Author: Blessing Chinelo Akah, Brad Barhorst, Daniel W. Neck, David J. Perrin, Chad Robertson, Sotirios Stathakis 👨🔬
Affiliation: Mary Bird Perkins Cancer Center 🌍
Purpose: To evaluate the performance and radiation isocenter of Elekta and Varian linear accelerator using Aktina Isopoint.
Methods: The Aktina Isopoint was used to measure the physical and radiation isocenter. The physical isocenter considers the radius of the collimator, gantry and couch rotations using an optical stereoscopic device. The system optimizes the intersection of the circles and guides the Lutz-sphere (ball bearing) at the optimized position. The radiation isocenter is defined with Linac EPID to acquire images of the ball bearing using a 6 MV, 50 MU and 4x4 cm2 field at various gantry, collimator and couch angles, similar to Winston-Lutz. For this study 1 VersaHD, 3 Elekta Infinity, 1 Varian TrueBeam and 1 Varian VitalBeam LINACs were evaluated at 4 centers.
Results: The physical isocenter radii were 0.63 mm and 0.70 mm for Elekta; 0.68 mm for VersaHD; 0.27 mm and 0.26 mm for TrueBeam and VitalBeam. The radiation isocenters were 0.58 mm and 0.52 mm; 0.60 mm; 0.39 mm and 0.27 mm (figure 1: left) radii. The maximum couch induced marker shift for both Elekta Infinity were 0.86 mm; 0.88 mm (figure 1: right) for VersaHD; 0.48 mm and 0.64 mm for Varians. The couch shift represents the amount the marker (tumor) shifted away from the reference position (CT data) as the couch rotates.
Conclusion: The Aktina Isopoint system proved effective for comparing LINAC performance across vendors. Varian machines demonstrated superior precision with tighter radiation isocenter, making them ideal for SRS treatments. Tumor shift was higher in Elekta compared to Varian, with maximum shifts observed at couch 90 degrees. The system aids in assessing linac performance and avoiding couch position where uncertainty is high. The system is valuable for SRS commissioning, LINAC installation, and periodic QA of the LINAC isocenter to assess any shifts to provide corrections.