ESTRO 2025 - Abstract Book

S3664

Physics - Quality assurance and auditing

ESTRO 2025

Analyzing collimator rotation, Halcyon LINACs exhibited greater deviations compared to TrueBeam systems. Halcyons demonstrated a more consistent and uniform pattern throughout the rotation, whereas TrueBeam LINACs showed considerable variation between 90º and 270º. This range was not present in Halcyons, since the collimator rotation is limited to 180º. Results beetween different TrueBeam LINAC systems had a higher variation than those among Halcyon systems. Conclusion: The study highlights the lower deviation of the radiation isocenter observed in TrueBeam LINACs during both gantry and collimator rotations. Despite their higher deviation Halcyon LINACs have demonstrated lower variability between the different models, throughout both, gantry and collimator rotation, in comparison to TrueBeam LINACs. Additionally, the Winston-Lutz test for TrueBeam STx and standard TrueBeam systems do not differ. References: - 1.Winston R, Lutz D. The alignment of the therapy beams of a linear accelerator. Med Phys. 1988;15(3):241-245. doi: 10.1118/1.596177. - 2. Du W, Johnson JL, Jiang W, Kudchadker RJ. On the selection of gantry and collimator angles for isocenter localization using Winston-Lutz tests. J Appl Clin Med Phys. 2016;17(1):167-178. doi: 10.1120/jacmp.v17i1.5792. - 3.Gao J, Liu X. Winston-Lutz-Gao test on the True Beam STx linear accelerator. Int J Med Phys Clin Eng Radiat Oncol. 2019;8:9-20. doi:10.4236/ijmpcero.2019.81002 - 4. Kerns JR. Pylinac: Image analysis for routine quality assurance in radiotherapy. J Open Source Softw. 2023;8(92):6001. doi: 10.21105/joss.06001 Keywords: Winston-Lutz test, Radiation isocenter accuracy

3403

Digital Poster QA strategy for online-Adaptive radiotherapy: Continuous monitoring and expanding of a commissioned space based on complexity metrics

Nicky van Lobenstein, Anna M Dinkla, Daan Hoffmans, Desiree HJGD van den Bongard, Duncan den Boer Radiotherapy, Amsterdam UMC, Amsterdam, Netherlands

Purpose/Objective: In online-adaptive radiotherapy (oART), there is no opportunity for plan-specific QA measurements between plan optimization and delivery. Online reoptimized plans might differ from plans in the class solution. To ensure the deliverability of these plans we propose a strategy based on a “commissioned complexity space" where plan deliverability is monitored using plan complexity parameters. To illustrate the use of this strategy we applied it to a new treatment indication in our department: online adaptive treatment in breast cancer patients. Material/Methods: Four metrics were selected that cover a variety of complexity aspects: Monitor Units per gray (MU/Gy), Mean Aperture Distance (MAD) 1,2 , Small Aperture Score 5mm (SAS5) 1,2 and Edge Metric (EM) 3 . These complexity metrics were calculated using an in-house written Matlab (The MathWorks Inc.) code. Treatment plans were selected from patients accrued in the BREAST-ART study (NCT05727553) using a tangential 4-5 beam IMRT- technique on the Ethos system (Varian a Siemens Healthineers Company). To define an initial commissioned space based on measurements, 13 treatment plans that encompass a wide range of complexity were selected. This space is updated by checking if the adaptive plans fall within it; if not, new measurements should be performed to expand the commissioned space. This resulted in 6 adaptive plans that were measured.