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 of ks directly 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, which can also be correlated with the average dose rate under Ultra High Dose Rates (UHDR) conditions. A DGIC prototype with electrode distances of 1 and 0.6 mm was developed and tested using different beam qualities: (1) a 240 MeV/n clinical carbon ion beam at conventional dose rates, (2) a 226 MeV continuous proton beam with a current between 5 and 800 nA, where the maximum approximately corresponds to 200 Gy/s in the treatment room and (3) a 9 MeV electron beam with a DPP from 0.03 to 4.2 Gy. ks-factors were derived for the top cavity using the DGIC method and compared against the following: For proton and carbon ions, comparisons were made with the Jaffe plot method. For the electron beam, it was compared with a dose rate independent device, a flashDiamond detector. 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 average dose rates of 200 Gy/s and in UHDPP electron beams tested up to a DPP of 4.2 Gy. A DGIC design and its inherent method provides a practical and accurate way of determining dose and dose rate in emerging radiotherapy treatment modalities.
Orts et al. (Tue,) studied this question.