(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: At higher energies, protons may be delivered with a broad, collimated beam. A magnetic-lens based collimation scheme and a tight dose deposition profile could allow patients to be treated from 360º, mitigating the entry dose, and creating a stereotactic delivery system.
Methods: An 800-MeV proton beamline is modeled in Geant4. The beam interacts with a 4"-thick tungsten multi-leaf collimator (MLC) at the collimation location. Protons are transported through a magnetic-refocusing imaging lens, with a 3-mrad angular system acceptance, as defined by an additional collimation stage located at the Fourier plane, within the magnetic lens. The dose distribution is measured by a voxelized dosimeter at the patient location, 9.4 m downstream of the collimation location.
Results: A proton distribution matching the opening within the MLC is visualized at the patient location. The ratio of dose within the open field to the closed field is 64:1, due mainly to neutron activation, as protons interacting with the MLC are removed with essentially 100% efficiency. At a depth of 20 cm in tissue, the dose-deposition profile (including secondary particles) from an 800-MeV proton pencil beam has a full-width half-maximum of 2.6 mm.
Conclusion: The ability to collimate a proton distribution, combined with the tightly-constrained dose-deposition profile of a high-energy proton beam, could enable intensity modulated proton therapy. While the ability to target a tumor utilizing the known stopping distances of lower-energy protons would be lost, the ability to rotate the patient while delivering a tightly collimated dose distribution from 360º drastically reduces the dose through the entry site, while mitigating the effects of radiation dose to sensitive tissues by distributing the collateral dose throughout the patient. A better choice of low-Z collimation materials could reduce neutron dose. This technique could reduce treatment times by delivering a full field, or lead to improved accuracy.
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