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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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