Author: Blessing Chinelo Akah, Daniel W. Neck, David J. Perrin, Sotirios Stathakis 👨🔬
Affiliation: Mary Bird Perkins Cancer Center 🌍
Purpose: To assess the reliability of the Aktina Isopoint software in evaluating different components of the linear accelerator for optimized quality assurance and linac performance
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 system. The system optimizes the intersection of the circles and guides the Lutz-sphere (ball bearing) at the optimized position. We defined the radiation isocenter with the 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. In this project, 1 VersaHD was evaluated five times under same conditions.
Results: The physical isocenter radii were 0.68 mm, 0.67 mm, 0.78 mm, 0.75 mm, and 0.74 mm. The radiation isocenters were calculated to have 0.60 mm, 0.63 mm, 0.65 mm, 0.65 mm, and 0.65 mm (figure 1) radii. The maximum couch walkout (figure 2) occurred at clockwise and collimator 0 degrees combination; and the maximum induced marker (tumor) shifts (figure 2) were 0.88, 0.81, 0.87, 0.88, and 0.89 (units in mm).
Conclusion: The Aktina Isopoint system proved reliable in evaluating different components of the linac. Th results were more stable for last three measurements, indicating possibly human error in the first two measurements. The system provided useful information to make the necessary adjustments to minimize the distance between isocenters of the linac by indicating the optimal precise adjustment for the couch. Tumor (marker) shift occurred mostly at couch -90 degrees. The system aids in evaluating linac stability, and avoiding couch, gantry and collimator directions where uncertainty is high. The system is valuable for periodic QA of the LINAC components to assess the linac performance to provide corrections.