ESTRO 2025 - Abstract Book

S2585

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

1377

Digital Poster Optimization of a flattening filter-free beam model and its validation using patient-specific dose quality assurance Mohammad Javad Safari, Roel Shpani, Katrin Straub, Catrin Rodenberg, Michael Reiner Clinic and Polyclinic for Radiotherapy and Radiation Oncology, Ludwig Maximilian University Hospital, Munich, Germany Purpose/Objective: The purpose of this study is to investigate and validate the accuracy of a Flattening Filter-Free (FFF) beam model (BM) with optimized MLC parameters based on experimental measurements. We aim to demonstrate the impact of optimized MLC parameters on the treatment and dose delivery accuracy. Material/Methods: For this study, the Agility™ MLC parameters of a 6MV FFF BM for the Monaco (Elekta) treatment planning system were optimized and validated against measurements of synchronous (SG) and asynchronous sweeping gaps (aSG) [1]. The impact of the optimized MLC parameters was demonstrated by comparing several in-house-designed and vendor provided fields and seven randomly selected stereotactic body radiation therapy (SBRT) plans that underwent hypofractionation treatment schemes in our clinic. This comparison was performed using the Gamma Index (GI) with the myQA SRS (IBA Dosimetry). Results: The comparison of measured and calculated dose of SG fields showed the largest deviation of 3.4% for 5x5 cm field size (FS) and for aSG fields the largest deviation among 10, 20, and 40 mm shifts was observed for 20 mm with 4.9%. These were considerably less than the deviations found using the default MLC parameters provided by Elekta. There was a substantial improvement for the MLC transmission of using the optimized BM, reducing the deviation from 156.1% to 9.6%. A high level of agreement was observed between the calculated dose and the measured dose using the optimized BM for several in-house-designed fields based on GI. The averaged GI for the SBRT cases was 99.4% ± 0.6% for 2mm/2% criteria. Conclusion: Throughout this study, the MLC parameters in Monaco were optimized and validated using a set of in-house designed and vendor-provided fields, enhancing the MLC parameter optimization process. Furthermore, the use of an optimized BM demonstrated favorable clinical outcomes across seven randomly selected SBRT plans. These findings emphasize on the importance of optimizing the MLC parameter configuration in Monaco-BMs received from Elekta after processing of the beam data to fit the true exit fluence from the linac head. In addition to point dose measurements, it is recommended to assess the accuracy of the model using various exposure fields and patient plans before implementing the optimized BM in clinical practice.

Keywords: MLC Parameters Optimization, FFF, Plan QA

References: 1. Hernandez, V., Angerud, A., Bogaert, E., Hussein, M., Lemire, M., García-Miguel, J., & Saez, J. (2022). Challenges in modeling the Agility multileaf collimator in treatment planning systems and current needs for improvement. Medical physics, 49(12), 7404–7416. https://doi.org/10.1002/mp.16016