Performance evaluation of gated volumetric modulated arc therapy

Background: Aim of this study is to evaluate the accuracy of the gated volumetric modulated arc therapy (VMAT/RapidArc) using 2D planar dosimetry, DynaLog files and COMPASS 3D dosimetry system. Materials and Methods: Pre-treatment quality assurance of 10 gated VMAT plans was verified using 2D array and COMPASS 3D dosimetry system. Advantage of COMPASS over 2D planar is that it provides the clinical consequence of error in treatment delivery. Measurements were performed for non-gated and different phase gating window level (80%, 50%, 30% & 20%) to know the impact of gating in VMAT dose delivery. Results: In 2D planar dosimetry, gamma agreement index (GAI) for all measurements were more than 95%. DynaLog file analysis shows the average deviations between actual and expected positions of monitor units, gantry and multi-leaf collimator. The STDVs MU and gantry position were less than 0.10 MU and 0.33° respectively. Root mean square (RMS) of the deviations of all leaves were less than 0.58 mm. The results from COMPASS show that 3D dose volume parameters for ten patients measured for different phase gating window level were within the tolerance level of ±5%. Average 3D gamma of PTV and OAR’s for different window level was less than 0.6. Conclusion: The results from this study show that gated VMAT delivery provided dose distributions equivalent to non-gated delivery to within clinically acceptable limits and COMPASS along with MatrixEvolution can be effectively used for pretreatment verification of gated VMAT plans.