Low noise optimization of an electron beam current transformer for conventional radiotherapy up to ultra high dose rate irradiations

Lahaye, Chloe; Fontbonne, J.M.; Salvador, S.

doi:10.1088/1748-0221/17/08/p08018

Cited by 2 publications

(2 citation statements)

References 10 publications

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“…in solid water should be discouraged if a Faraday shield is not used, as our results indicate that the presence of a connected ion chamber can reduce the BCT signal perturbation caused by solid water. Finally, for BCTs with a wide enough dynamic range to cover conventional and UHDR irradiations (Lahaye et al 2022, Vojnovic et al 2023, outcome comparisons between the two regimes based on the BCT signal would likely be biased if the effect of electric fields is not taken into account.…”

Section: Discussionmentioning

confidence: 99%

The effect of electron backscatter and charge build up in media on beam current transformer signal for ultra-high dose rate (FLASH) electron beam monitoring

Pageot,

Zerouali,

Guillet

et al. 2024

Phys. Med. Biol.

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Objective: Beam current transformers (BCT) are promising detectors for real-time beam monitoring in ultra-high dose rate (UHDR) electron radiotherapy. However, previous studies have reported a significant sensitivity of the BCT signal to changes in source-to-surface distance (SSD), field size, and phantom material which have until now been attributed to the fluctuating levels of electrons backscattered within the BCT. The purpose of this study is to evaluate this hypothesis, with the goal of understanding and mitigating the variations in BCT signal due to changes in irradiation Approach: Monte Carlo simulations and experimental measurements were conducted with a UHDR-capable intra-operative electron linear accelerator to analyze the impact of backscattered electrons on BCT signal. The potential influence of charge accumulation in media as a mechanism affecting BCT signal perturbation was further investigated by examining the effects phantom conductivity and electrical grounding. Finally, the effectiveness of Faraday shielding to mitigate BCT signal variations is evaluated. Main Results: Monte Carlo simulations indicated that the fraction of electrons backscattered in water and on the collimator plastic at 6 and 9 MeV is lower than 1%, suggesting that backscattered electrons alone cannot account for the observed BCT signal variations. However, our experimental measurements confirmed previous findings of BCT response variation up to 15% for different field diameters. A significant impact of phantom type on BCT response was also observed, with variations in BCT signal as high as 14.1% when comparing measurements in water and solid water. The introduction of a Faraday shield to our applicators effectively mitigated the dependencies of BCT signal on SSD, field size, and phantom material.Significance: Our results indicate that variations in BCT signal as a function of SSD, field size, and phantom material are likely driven by an electric field originating in dielectric materials exposed to the UHDR electron beam. Strategies such as Faraday shielding were shown to effectively prevent these electric fields from affecting BCT signal, enabling reliable BCT-based electron UHDR beam monitoring.

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Section: Discussionmentioning

confidence: 99%

The effect of electron backscatter and charge build up in media on beam current transformer signal for ultra-high dose rate (FLASH) electron beam monitoring

Pageot,

Zerouali,

Guillet

et al. 2024

Phys. Med. Biol.

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“…Furthermore, as the pulse width is reduced, the BCT's SNR for a constant charge pulse improves. A significantly better approach is to increase the BCT L s / R l ratio by following it with a transimpedance (virtual earth) amplifier, as recently highlighted [57]. The low number of turns (<10 typ.)…”

Section: Figurementioning

confidence: 99%

Monitoring beam charge during FLASH irradiations

Vojnovic,

Tullis,

Newman

et al. 2023

Front. Phys.

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In recent years, FLASH irradiation has attracted significant interest in radiation research. Studies have shown that irradiation at ultra-high dose rates (FLASH) reduces the severity of toxicities in normal tissues compared to irradiation at conventional dose rates (CONV), as currently used in clinical practice. Most pre-clinical work is currently carried out using charged particle beams and the beam charge monitor described here is relevant to such beams. Any biological effect comparisons between FLASH and CONV irradiations rely on measurement of tissue dose. While well-established approaches can be used to monitor, in real time, the dose delivered during CONV irradiations, monitoring FLASH doses is not so straightforward. Recently the use of non-intercepting beam current transformers (BCTs) has been proposed for FLASH work. Such BCTs have been used for decades in numerous accelerator installations to monitor temporal and intensity beam profiles. In order to serve as monitoring dosimeters, the BCT output current must be integrated, using electronic circuitry or using software integration following signal digitisation. While sensitive enough for FLASH irradiation, where few intense pulses deliver the requisite dose, the inherent insensitivity of BCTs and the need for a wide detection bandwidth makes them less suitable for use during CONV “reference” irradiations. The purpose of this article is to remind the FLASH community of a different mode of BCT operation: direct monitoring of charge, rather than current, achieved by loading the BCT capacitively rather than resistively. The resulting resonant operation achieves very high sensitivities, enabling straightforward monitoring of output during both CONV and FLASH regimes. Historically, such inductive charge monitors have been used for single pulse work; however, a straightforward circuit modification allows selective resonance damping when repetitive pulsing is used, as during FLASH and CONV irradiations. Practical means of achieving this are presented, as are construction and signal processing details. Finally, results are presented showing the beneficial behaviour of the BCT versus an (Advanced Markus) ionisation chamber for measurements over a dose rate range, from <0.1 Gys−1 to >3 kGys−1.

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Low noise optimization of an electron beam current transformer for conventional radiotherapy up to ultra high dose rate irradiations

Cited by 2 publications

References 10 publications

The effect of electron backscatter and charge build up in media on beam current transformer signal for ultra-high dose rate (FLASH) electron beam monitoring

The effect of electron backscatter and charge build up in media on beam current transformer signal for ultra-high dose rate (FLASH) electron beam monitoring

Monitoring beam charge during FLASH irradiations

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