July 30, 2017

The Use of Large-Volume Ion Chambers in Out-Of-

Field Locations for Radiotherapy Beam Scanning

59th Annual Meeting of the American Association of Physicists in Medicine

Denver, CO

Wesley Culberson

Department of Medical Physics, University of Wisconsin – Madison

University of Wisconsin Medical Radiation Research Center (UWMRRC)

Disclosures

• None

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Background

• Linac output fluctuates

• When acquiring commissioning scans, removing the effects of

output fluctuations helps

• Internal monitor chambers too difficult to utilize

• “Reference detectors” and a second electrometer used to

account for beam fluctuations

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Remove linac

fluctuations

Smoother

data

Better TPS

calculations

PDD

without

reference

detector

Profile

without

reference

detector

Output Fluctuations

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PDD with

reference

detector

Profile with

reference

detector

Challenges

• Reference detector location must be continually adjusted

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Challenges with small fields

• Reference detector location not available for very small fields

• Some linacs have access for reference detectors in the head,

but this can be tricky

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Possible Elekta Infinity

reference detector location

Project Goal

• Test locations around a linac with large-volume chambers

• Quantify ion chamber current for scanning feasibility

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Methods

• Three large-volume ion chambers used

1) Exradin A4 – 30 cc

2) Exradin A5 – 100 cc

3) Custom parallel-plate – 30 cc

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Image source: www.standardimaging.com

Methods (cont)

• Varian 21EX

• 6 MV, 600 MU/min

• Multiple locations outside of the primary beam path tested

• Field sizes: 1 x 1 cm2 -> 10 x 10 cm2

• Ion recombination checked

• Chambers relocated twice for repeatability testing

• Radiochromic film measurements for hot spots

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Results

• Green cells indicate beam currents > 138 pA (adequate for

beam scanning)

• Recombination <2% for all tests

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Hot Spots

• Varian 21EX

• Varied little with field size

• Gradient present

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Hot

spot

Hot Spots

• Varian TrueBeam™

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Summary

• Out of field locations tested for different chambers and locations

around a clinical linear accelerator

• It is possible to use a reference detector in an out-of-field

location

• This means fewer trips into the vault

• Very useful for small fields where reference detector location is

limited

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Acknowledgements

• Zac Labby, PhD

• UWMRRC students and staff

• UWADCL customers for their continued support

Questions?