(Sunday, 7/29/2018) 4:00 PM - 4:55 PM
Room: Karl Dean Ballroom A1
Purpose: A spatially-modulated proton minibeam has been developed on the preclinical small animal proton platform at our institution. Given the strong modulation, dosimetric verification is challenging, especially for lateral dose profiles. Here we characterize and dosimetrically verify this beam with three different methods.
Methods: A 3cmx3cm steel collimator was built for the 50MeV proton beamline to generate 300µm wide planar minibeams with a center-to-center spacing of 1mm. Physical dosimetry was measured using Gafchromic EBT3 film of both the minibeam and broad beam (no collimator). EBT3 film was placed at seven different depths. For the broad beam, the film measurements were compared to percentage depth dose measurement acquired with a microDiamond detector (sensitive volume of 0.004mm^3) and Monte Carlo (MC) (TOPAS) simulations (0.05mm/voxel). The peak-to-valley dose ratio (PVDR) on film at each depth was compared with MC simulations. We obtained lateral profiles of the minibeam at the entrance (1.8mm) and Bragg peak (BP, d=17.0mm) from film (0.021mm/voxel) and MC simulations. Corrections were applied to account for the energy dependency of the film.
Results: The film measurements of the broad beam revealed an energy dependence of the film at the depth of the BP (LET=15keV/µm) amounting up to 37%. This high LET dependence has been previously reported in the literature. PVDRs were 1.08 to 3.82, depending on depth. Overall, the PVDR determined by film matched that derived from MC simulations within 7%. The lateral profiles from the film and simulations were consistent with each other at entrance and BP of the minibeam.
Conclusion: We have developed a highly-modulated proton beam (300µm/1mm spacing). With a BP at depth of 17.0mm, this beam is ideal for preclinical work with small animals. The film measurements verified the simulated beam profiles and confirmed the strong energy dependence of radiochromic film close to the BP.
Funding Support, Disclosures, and Conflict of Interest: This work is supported by University of Washington Royalty Research Fund (05/2017 - 04/2018).
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