the use of large-volume ion chambers in out-of- field ......july 30, 2017 the use of large-volume...
TRANSCRIPT
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
2 of 15
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
3 of 15
Remove linac
fluctuations
Smoother
data
Better TPS
calculations
PDD
without
reference
detector
Profile
without
reference
detector
Output Fluctuations
4 of 15
PDD with
reference
detector
Profile with
reference
detector
Challenges
• Reference detector location must be continually adjusted
5 of 15
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
6 of 15
Possible Elekta Infinity
reference detector location
Project Goal
• Test locations around a linac with large-volume chambers
• Quantify ion chamber current for scanning feasibility
7 of 15
Methods
• Three large-volume ion chambers used
1) Exradin A4 – 30 cc
2) Exradin A5 – 100 cc
3) Custom parallel-plate – 30 cc
8 of 15
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
9 of 15
Results
• Green cells indicate beam currents > 138 pA (adequate for
beam scanning)
• Recombination <2% for all tests
10 of 15
Hot Spots
• Varian 21EX
• Varied little with field size
• Gradient present
11 of 15
12 of 15
Hot
spot
Hot Spots
• Varian TrueBeam™
13 of 15
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
14 of 15
Acknowledgements
• Zac Labby, PhD
• UWMRRC students and staff
• UWADCL customers for their continued support
Questions?