SU‐F‐T‐643: Feasibility of Performing Patient Specific VMAT QA On Single Linac for Plans Treated in Beam‐Matched Elekta Agility Linacs

Abstract

Purpose:The increasing application of VMAT demands a more efficient workflow and QA solution. This study aims to investigate the feasibility of performing VMAT QA measurements on one linac for plans treated on other beam‐matched Elekta Agility linacs.Methods:A single model was used to create 24 clinically approved VMAT plans (12 head‐and‐neck and 12 prostate using 6MV and 10MV respectively) on Pinnacle v9.10 (Philips, Einhoven, Netherlands). All head‐and‐neck plans were delivered on three beam‐matched machines while all prostate cases were delivered on two beam‐matched 10MV Agility machines. All plans were delivered onto PTW Octavius 4D phantom with 1500 detector array (PTW, Freiburg, Germany). Reconstructed volume doses were then compared with the Pinnacle reference plans in Verisoft 6.1 under 3%/3mm gamma criteria at local dose. Plans were considered clinically acceptable if >90% of the voxels passing the gamma criteria.Results:All measurements were passed (3D gamma passing rate >90%) and the result shows that the mean difference of 3D gamma of 12 head‐and‐neck cases is 1.2% with standard deviation of 0.6%. While for prostate cases, the mean difference of 3D gamma is 0.9% with standard deviation of 0.7%. Maximum difference of 3D gamma of all measurements between beam‐matched machines is less than 2.5%. The differences of passing rates between different machines were statistically insignificant (p>0.05). Conclusion. The result suggests that therConclusion:The result suggests that there exists a 3D gamma threshold, in our case 92.5%, above which the VMAT QA performed in any one of beam‐matched machine will also pass in another one. Therefore, VMAT QA efficiency may be increased and phantom set up time can be saved by implementing such method. A constant performance across all beam matched machines must be maintained to make this QA approach feasible.