Author: Seung Yeon Kim, Grant A. McAuley, Ju A Park, Jerry D Slater 👨🔬
Affiliation: Loma Linda University 🌍
Purpose: The production of proton minibeams in standard PBS nozzles is challenging due to the effects of finite source beam divergence and multiple Coulomb scattering. Herein we investigate the use of permanent multipole magnets to create proton minibeams from low emittance source beams.
Methods: Monte Carlo simulations were performed in a previously proposed PBS nozzle that incorporates a doublet of octupole magnets and a single quadrupole modeled as Halbach cylinders. A combination of octupole tail folding and quadrupole focusing is used to create proton minibeams. Single symmetric and planar beamlet dose distributions were scored in a water phantom. Symmetric beamlets were produced by continuous axial rotation of the magnets. Spatially fractionated dose distributions were produced by laterally shifting single beamlet distributions in the water tank in one or two dimensions, and PVDR and beamlet FWHM were determined. Particle energies covered a range typical in clinical applications (70 MeV, 100 MeV, 150 MeV and 225 MeV). A symmetric Gaussian beam model with σ=1mm and divergence=0.5mrad was used to simulate low-emittance source beams as may potentially be produced by a proton linac.
Results: Single symmetric beamlets created from 150 MeV source beams showed FWHM was as low as 0.8mm. PVDR was as high as 53, about 4 times larger than previous results using a more conventional beam model (σ=3 mm and divergence=2 mrad). It is noteworthy that minibeam spot FWHM of < 1 mm was achieved ~1 m downstream of nozzle entrance for the 150 MeV linac source beam.
Conclusion: Our data suggests that the proposed PBS nozzle is able to produce spatially fractionated proton minibeams with minimum FWHM < 1 mm, and PVDR values at least 4x higher compared to the conventional beam model. Such beams appear suitable for clinical or preclinical applications.