Purpose: The purpose of this work was to optimize beam shaping devices to produce small volumes (up to ~8 cm³) of uniform dose in a spread-out Bragg peak to achieve ultrahigh dose rates appropriate for FLASH experiments. We modified the characteristics of our ocular beamline to spill the accelerated protons in times as short as 2 milliseconds.
Methods: Monte Carlo simulations were employed to design custom beam modifiers. These included a scattering foil and a conical flattening filter to maximize the flux of protons into the region of interest, ridge filters, range compensators, and apertures. Shapes, sizes and positions of the last three components were optimized to provide various field sizes and SOBPs in preparation of experiments to be performed. The stability and reliability of the beamline was ascertained, and dose distributions computed with Monte Carlo simulations for each configuration have been validated experimentally.
Results: With the modified devices we produced circular field sizes of 10, 15 and 20 mm diameter and SOBP modulation widths of 10, 15 and 20 mm. System tests revealed excellent stability and flatness of lateral dose profiles at the center of the SOBP was within ± 3%.Assessment of systematic uncertainties, such as impact of misalignments and positioning uncertainties was performed using simulations and the results used to make appropriate adjustments.
Conclusion: As a result of this project, the modified beamline is now capable of delivering proton flash beams for in vitro and in vivo experiments.