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The Sensitivity of Gamma Index to Detect MLC Positioning Errors Using Varian EPID and ArcCHECK for Patient-Specific VMAT QA

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J Darko

B Maraghechi1 , J Darko1,2,3*, J Davis2 , N Mitchell2 , M Shah2 , A Fleck1,3, E Osei1,2,3 , (1) Grand River Regional Cancer Centre, Kitchener, Ontario, Canada (2) University of Waterloo, Waterloo, Ontario, Canada (3) University of Guelph, Guelph, Ontario, Canada


SU-I-GPD-T-260 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: The objective of this study is to compare the sensitivity of the Varian TrueBeam Electronic Portal Imaging Device (EPID) dosimetry and ArcCHECK in detecting errors that were deliberately introduce in MLC positions using a single- and multi-gamma criteria techniques.

Methods: Seven intact prostate VMAT plans were randomly selected and modified. The active MLC positions were randomly changed with a mean of 0.25, 0.5, 1 and 2 mm and a standard deviation of 0.1 mm (randomly opened or closed) on 25%, 50%, 75% and 100% of the 177 control points. The gamma criteria of 3%/3mm, 3%/2mm, 3%/1mm, 2%/2mm, 2%/1mm, and 1%/1mm were considered individually and as a group for the single- and the multi-gamma criteria techniques, respectively. The effect of the introduced MLC errors on the dose distributions to the PTV and OAR were also analyzed.

Results: When the error was introduced to 25%, 50%, 75% and 100% of control points, the EPID was able to detect errors of 2 mm, 1 mm, 1 mm and 1 mm using the criterion of 2%/2mm and 2 mm, 1 mm, 0.5 mm and 0.5 mm by using the multi gamma criteria technique. With the ArcCHECK and using the multi gamma criteria technique, when MLC errors were applied on 50%, 75% and 100% of the control points the smallest detectable errors were 2 mm, 2 mm and 1 mm. The ArcCHECK was not able to detect any of the errors when 25% of control points were modified. The single gamma criterion results were similar to those of multi gamma criteria technique for the ArcCHECK.

Conclusion: The Varian TrueBeam EPID dosimetry was more sensitive in detecting MLC positioning errors compared to the ArcCHECK. Using a multi-gamma criteria approach improves this sensitivity for EPID.

Funding Support, Disclosures, and Conflict of Interest: Financial support provided by TELLUS Ride For Dad and the Prostate Cancer Fight Foundation.

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