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Evaluating VMAT Dose Accuracy Using 3D Detector Systems:A Comparative Study with Intentional MLC Errors

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C Johnson

C Johnson1*, R Varadhan2 , P Higgins3 , (1) Minneapolis Radiation Oncology, Edina, MN, (2) Minneapolis Radiation Oncology, Fridley, MN, (3) Univ Minnesota, Minneapolis, MN


SU-E-T-156 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

The purpose of this study was to investigate the sensitivity of commercially available 3D detectors (ArcCHECK and Delta4) to detect introduced errors on the Elekta MLCi head during Volumetric Modulated Arc Therapy (VMAT).
Six randomly selected VMAT plans were studied including two Head and Neck, three Prostate and one SBRT lung. Baseline measurements were taken, and then the known MLC errors were applied in both IN and OUT directions individually to each MLC leaf bank in increments of 0.5 mm and 1.0 mm symmetrically and also together in increments of 0.25 mm and 0.5 mm. The measurements were analyzed using various local gamma and DTA analysis criteria and compared to baseline measurements. The ArcCHECK was evaluated both with and without the acrylic plug.
There was considerable variation in QA passing rates for both 3D detector systems ranging from 99.9% to 60.9% for all measurements with local gamma analysis. The ArcCHECK and Delta4 recorded a 10% decrease in passing rates when MLC leafbank were shifted 1.0 mm IN and OUT directions respectively. When the acrylic plug was inserted into the ArcCHECK, there was a maximum increase in gamma pass rate of 10.4% and an average increase of 3.9% respectively. For the Head and Neck VMAT plans, both devices detected only half of the introduced errors using conventional QA metrics.
There was a poor correlation between gamma pass rates and the introduced MLC errors for the 3D detectors studied. It is recommended to use the Acrylic plug insert in ArcCHECK for highly modulated VMAT plans as it improved passing rates by 3.1%. The Delta4 showed a higher sensitivity to the introduced errors when a local gamma analysis of 2%/2 mm was used and therefore that analysis criterion is recommended to detect any possible MLC errors during VMAT delivery.

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