The dual gap ionization chamber: a novel ionization chamber design for reference dosimetry to automatically correct for recombination losses in emerging radiotherapy modalities

<i>Objective.</i>Current dosimetry protocols typically recommend multiple measurements to determine recombination correction factors (ks), increasing the time required for dose measurements in the quality assurance workflow. We propose a novel dual-gap ionization chamber (DGIC) design for reference dosimetry featuring two air gaps of different thicknesses within a single device. This design enables the determination ofksdirectly from the same measurements required to determine absorbed dose-to-water. Thus, eliminating the need for separate measurements to correct for recombination losses. The approach relies on analyzing the charge ratio between the two gaps, under ultra high dose rates (UHDR) and dose per pulse (DPP) under ultra high DPP (UHDPP) conditions.<i>Approach.</i>A DGIC prototype with electrode distances of 1 and 0.6 mm was developed and tested using different beam qualities: (1) a 240 MeV u^-1clinical carbon ion beam at conventional field dose rates of 25 Gy min^-1, (2) a 226 MeV continuous proton beam with a current between 5 and 800 nA at the cyclotron exit, where the maximum approximately corresponds to 200 Gy s^-1in the treatment room and (3) a 9 MeV electron beam with a DPP from 0.03 to 4.2 Gy, a frequency of 60 Hz and a pulse duration between 0.7-3.9μs.ks-factors were derived for the top cavity of 1 mm gap using the DGIC method and compared against the following: for proton and carbon ions, comparisons were made with the Jaffé plot method. For the electron beam, it was compared with a dose rate independent device, a flashDiamond detector, and the integrated current transformer of the LINAC.<i>Main results.</i>A DGIC prototype was able to successfully correct for recombination losses under different beam modalities: for initial recombination in a clinical carbon ion beam, volume recombination in UHDR proton beam with field dose rates of 200 Gy s^-1and in UHDPP electron beams, where four pulses were delivered with DPP up to 4.2 Gy (this DPP corresponds to an effective pulse duration of 3.9μs).<i>Significance.</i>A DGIC design and its inherent method provides a practical and accurate way of determining dose and dose rate in emerging radiotherapy treatment modalities.