SU‐E‐T‐125: VMAT Monthly Quality Assurance Using a 2D Ionization Chamber Array

Purpose: Periodic quality assurance is essential for the accurate and consistent operation of dynamic multileaf collimators (DMLC) for volume modulated arc therapy (VMAT). Varian RapidArc is a system for delivering VMAT that can vary gantry speed and dose rate as the DMLCs quickly change position in order to obtain a conformal dose. A more time efficient and reproducible method for verifying the RapidArc capabilities has been developed building upon previous work by Ling et. al. using a 2D ionization chamber array. Methods: The PTW seven29 2D‐ARRAY was used in conjunction with an isocentric gantry mount, removing directional dependence, to complete three tests. Tests included evaluating machine performance while varying dose rate and gantry speed, while varying DMLC speed and the dose rate, and DMLC position accuracy. Reproducibility of each test was also evaluated. Results: Dose rate (111–600 MU/min) and gantry speed (5.5–4.3 degrees/s) capability was studied with a RapidArc picket fence pattern consisting of 7 different sections of combinations and compared to a corresponding open field and was found to agree within 0.84%. DMLC speed (0.46, 0.92, 1.84, and 2.76 cm/s) evaluation was performed similarly and agreed within 3.66% (2.93–3.66%). A RapidArc picket fence was compared with a static gantry picket fence to evaluate DMLC position accuracy to within 2.91%. Reproducibility of each of the three RapidArc tests was found to be within 0.57%, 1.78%, and 2.70% respectively. Conclusions: The seven29 detector was able to perform the RapidArc quality assurance tests efficiently and accurately and the results were reproducible. Periodic verification of DMLC movement, dose rate variation, and gantry speed variation relating to RapidArc delivery can be completed in a timelier manner using this equipment. This work partially supported by PTW, Freiburg, Germany.