Optimization of hexagonal‐pattern minibeams for spatially fractionated radiotherapy using proton beam scanning. Issue 8 (11th May 2020)

• Record Type: Journal Article
• Authors: Charyyev, Serdar; Artz, Mark; Szalkowski, Gregory; Chang, Chih‐Wei; Stanforth, Alexander; Lin, Liyong; Zhang, Rongxiao; Wang, C.‐K. Chris
• Is Part Of: Medical physics. Volume 47:Issue 8(2020)
• Page Start: 3485
• Page End: 3495
• Publication Date: 2020-05-11
• Subjects: Monte Carlo -- peak‐to‐valley dose ratio (PVDR) -- pencil beam scanning (PBS) -- proton minibeam therapy -- spatially fractionated radiotherapy (SFRT); Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1002/mp.14192
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: In this study, we investigated computationally and experimentally a hexagonal‐pattern array of spatially fractionated proton minibeams produced by proton p encil b eam s canning (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. Methods: 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 1–3 mm 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). Results: Optimized hexagonal‐pattern minibeams having PVDRs of >10 at phantom surface and of >3 at depths up to 6 cm were achieved with 2 mm diameter modulated proton minibeams (with proton energies between 120 and 140 MeV) corresponding to a spread‐out‐Bragg‐peak (SOBP) over the depth of 10–14 cm. The results of the film measurements agree with the MC results within 10%. The TA of the 2 mm minibeams against the uniform PBS is >3 from phantom surface to the depth of 5 cm and then smoothly drops to ~1.5 as it approaches the proximal edge of the SOBP. For 2 mm minibeams and 6 mm c‐t‐c distance, we delivered 1.72 Gy at SOBP for 7.2 × 7.2 × 4 cm 3 volume in 48 s. Conclusions: We conclude that it is feasible to implement the optimized hexagonal‐pattern 2 mm proton minibeam radiotherapy at the existing proton clinics, because desirable PVDRs and TAs are achievable and the treatment time is reasonable. Abstract :