direct calibration of ion chambers in linac electron beams
TRANSCRIPT
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Direct calibration of ion chambers in linac electron beamsMalcolm McEwen & Carl Ross
Workshop on Absorbed Dose and Air Kerma Primary Standards
LNE‐LNHB, Paris9‐11 May 2007
Ionizing Radiation Standards National Research Council
Ottawa, Canada
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Outline of project
• Aim - to obtain absorbed dose calibration coefficients for ion chambers in megavoltage electron beams from a clinical linac
• Follow on from photon beam measurements completed in 2005
In general, electrons are more troublesome
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Water calorimetry - electrons
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Water calorimetry - electrons
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Water calorimetry - electrons
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Water calorimetry - electrons
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Water calorimetry - electrons
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Issues for calorimetry
LINAC - Performance of monitor chamber
CALORIMETER – Vessel geometry
CHAMBER – Type, Ion recombination
Most of the time in calorimetry is spent measuring ΔT
But there are a number of other factors:
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• All clinical linacs use a multi-element ion chamber to control output – doserate, stability, uniformity
• Any variability in the performance of this ion chamber will increase the uncertainty in dose measurements
• Short-term very good –standard deviation on a set of 100 MU runs is 0.06%
• Need ± 0.1% stability over the course of a day –generally meet this requirement, worst case drift ~ 0.3% over 8 hours
Monitor reproducibility – within day
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Monitor reproducibility – day-to-day
Linac monitor
0.9900
0.9950
1.0000
1.0050
1.0100
1.0150
13-Apr-06 3-May-06 23-May-06 12-Jun-06 2-Jul-06 22-Jul-06 11-Aug-06
22 MeV
18 MeV
12 MeV
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Monitor reproducibility – day-to-day
Linac monitor
0.9900
0.9950
1.0000
1.0050
1.0100
1.0150
13-Apr-06 3-May-06 23-May-06 12-Jun-06 2-Jul-06 22-Jul-06 11-Aug-06
22 MeV
18 MeV
12 MeV
Use NE2571 chamber mounted on Linac head to normalise day-to-day variations
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Comparison of vessels
1. Standard parallel-plate vessel
2. Sealed-glass cylindrical vessel
3. Angled port vessel
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Vessel perturbation
Perturbation
Two methods investigated:
Dummy vessel
Measurements with glass plate
Two effects involved:
i) shift in reference depth
ii) fluence perturbation
Notes:
1. Dummy vessel can only be
used for standard parallel vessel
2. Glass plate can be used to look
at effect of different walls
separately
3. Method was used successfully
for photon beams
To compare vessels need to determine vessel perturbation and heat conduction corrections
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Vessel perturbation
Back and side walls have no measurable effect for NRC vessels:
0.998
1.000
1.002
1.004
1.006
1.008
1.010
1.012
1.014
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
d (glass - diode) cm
back
scat
ter d
ose
(nor
mal
ised
)
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Vessel perturbation
Front wall effect varies with wall thickness and position relative to detector:
0.9960
0.9980
1.0000
1.0020
1.0040
1.0060
1.0080
1.0100
1.0120
1.0140
1.0160
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
d (glass - diode) cm
rela
tive
dose
1.6 mm plate
1 mm plate
"scaled from 1.6 mm"
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kp factors for different vessels
Enom
(MeV)Vessel kp
(full)kp
(ddose)
12 Angled 1.0049
0.99771.00081.00251.0029
18AngledStandard
1.0000
1.0015
22Standard
Cyl1.0032
Get reasonable consistency between dummy vessel and plate methods
Depth-dose correction cannot correctly distinguish between vessels
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Comparison of vessels - results
1. Standard parallel-plate vessel compared to sealed-glass cylindrical vessel (22 MeV):
parallel-plate/cylindrical = 0.9994
2. Standard parallel-plate vessel compared to angled-probe vessel (18 MeV):
parallel-plate/angled probe = 0.9996
Standard uncertainty on calorimeter ratio ~ 0.2%
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Ion recombination
0.9950
1.0000
1.0050
1.0100
1.0150
1.0200
1.0250
1.0300
1.0350
0.000 0.005 0.010 0.015 0.020 0.025 0.030
1/V
1/R
NACPPTW Roos
Some non-linearity above 150 V
Chambers operated at 100 V
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Uncertainties
SourceSource
Calorimeter reproducibility
Chamber measurements
Linac stability
Calorimeter corrections
Chamber corrections
Overall standard uncertaintyOverall standard uncertainty
Standard Uncertainty (%)Standard Uncertainty (%)
0.26
0.1
0.05
0.24
0.14
0.40.4
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ND,w factors
• Measurements carried out in 3 electron beams –12, 18, 22 MeV
• 10 cm x 10 cm field
• Doserate = 250 MU/min
• 3 chamber types – NACP-02, PTW Roos, NE2571
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ND,w factors for NACP-02
Enom
(MeV)R50,D
(cm)ND,w
(cGy/nC)ND,w /
ND,w,TG‐51
12 4.8 16.29
15.85
15.75
18 7.1
1.016
1.006
22 9.0 1.009
Consistent difference between calorimeter and TG-51?
Data for NE2571 also shows similar difference (~ 1%)
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Comparison with other results
R50,D (cm)
3 4 5 6 7 8 9 10
Dos
e cal/D
ose TG
-51
1.004
1.006
1.008
1.010
1.012
1.014
1.016
1.018
NRC - water calorimeterNPL - graphite calorimeter
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A comment on chamber perturbation
Enom
(MeV)R50,D
(cm)ND,w /
ND,w,TG‐51
12 4.8
18 7.1
1.016
1.006
22 9.0 1.009
Can this data say anything about pQ of chambers?
Absolute ratio – need uncertainty in dose:
1. Calorimeter = 0.4%
2. TG-51 ~ 1% (kecal dominates)
3. Combined ~ 1.1%
Energy dependence – need uncertainty in
kQ ratio:
1. Calorimeter – very little is correlated between
energies (0.35%)
2. TG-51 – basically k’R50 (0.2% - 0.3%)
3. Combined ~ 0.45%
=> Can’t really say much at this stage
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A comment on chamber perturbation
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A comment on chamber perturbation
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A comment on chamber perturbation
Enom
(MeV)R50,D
(cm)ND,w /
ND,w,TG‐51
12 4.8
18 7.1
1.010
1.002
22 9.0 1.005
Apply results for Varian to NRC Elekta:
All values > unity
Indicates that pQ(NACP) may be greater
than ~ 0.5% calculated by MC?
Or larger linac-to-linac variations?
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Chamber perturbation contd.
NACP/Roos ratio in water
0.9960
0.9970
0.9980
0.9990
1.0000
1.0010
1.0020
0 2 4 6 8 10
R50,d (cm)
Nor
mal
ised
var
iatio
n
NPL data (2001)NRC data
Empirical model => Doesn’t appear to be rear wall
Side wall or pcav? MC required
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ConclusionCalibration factors for three types of ion chamber have
been obtained for 12, 18, 22 MeV electron beams
Initial results show significant differences between this
direct method and TG-51 – requires further investigation
There is direct traceability (same standard) between Co-60
and high energy electron beams
Future work will focus on the lower energies, repeatability
issues and relative chamber measurements
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