Author: Emily Y. Hirata, Xiaoyu Sherry Liu, Evan Porter, Manju Sharma 👨🔬
Affiliation: University of California San Francisco, University of California, San Francisco, University of California 🌍
Purpose: Introduced in 2018, the Varian Halcyon platform has increased in popularity due to its reduced cost, user-friendly design, and operational efficiency. As this platform ages, major repairs, including central beamline, will become more frequent. This study compares the Halcyon performance following central beamline replacement, highlighting the importance of mechanical adjustment to the beam assembly after installation and the potential deviation tolerances observed while recovering from major repairs.
Methods: A Halcyon unable to produce a stable beam was repaired by replacing both the magnetron and central beamline (accelerating waveguide). After replacement of the magnetron and waveguide, the dose wand was tuned to yield a stable beam. Once stable, machine performance check was performed, prior to any additional corrections or waveguide alignment, then run again after alignment using the six set screws (no other beam tuning, output or calibrations performed). Next, IC Profiler and water tank were used to measure energy constancy, off-axis ratio, and symmetry. Output verification was performed using a PTW 30013 in solid water at a depth of 1.5cm. Machine isocentricity was quantified using MPC and WL-3 cube.
Results: After repair, and prior to beamline alignment, MPC recorded a MV offset deviation of 1.32mm (+1.17mm), with the misalignment increasing isocenter by 0.18mm and output by 0.53%. Following realignment of the beamline, the machine returned to pre-repair MPC performance (output -0.21%, isocenter +0.10mm, uniformity +0.28%) and within annual tolerances for physics checks (isocenter +0.02mm, PDD10 -0.60%, output -0.28%).
Conclusion: Despite Varian having designed the central beamlines to be drop-in replacements, ideally requiring no mechanical re-alignment, our clinical experience highlights the exact opposite – the most crucial post-repair adjustment was correcting the set screws for the central beamline. Therefore, physicists should request engineering pay particular attention to beam alignment, and the physicist should verify alignment prior to adjusting machine output.