ESTRO 2025 - Abstract Book

S2635

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

3506

Digital Poster Combined ion recombination and polarity effect corrections on FFF scanning measurements Antonella Fogliata 1 , Pasqualina Gallo 1 , Andrea Bresolin 1 , Marco Pelizzoli 1 , Francesco La Fauci 1 , Luca Cozzi 1 , Stefano Tomatis 1 , Marta Scorsetti 1,2 , Giacomo Reggiori 1 1 Radiosurgery and Radiotherapy Department, IRCCS-Humanitas Research Hospital, Milan, Italy. 2 Department of Biomedical Sciences, Humanitas University, Milan, Italy Purpose/Objective: The ion recombination k s depends on the dose per pulse, which varies steadily along a depth dose (PDD) acquisition due to the dose rate decrease from d max to large depth. For small k s factors, its variation on PDD measurements is negligible. However, with flattening filter-free beams (FFF) the dose per pulse is larger, and the k s is higher. This work aims to determine a potential correction factor curve, depth-dependent, to apply to a PDD in particular for FFF beams for three ion chambers, combining both ion recombination and polarity k pol effects. The same concept is applied to profile measurements. Material/Methods: k s and k pol were estimated at each measuring point of a PDD according to TRS-398 (two-voltage method for k s ), for three ion chambers: PTW Semiflex-3D-31021, PinPoint-3D-31022, and Semiflex-31010. PDD were acquired from d max to 32 cm depth, SSD=90 cm, at four voltages (± 400V or ± 300V, and ± 100V) and for field sizes 4×4, 10×10, 20×20, and 40×40 cm 2 . Profiles in the same conditions and field sizes were acquired at d max , 10 and 30 cm depth. The k s is expected to increase linearly with the dose rate. 6FFF and 10FFF beams from a Varian TrueBeam linac were used. The k s values were computed after the k pol correction at each applied voltage for each depth of PDD and each position of profiles. The ratio of the k s and k pol corrected and uncorrected PDD is proposed as a depth-dependent correction factor, which can be fitted for easy application. A similar approach was explored for profiles, evaluating the combined k s and k pol variation with the off-axis position for different field sizes and depths. Results: Correction factors to apply to a PDD for a 10x10 cm 2 10FFF field are shown in the plot (as an example), for the three ion chambers, with fit overlaid. The second plot is the correction factor to apply for the same beam at 10 cm depth.