ESTRO 2025 - Abstract Book

S3627

Physics - Quality assurance and auditing

ESTRO 2025

1868

Digital Poster Analysis of PSQAs by testing the new enhanced leaf model applying the AAPM TG-218 report Francesca Dusi, Fabio Busato, Alessandro Testolin, Edoardo Gozzelino, Davide Fiorentin, Mattia Nicoletto, Marco Fusella Radiation Oncology, Policlinico Abano Terme, Abano Terme, Italy Purpose/Objective: The implementation of patient-specific quality assurance (PSQA) is crucial role in evaluating the congruence between intended and actual delivered doses. TG-218 [1] recommends the adoption of tolerance (TL)-and action limit (AL)-based workflows for evaluate the clinical deliverability of a plan. Furthermore, it suggests to establish customized site-specific tolerance and intervention limits to enhance consistency and reliability. Within this work, we propose the site-specific tolerance limit (TLs) and action limit (ALs), to be adopted for the Halcyon and TrueBeam platform coupled to the latest version of Varian's treatment planning system Eclipse v.18 equipped with the new enhanced leaf modelling (ELM), a more advanced and detailed modeling of the MLC [2]. Material/Methods: Between July 2024 and November 2024 a total of 50 volume-modulated arc therapy (VMAT) plans were generated for each Halcyon and TrueBeam (equipped with Millenium 120-leaf multileaf collimator) linacs. Dose calculations were performed in an Eclipse environment utilizing Acuros version18.0.1 algorithm. The treatments were related to a large number of different sites (brain, H&N, thorax and pelvis) and exhibited diverse levels of complexity. The plans were delivered and measured with PTW Octavius-4D phantom (OCT4D) together with the Octavius 1500 2D array (PTW, Freiburg, Germany). Measured and calculated dose distributions were compared by means of 3D global gamma analysis using a 10% dose threshold for 3%/2mm and 2%/2mm gamma criteria. The ALs and TLs were calculated for our clinic for both the treatment machines following TG-218 recommendations [1]. Results: The mean values of GP% (3%G/2 mm) are: 98.1%±1.4% and 98.6%±1.1%, for TrueBeam and Halcyon respectively. The proposed ALs and TLs on the GP% computed with (3%,2 mm) and (2%/2mm) global normalization, after a period of five month monitoring, were reported in the following table.

Conclusion: The TLs and ALs of both machines were more stringent than the commons standard as defined by the TG218 report , which are 95% and 90% respectively, on the GP% computed with (3%, 2 mm), global normalization criteria. Over the entire collection period, any plans failed to meet the universal AL defined by the TG-218 report. Following the TG-218 methodology, we calculated ALs an TLs for VMAT plans calculated with the new enhanced leaf model for both TrueBeam and Halcyon treatment delivery systems, these values be used as reference for the monitoring of the PSQA procedure.

Keywords: Enhanced Leaf Model (ELM), AAPM TG-218

References: [1] Miften M, Olch A, Mihailidis D, Moran J, Pawlicki T, Molineu A, et al. Tolerance limits and methodologies for IMRT measurement-based verifcation QA: recommendations of AAPM Task Group No. 218. Med Phys. 2018;45:e53-83. https://doi.org/10.1002/mp.12810