Characterization of a shielded beam current transformer for ultra-high dose rate (FLASH) electron beam monitoring and dose reporting

Abstract

Background: Real-time beam monitoring and accurate dose reporting is challenging in ultra-high dose rate (UHDR) electron beams. Although beam current transformers (BCTs) can effectively track parameters such as pulse width (PW) and repetition frequency for UHDR electron beams, recent work has highlighted their sensitivity to electric fields induced by transient charge buildup in irradiated media under UHDR conditions.

Purpose: This study evaluates the performance of a novel electrostatically shielded BCT for real-time, high-accuracy dose monitoring in UHDR electron beams.

Methods: Irradiations were conducted using the Mobetron linear accelerator configured for UHDR electron beams with energies of 6 and 9 MeV. A shielded BCT was implemented to monitor beam delivery, with dose calibration established using alanine dosimeters in solid water phantoms. Dose stability was assessed over short (7-day) and long (16-week) periods. The BCT's response to variations in PW, pulse number, and pulse repetition frequency was also evaluated to determine its robustness across beam configurations.

Results: The BCT showed high reproducibility and accuracy, with standard deviations of the difference between BCT-predicted and alanine-measured doses within 0.21% over short-term measurements and 0.57% over long-term measurements, even when subject to large (10%) machine output adjustments. When varying beam parameters, the BCT maintained accurate dose prediction within 1.0% and 1.4% of alanine measurements for 6 and 9 MeV, respectively, with high linearity ( $R^2\ge$ 0.9997) across total doses.

Conclusion: Shielded BCTs provide a stable and accurate solution for real-time dose monitoring in FLASH radiotherapy, demonstrating robustness against output fluctuations and beam parameter variations. While further calibration standardization is required, this study supports the feasibility of using shielded BCTs for reliable UHDR dose monitoring, facilitating safe and precise implementation of FLASH radiotherapy in preclinical and clinical settings.

Keywords: FLASH radiotherapy; beam current transformer; electron beams.

FIGURE 1

Schematic of the shielded BCT setup for UHDR beam monitoring.

FIGURE 2

Alanine dosimeters used for dosimetric calibration.

FIGURE 3

Experimental setup for the BCT calibration and evaluation of its short‐ and long‐term stability. The PEEK holder, containing the alanine pellets, is placed within a machined solid water slab.

FIGURE 4

(a) Short‐ and (b) long‐term tracking of the BCT calibration.

FIGURE 5

BCT signal for the 9 MeV irradiation on Week 11.

FIGURE 6

Relative difference in dose predicted by the calibrated BCT and that measured by the alanine as a function of variable (a) pulse width, (b) number of pulses, and (c) pulse repetition frequency. The reference condition for each beam parameter is identified with a vertical dashed red line.

FIGURE 7

Dose predicted by the calibrated BCT versus measured by the alanine as a function of the total dose delivered for UHDR beams of (a) 6 MeV and (b) 9 MeV. Error bars not shown as they are substantially smaller than the symbols.

FIGURE A1

Dose predicted by the calibrated BCT versus measured by the Plastic Scintillation Detector (PSD) as a function of the total dose delivered for UHDR beams of (a) 6 MeV and (b) 9 MeV. Error bars not shown as they are substantially smaller than the symbols.