Abstract
In this study, we investigated computationally and experimentally a hexagonal-pattern array of spatially fractionated proton minibeams produced by proton pencil beam scanning (PBS) technique. Spatial fractionation of dose delivery with millimeter or submillimeter beam size has proven to be a promising approach to significantly increase the normal tissue tolerance. Our goals are to obtain an optimized minibeam design and to show that it is feasible to implement the optimized minibeams at the existing proton clinics. An optimized minibeam arrangement is one that would produce high peak-to-valley dose ratios (PVDRs) in normal tissues and a PVDR approaching unity at the Bragg peak. Using Monte Carlo (MC) code TOPAS we simulated proton pencil beams that mimic those available at the existing proton therapy facilities and obtained a hexagonal-pattern array of minibeams by collimating the proton pencil beams through the 1-3mm diameter pinholes of a collimator. We optimized the minibeam design by considering different combinations of parameters including collimator material and thickness (t), center-to-center (c-t-c) distance, and beam size. The optimized minibeam design was then evaluated for normal tissue sparing against the uniform pencil beam scanning (PBS) by calculating the therapeutic advantage (TA) in terms of cell survival fraction. Verification measurements using radiochromic films were performed at the Emory proton therapy center (EPTC). Optimized hexagonal-pattern minibeams having PVDRs of >10 at phantom surface and of >3 at depths up to 6cm were achieved with 2mm diameter modulated proton minibeams (with proton energies between 120 and 140MeV) corresponding to a spread-out-Bragg-peak (SOBP) over the depth of 10-14cm. The results of the film measurements agree with the MC results within 10%. The TA of the 2mm minibeams against the uniform PBS is >3 from phantom surface to the depth of 5cm and then smoothly drops to ~1.5 as it approaches the proximal edge of the SOBP. For 2mm minibeams and 6mm c-t-c distance, we delivered 1.72Gy at SOBP for 7.2×7.2×4cm3 volume in 48s. We conclude that it is feasible to implement the optimized hexagonal-pattern 2mm proton minibeam radiotherapy at the existing proton clinics, because desirable PVDRs and TAs are achievable and the treatment time is reasonable.
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