Congress on Medical Physics and Biomedical EngineeringIFMBE Proc. 25/I (2009), 85-88

Kapsch, R.-P., Krauss, A.,World Congress on Medical Physics and Biomedical EngineeringIFMBE Proc. 25/I (2009), 85-88


kQ for cylindrical chambers

PTW 30013 PTW 31013

2938 2939 2940 2941 2942 2943 2944 2945 2946 29480.955

0.960

0.965

0.970

0.975

0.980

0.985

k Q

Serial number

8 MV 16 MV

0.9628

0.9794

1191 1192 1193 1194 1195 1196 1197 1198 1199 12000.955

0.960

0.965

0.970

0.975

0.980

0.985

k Q

Serial number

8 MV 16 MV

0.9629

0.9777

beam quality correction factors for plane-parallel chambers in

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