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November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
Ralf-Peter Kapsch1, Igor Gomola2
1 Physikalisch-Technische Bundesanstalt, Braunschweig, Germany2 IBA Dosimetry, Schwarzenbruck, Germany
ralf-peter.kapsch@ptb.de
Beam quality correction factors forplane-parallel chambers in photon beams
Beam quality correction factors forplane-parallel chambers in photon beams
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
The beam quality correction factor kQ
• Cylindrical chambers:– Chamber type specific values
IAEA TRS-398
AAPM TG-51
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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.
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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,
Q
D w
DQ
w
M Nk
M Nk =
Method:
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
Step 1: Calibration in 60Co
refrefpp re
p,p,
fpp
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
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
orPPC05
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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
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
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
Step 3: Calculation of kQ
ref refpp ,
pref
p pp,
Q D wQ Q
Q D w
M Nk
M Nk=
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
Step 3: Calculation of kQ
ref refpp ,
pref
p pp,
Q D wQ Q
Q D w
M Nk
M Nk=
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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 %
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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 %
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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 %
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
Summary and Conclusion
Here:
variations of less than 0.7 %Here:
variations of less than 0.7 %
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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
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
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
342
344
507
509
510
512
808
809
810
811
80
82
84
86
88
90
92
serial number of chamber
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
serial number of chamber
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
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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
serial number of chamber
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
serial number of chamber
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
serial number of chamber
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
serial number of chamber
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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
November 2010Ralf-Peter Kapsch (PTB) IAEA Symposium
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