Purpose: To validate non-isocentric delivery and analysis of VMAT QA plans using ArcCHECK
Methods: ArcCHECK (Sun Nuclear Corp., Melbourne, FL) is routinely used in rotational delivery for patient pre-treatment QA. A CavityPlug insert with ion chamber is used for isocenter dose measurement. Plans with high dose targets not at isocenter typically require one delivery with isocentric setup for gamma analysis and another with ArcCHECK shifted to position the ion chamber at a high dose region for dose measurement only. The shifted non-isocentric ArcCHECK measurements, particularly in transverse directions, are not standard and not recommended by the manufacture for gamma analysis. In this study, we demonstrated and validated non-isocentric delivery and analysis of VMAT QA plans using ArcCHECK. Ten clinical VMAT plans with the isocenter outside high dose targets were used for the study. All plans were generated in Varian Eclipse TPS with HD120 and Millennium 120 MLCs using 6FFF, 6X, 10FFF and 10X photon energies. For each clinical plan, two QA plans were created, one without and one with shifts from the AckCHECK isocenter. QA plans delivered without shifts were used for gamma analysis as baseline. QA plans delivered with shifts were used for both absolute dose measurement and gamma analysis. The average gamma passing rates for non-isocentric delivery were compared with the corresponding results of isocentric delivery.
Results: With 10% threshold and local 3%/3mm passing criteria, the average gamma passing rate for non-isocentric and isocentric deliveries were (98.7 ± 1.4)% vs. (97.0 ± 2.4)%, respectively. With 3%/2mm passing criteria, they were (95.2 ± 4.0)% vs. (90.2 ± 5.7)%, respectively. The absolute dose measurements of the non-isocentric deliveries were all within ± 3% of the calculated dose.
Conclusion: The non-isocentric delivery and analysis of VMAT QA plans using ArcCHECK were demonstrated and validated for patient pre-treatment QA.
Not Applicable / None Entered.