First Pulse-Width–Specific Phase Space Characterization Universal Beam Modeling for Flash-RT Treatment Planning Systems
Abstract
Purpose
FLASH radiotherapy (>40 Gy/s) demonstrates normal tissue sparing while maintaining tumor control, but pulse width variations systematically alter electron beam energy through waveguide loading effects, creating treatment planning system (TPS) challenges. This work establishes pulse-width-specific phase space characterization across the Mobetron UHDR's clinical range and proposes a universal beam model for implementation.
Methods
Using experimentally characterized regression relationships between pulse width and beam penetration, we developed an iterative optimization process for phase space parameter refinement. Starting with a 6cm diameter aperture, we systematically adjusted mean energy and energy spread by minimizing discrepancies between computation and measurements for R50 (fall-off region), surface dose and build-up gradient (build-up region), and integrated dose accuracy. Convergence required R50 and FWHM matching within 0.5mm.
Results
Pulse-width-specific phase space files were successfully generated for 1.2-4.0μs across all apertures. For the 6cm aperture, mean energy decreased exponentially from 9.58 MeV (1.2μs) to 9.04 MeV (4.0μs), while energy spread increased quadratically from 0.173 to 0.287 MeV. Regression analysis established exponential decay for mean energy (R2=0.9946) and quadratic growth for energy spread (R2=0.9974) with pulse width. Statistical analysis revealed strong linear correlation between mean energy and energy spread (r=-0.98, p=0.018), with 1.81-3.17% beam quality degradation across the pulse width range. Based on beam loading physics, the geometric mean of experimental conditions (2.28μs) was selected as the universal reference, corresponding to 9.32 MeV mean energy and 0.214 MeV energy spread.
Conclusion
This work establishes the first comprehensive pulse-width-specific phase space characterization for FLASH TPS development, with validated regression models enabling parameter prediction at arbitrary pulse widths. The universal pulse width approach demonstrates maximum depth-dose parameter deviation of 2.0mm and mean deviation of 0.8mm across experimental extremes, representing <1% of therapeutic depth range. This universal beam model provides clinically acceptable accuracy while eliminating pulse-width-specific requirements, facilitating practical FLASH TPS implementation.