Development of 2-3 mm proton minibeams as a new form of GRID radiotherapy
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In this study, we investigated, computationally and experimentally, the feasibility of a new form of proton GRID therapy based on an array of proton minibeams developed at the existing proton therapy clinics. The diameter of the proton minibeams is in the range of 2-3 mm. The purpose of this new form of proton GRID therapy is to further enhance normal tissue sparing during the treatment. The optimal design of the proton minibeam array is based on the figures-of-merit parameters including the peak-to-valley dose ratio (PVDR), dose rate at the Bragg peak and the unwanted neutron dose. Using Monte Carlo code TOPAS we simulated proton pencil-beams that mimic those available at cyclotron-based facilities. We achieved parallel beams of 2-3 mm diameter using physical collimator made of dense materials. The beams are produced via the open holes of the collimator. The spatial pattern of the beam array follows the hexagonal arrangement. We optimized the proton minibeam design by considering different combinations of parameters like beam size, collimator material, center-to-center distance, phantom to collimator distance, and collimator thickness. Verification measurements of the PVDRs using radiochromic films and neutron dose using WENDI-II neutron detector were conducted at two proton therapy facilities: the University of Florida Proton Therapy Institute and the Emory Proton Therapy Center. Results show that using the existing proton pencil beam scanning technique, the optimized proton minibeams can achieve high PVDR values at the entrance of the water phantom and at the same time maintain clinically acceptable dose rates at the tumor depth. Although the neutron dose increases by 20-30 folds (on average) with the use of a collimator, it is still less than the neutron dose produced with double scattered proton beam. Accordingly, the results suggest that it is feasible to develop an array of proton minibeams to further enhance normal tissue sparing during the treatment.