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Gamma Passing Rates for IMRT QA and VMAT QA

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E Kremmel

E Kremmel*, T Giaddui , J Keller , M Fu , C Peng , L Doyle , Y Yu , A Harrison , Thomas Jefferson University, Philadelphia, PA


SU-E-T-441 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:This study compares gamma passing rates for a cohort of similar intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) quality assurance (QA) plans to determine the equivalence of the patient specific QA plan delivery. The passing criterion is 90% gamma index with 3% dose difference (DD), 3mm distance-to-agreement (DTA) and a dose threshold of 10%.

Methods:Gamma passing results of previously treated IMRT QA plans, delivered to Sun Nuclear MapCheck phantom, and VMAT QA plans, delivered to ScandiDos Delta4 phantom, are organized by anatomical site and treatment machine. Both Varian and Elekta machines are included. Pairs of IMRT and VMAT plans are matched based on site, machine, and PTV volume to ensure similar plan cohorts. A two-tailed t-test analysis of the data with an alpha of 0.05 determines if there exists a statistically significant difference. Power was calculated to detect a difference of 4%; all data sets were strong with above a 0.8 power.

Results:The first data set consisting of 20 matched pairs of prostate plans was statistically insignificant (p-value=0.90, Power=0.99). The 14 matched pairs set of head and neck plans has a statistically significant result (p-value=0.028, Power=0.88). The head and neck IMRT gamma indexes have a mean of 93.1% and range of 82%-100% while the VMAT gamma indexes have a mean of 96.7% and range of 92%-100%. The two combined data sets of matched plans had a statistically insignificant result (p-value=0.073, Power=0.99).

Conclusion:Overall, IMRT and VMAT have equivalent passing rates when comparing the gamma analysis using a passing criterion of 3% DD and 3mm DTA. When separated by site, prostate IMRT and VMAT plans have equivalent passing rates while head and neck plans have a statistically significant variation of passing rates. The passing rates for the two modalities are independent of delivery machine for matched PTV target volumes.

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