ESTRO 2025 - Abstract Book

S2652

Physics - Detectors, dose measurement and phantoms

ESTRO 2025

3828

Proffered Paper Dual gap ionization chamber as an automated tool for real-time ion recombination correction and dose rate measurements. Marina Orts Sanz 1 , Severine Rossomme 2 , Kevin Souris 2 , Edmond Sterpin 1,3,4 1 Institut de Recherche Expérimentale et Clinique (IREC), Université catholique de Louvain, Brussels, Belgium. 2 IBA, Ion Beam Applications, Louvain-La-Neuve, Belgium. 3 PARTICLE, Particle Therapy Interuniversity Center Leuven, Leuven, Belgium. 4 Department of Oncology, Laboratory of experimental radiotherapy, KULeuven, Leuven, Belgium Purpose/Objective: Emerging radiation therapies present new challenges for physicists to ensure accurate dose determination, particularly with ionization chambers, where ion recombination losses must be considered. Under ultra-high dose rate (UHDR) conditions, volume recombination induces a significantly large correction factor due to its strong dependence on dose rate. In heavy ion beam therapy, initial recombination, influenced by the Linear Energy Transfer (LET), must also be accounted for. Current protocols typically recommend multiple measurements to determine recombination correction factors (ks), increasing the time required for dose measurements in the quality assurance workflow. To address these challenges, we propose a novel dual-gap ionization chamber (DGIC) featuring two air gaps of different thicknesses within a single device. This design allows the determination of ks from a single measurement by analyzing the charge ratio between the two gaps. Additionally, the charge ratio variation can be correlated with the dose rate in UHDR conditions Material/Methods: We developed a DGIC prototype with air gap thicknesses of 1 and 0.6 mm. The prototype was tested using different beam qualities: (1) a 226 MeV proton produced by a isochronous cyclotron (IBA Proteus®PLUS) with an current at the exit of the cyclotron between 5 nA and 800 nA, where 800 nA approximately corresponds to 200Gy/s, (2) a 9 MeV electron beam with a dose per pulse from 0.003 to 2 Gy, a frequency of 30 Hz and a pulse duration between 0.5 and 4 μs and (3) a 120 MeV/n clinical carbon ion beam. Ks-factors were derived for the 1 mm gap. For proton and carbon ions, comparative analyses were conducted against Jaffe plot method, as endorsed in the recent update of TRS-398. For the electron beam, independent devices such as an IBA Razor diode and an integrated current transformer (ICT) were used in combination with EBT3 film dosimetry. Results: The DGIC demonstrated excellent agreement with the Jaffe plot method for obtaining ks in UHDR proton and Carbon ion beams, as illustrated in Figure 1. Figure 2 shows the comparison of the dose measured with different devices in an UHDR electron beam. Not correcting for ks can lead to an underestimation of the dose measured of a 53% at the highest dose per pulse. The DGIC determined the DPP with 5% agreement after proper calibration.