Author: Cambridge L Bui-Nguyen, Alexander S. Nguyen, Liqiang (Lee) Tao 👨🔬
Affiliation: Varian AOS @Epic Care, UC Berkeley 🌍
Purpose: Dynamic Winston-Lutz (WL) testing provides a comprehensive framework for evaluating the accuracy of isocenter alignment. This study investigates the impact of dynamic beam modulation, such as volumetric modulated arc therapy (VMAT), on isocenter accuracy for both 6MV flattening filter-free (FFF) and conventional 6MV beams. Testing was conducted on the Elekta VersaHDTM linear accelerator equipped with the Hexapod 6D couch, with real-time tracking enabled through the electronic portal imaging device (EPID). The analysis was performed using DoseLabTM version 7.0 software.
Methods: An Sun NuclearTM WL Phantom was selected to use for measurements. Initial baseline alignment was established using static WL tests with gantry angles incremented by 45 degrees. Dynamic WL was then performed under continuous gantry motion to simulate clinical VMAT conditions. Since the central focus is the dynamic nature of beam delivery, both couch and collimator are kept at 0. Real-time tracking with the EPID was employed to monitor and record isocenter deviations frame by frame during beam delivery. The shift between the phantom’s radiopaque marker and the radiation field center was analyzed to quantify isocenter accuracy dynamically.
Results: Isocenter deviations for both beam types remained within clinically acceptable limits (≤1 mm). However, the 6MV FFF beam exhibited slightly greater sensitivity to mechanical motion, particularly during rapid gantry rotations, likely due to its higher dose rate and reduced beam flatness. Real-time EPID tracking provided detailed temporal resolution of isocenter shifts, revealing small, transient deviations during dynamic delivery.
Conclusion: This study demonstrates the importance of dynamic WL testing to identify subtle deviations and ensure treatment precision. The findings highlight the robustness of the Elekta VersaHDTM system for dynamic treatments and emphasize the value of incorporating dynamic WL testing into routine quality assurance protocols. Future research may explore integrating automated real-time correction strategies for enhanced alignment accuracy.