ESTRO 2025 - Abstract Book

S3589

Physics - Quality assurance and auditing

ESTRO 2025

(MLC) aperture adjustments address motion along the IEC-X and IEC-Z. Although the system is effective, compensation accuracy is constrained by the binary MLC resolution, an aspect that has received little attention. This study evaluates the robustness of target offset compensation in the Radixact Synchrony system using a Delta4 Phantom+ on a Hexamotion stage. Material/Methods: A spherical planning target volume (PTV) was created at the center of the Delta4 Phantom+ and positioned at the isocenter (IC). A treatment plan simulating stereotactic body radiotherapy (SBRT) for lung cancer was developed. To assess the accuracy of target offset compensation, the phantom was offset by 1 to 5 mm in 1 mm increments along the IEC-X, Y, and Z. The plan was delivered using the Synchrony tracking system, with respiratory motion simulated by the Hexamotion stage. Dose distributions were measured in sagittal and coronal planes and analyzed using gamma criteria (3%/3 mm, 10% threshold). To determine the threshold for binary MLC compensation, additional measurements were conducted with the phantom offset by 4.1 to 4.5 mm in 0.1 mm increments along the IEC-X direction. Results: For the IEC-X and Z, gamma pass rates (GPRs) were above 95% for offsets up to 2 mm, but fell below 95% at 3 mm and 90% at 4 mm. Binary MLC compensation improved GPRs to above 95% at 5 mm. Along the IEC-Y direction, GPRs consistently reached 100% under all conditions. The threshold for binary MLC compensation was determined to be approximately 4.3 mm, as the dose profile in the IEC-X direction was compensated when the target offset exceeded this threshold. Conclusion: For the Radixact Synchrony system, GPRs for target offsets in the IEC-X and IEC-Z decreased before binary MLC compensation was triggered but improved once compensation was applied. In contrast, jaw compensation for IEC-Y offsets demonstrated consistent performance and maintained high GPRs. Contrary to expectations that a leaf shift would occur for offsets exceeding half the leaf width (3.125 mm), this study revealed that binary MLC compensation is activated at approximately 4.3 mm. These findings have implications for defining GTV-PTV margins. To maintain accurate radiation delivery, we recommend using comet graphs to monitor setup error and baseline shift and reregistering for significant transverse offsets. References: [1] Schnarr E, Beneke M, Casey D, Chao E, Chappelow J, Cox A, et al.. Feasibility of real ‐ time motion management with helical tomotherapy. Medical Physics 2018;45:1329–37. [2] Ferris WS, Culberson WS, Smilowitz JB, Bayouth JE. Effects of variable ‐ width jaw motion on beam characteristics for Radixact Synchrony®. Journal of Applied Clinical Medical Physics 2021;22:175–81. [3] Chen Q, Rong Y, Burmeister JW, Chao EH, Corradini NA, Followill DS, et al.. AAPM Task Group Report 306: Quality control and assurance for tomotherapy: An update to Task Group Report 148. Medical Physics 2023;50. Keywords: Radixact, Synchrony, tracking

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Proffered Paper Automated prospective tumor segmentation and treatment plan quality assurance in the hypo-FLAME 3.0 trial Joost Surewaard 1 , Ruben Bosschaert 1 , Herbert Beemster 1 , Bas Schipaanboord 1 , Matthijs Dassen 1 , Barry Doodeman 1 , Fons van den Bergh 1 , Martina Kunze-Busch 2 , Robin De Roover 3 , Cédric Draulans 4,5 , Karin Haustermans 3 , Danny Schuring 6 , Hendrik Piersma 7 , Ans Pelgrims 5 , Ann Coelmont 8 , Tomas Janssen 1 , Floris Pos 1 , Uulke van der Heide 1