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Purpose
This study addresses the critical challenge of real-time beam monitoring in ultra-high dose rate X-ray FLASH (XFLASH) radiotherapy, where conventional ionization chambers suffer from severe electron-ion recombination losses under extreme dose rates (≥40 Gy/s). We propose a low-pressure ionization chamber (LPIC) as a novel beam monitor, aiming to achieve accurate dose measurement while maintaining beam penetration characteristics required for clinical applications.
Methods
The LPIC was designed with two independent chambers housing high-voltage, collecting, and guard electrodes. Key parameters included a 1 mm electrode gap and a reduced chamber pressure (~5 kPa) to mitigate recombination effects. Theoretical analysis based on the Boag model and numerical simulations (using the numerical-ks-calculator program) quantified recombination loss dependency on pressure (P), electrode spacing (d), and voltage (Uc). MCNP simulations evaluated X-ray transmission through chamber windows (Be, Al, Ti) with thicknesses up to 1000 μm. A prototype LPIC was fabricated and tested on a 10 MeV XFLASH accelerator (dose rate: 80 Gy/s) for plateau characteristics, dose repeatability, linearity, and dose-rate response, following national standards (GB/T15213-2016).
Key Physical Results
1.Recombination Loss Suppression: Theoretical analysis based on the Boag model revealed that the recombination ratio R scales with P3, d2, and Uc−1, validated by numerical simulations (R=0.2256P3; R=0.0534Uc−1; R=0.00548d2). At1.1 Gy/pulse, recombination losses were maintained below 1% by optimizing parameters: P<0.3 atm for d=0.1 mm or P<0.04 atm for d=1 mm.
2.Beam Transmission Optimization: MCNP simulations demonstrated that X-ray transmission exceeded 90% for beryllium (Be), aluminum (Al), and titanium (Ti) windows with thicknesses ≤1000 μm. While 0.1 mm Be achieved the highest transmission (>99%), 1 mm Al (transmission ~95%) was selected as the optimal window material due to its clinical acceptability (<5% dose loss), cost-effectiveness, and ease of fabrication.
3.LPIC Performance Validation: The prototype exhibited stable plateau characteristics (ΔI/I<0.069% at Uc>40V), exceptional dose repeatability (coefficient of variation <0.5% across 10–250 Gy/s), and linearity (R2>0.999 for dose and dose-rate measurements). These results confirm compliance with national standards (GB/T15213-2016) and suitability for real-time XFLASH monitoring.
Conclusion
The LPIC demonstrates robust suppression of recombination losses and reliable performance under XFLASH conditions. Its design—optimized via theoretical modeling and simulations—ensures high precision (meeting GB/T15213-2016 requirements) while preserving beam penetration. The use of 1 mm Al windows balances cost and functionality, making the LPIC a practical solution for clinical translation. Future studies will focus on multi-channel LPIC arrays for 2D beam profiling. -
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