(Wednesday, 7/17/2019) 9:30 AM - 10:00 AM
Room: Exhibit Hall | Forum 7
Purpose: Energy degradation and refocusing of clinical proton beams to energies and dimensions relevant for small animal irradiation.
Methods: Geant4 Monte Carlo simulations were carried out to degrade a 75 MeV clinical proton beam by graphite to 20-60 MeV. A lattice based on permanent-magnet quadrupoles (PMQs) was designed using the beam-optics code elegant, to refocus the resulting large emittance beams to iso-center. The drift lengths and collimator openings were further optimized for the same range of energies while maintaining a small spot at iso-center, fulfilling additional design constraints. The PMQs were modeled in Radia and CST to define design parameters and tolerance levels.
Results: A triplet of PMQs was optimized for focusing 20-60 MeV proton beams at the iso-center, ~70 cm downstream the degrader. Two collimator assemblies, with two pairs of rectangular collimators each, dynamically defined the accepted emittance upstream the magnets. The strong magnetic field gradient of ~350 T/m required for the magnets was found feasible using Hallbach-type quadrupoles modeled using 16 NdFeB segments.By dynamically adjusting the collimators, spot sizes smaller than 1 mm FWHM were achieved for an energy spread up to 4 % with transmissions below 1 %. Compared to a collimator-only passive beam delivery, neutron fluence at the iso-center below 10 % was observed, along with improved entrance-to-peak and plateau-to-peak ratios of the simulated laterally-integrated dose distributions in a water phantom.
Conclusion: A permanent-magnet quadrupole triplet with high field gradients can be used to refocus energy degraded clinical proton beams for preclinical studies. Spot sizes below 1 mm FWHM can be achieved for degraded beams with energy spreads <4 % by beam shaping with collimators upstream the magnets. PMQ beamline results in reduced neutron fluence and improved laterally-integrated dose distributions.
Funding Support, Disclosures, and Conflict of Interest: This work is funded by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program through the grant agreement number 725539
Keywords
Protons, Beam Shaping, Collimation
Taxonomy
TH- External Beam- Particle therapy: Proton therapy - instrumentation
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