Determination of the ion collection efficiency of the Razor Nano Chamber for ultra‐high dose‐rate electron beams

Abstract

Background Ultra‐high dose‐rate (UHDR) irradiations (>40 Gy/s) have recently garnered interest in radiotherapy (RT) as they can trigger the so‐called “FLASH” effect, namely a higher tolerance of normal tissues in comparison with conventional dose rates when a sufficiently high dose is delivered to the tissue. To transfer this to clinical RT treatments, adapted methods and practical tools for online dosimetry need to be developed. Ionization chambers remain the gold standards in RT but the charge recombination effects may be very significant at such high dose rates, limiting the use of some of these dosimeters. The reduction of the sensitive volume size can be an interesting characteristic to reduce such effects. Purpose In that context, we have investigated the charge collection behavior of the recent IBA Razor™ Nano Chamber (RNC) in UHDR pulses to evaluate its potential interest for FLASH RT. Methods In order to quantify the RNC ion collection efficiency (ICE), simultaneous dose measurements were performed under UHDR electron beams with dose‐rate‐independent Gafchromic™ EBT3 films that were used as the dose reference. A dose‐per‐pulse range from 0.01 to 30 Gy was investigated, varying the source‐to‐surface distance, the pulse duration (1 and 3 μs investigated) and the LINAC gun grid tension as irradiation parameters. In addition, the RNC measurements were corrected from the inherent beam shot‐to‐shot variations using an independent current transformer. An empirical logistic model was used to fit the RNC collection efficiency measurements and the results were compared with the Advanced Markus plane parallel ion chamber. Results The RNC ICE was found to decrease as the dose‐per‐pulse increases, starting from doses above 0.2 Gy/pulse and down to 40% of efficiency at 30 Gy/pulse. The RNC resulted in a higher ICE for a given dose‐per‐pulse in comparison with the Markus chamber, with a measured efficiency found higher than 85 and 55% for 1 and 10 Gy/pulse, respectively, whereas the Markus ICE was of 60 and 25% for the same doses. However, the RNC shows a higher sensitivity to the pulse duration than the Advanced Markus chamber, with a lower efficiency found at 1 μs than at 3 μs, suggesting that this chamber could be more sensitive to the dose rate within the pulse. Conclusions The results confirmed that the small sensitive volume of the RNC ensures higher ICE compared with larger chambers. The RNC was thus found to be a promising online dosimetry tool for FLASH RT and we proposed an ion recombination model to correct its response up to extreme dose‐per‐pulses of 30 Gy.

Dosimetric response of Gafchromic™ EBT‐XD film to therapeutic protons

“…films have been used in many previous FLASH experiments with doses higher than 10 Gy. 15,16,18 Although their doses were out of the optimal dose response range of EBT-3 films, the authors reported no issues in dosimetry. Nevertheless, only EBT-XD films were selected in our study to take advantage of the higher optimal dose response range (up to 40 Gy).…”

“…average 2 Gy/min), as reference dosimeters to quantify the ion collection efficiency of ion chambers at FLASH dose rate by assuming dose-rate independence in these films. 15,16 However, a recent study reported the average doserate dependence of response of EBT-3 and EBT-XD films when the dose was higher than 10 Gy 17 with an isochronous cyclotron-based proton FLASH. These findings motivated us to investigate dose-rate dependence of the response of films for a synchrotron-based proton FLASH beam, which was the second aim of the current study.…”

Preliminary study on the correlation between accelerated current and dose in water for an electron-based LINAC

“…As the most promising beam-monitoring technique for electron FLASH LINACs is based on current transformers [43,44] which provides a direct current measurement at the LINAC exit window, the possibility of relating such measurement to dose is particularly interesting and may even represent a solution to the actual difficulties in dose monitoring. The interested reader could find details about the current FLASH dosimetric challenges in [43,45], together with some promising solutions: the FLASH diamond [46], FLASH ionization chambers [47-49], and using current technologies in the FLASH regimes [50,51]. This work presents and validates the determination of the absorbed dose to water in any clinical configurations by means of a purely Monte Carlo simulation, once the beam optics geometry is known and the beam current at the exit window is available.…”

FLASH radiotherapy and the associated dosimetric challenges

“…Typical unfavourable characteristics for charge-based detectors are their dose rateand energy dependence and non-tissue equivalence. Several promising modifications to meet the demands of the UHDR beam dosimetry have however recently been investigated for ion chambers and diamond detectors [23][24][25][26]. For instance, ion recombination correction model has been suggested by Cavallone et al 2022 for online dosimetry using a Nano Chamber, and Gómez et al 2022 demonstrated the ability to extend the dose rate operating range of an ultra-thin parallel plate ionization chamber by reducing the spacing between electrodes [23,24].…”

“…Several promising modifications to meet the demands of the UHDR beam dosimetry have however recently been investigated for ion chambers and diamond detectors [23][24][25][26]. For instance, ion recombination correction model has been suggested by Cavallone et al 2022 for online dosimetry using a Nano Chamber, and Gómez et al 2022 demonstrated the ability to extend the dose rate operating range of an ultra-thin parallel plate ionization chamber by reducing the spacing between electrodes [23,24]. Furthermore, novel approaches for the feasibility of a diamond detector for UHDR beam dosimetry have also been carried out recently.…”