Monte Carlo Comparison of Proton and Helium-ion Minibeam Generation Techniques

Tim Schneider,Yolanda Prezado,Annalisa Patriarca,Ludovic De Marzi

doi:10.3389/fphy.2021.595721

Abstract

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the normal tissue sparing of submillimetric, spatially fractionated beams with the improved dose deposition of protons. In contrast to conventional approaches which work with comparatively large beam diameters (5 mm to several centimetres) producing laterally homogeneous fields, pMBRT uses submillimetric minibeams to create a distinct spatial modulation of the dose featuring alternating regions of high dose (peaks) and low dose (valleys). This spatial fractionation can increase the tolerance of normal tissue and may allow a safe dose escalation in the tumour. Important quantities in this context are the valley dose as well as the peak-to-valley dose ratio (PVDR). Creating submillimetric proton beams for clinical applications is a challenging task that until now has been realized with mechanical collimators (metal blocks with thin slits or holes). However, this method is inherently inefficient, inflexible and creates undesirable secondary neutrons. We therefore recently proposed a method for obtaining clinical minibeams using only magnetic focusing. In this study, we performed Monte Carlo simulations in order to compare minibeams generated using the new method of magnetic focusing with two techniques involving mechanical collimators (collimator and broad beam irradiation, collimator and pencil beam scanning). The dose deposition in water was simulated and dosimetric aspects [beam broadening, depth-dose profiles, PVDR and Bragg-peak-to-entrance dose ratio (BEDR)] as well as irradiation efficiencies were evaluated. Apart from protons, we also considered helium ions which, due to their reduced lateral scattering and sharper Bragg peak, may present a promising alternative for minibeam radiation therapy. Magnetically focused minibeams exhibited a 20–60 times higher PVDR than mechanically collimated minibeams and yielded an increase in irradiation efficiency of up to two orders of magnitude. Compared to proton minibeams, helium ion minibeams were found to broaden at a slower rate and yield an even higher PVDR (at the same minibeam spacing) as well as a more favourable BEDR. Moreover, the simulations showed that methods developed for proton minibeams are suitable for the generation of helium ion minibeams.

Highlights

Technological advances in radiation therapy have led to a notable improvement of dose conformity in the tumour as well as a reduction of the dose given to organs-at-risk [1]
The goal of this study was to perform a thorough comparison of the three minibeam generation techniques and to assess the possible advantages and shortcomings of each method in terms of dose distributions, peak-to-valley dose ratio (PVDR) and efficiency
For the case of magnetic focusing, it should be noted that the quadrupole settings and beam source parameters were manually adapted as to always yield a beam size between 0.6 and 0.7 mm full width at half maximum (FWHM) at the phantom entrance

Summary

Introduction

Technological advances in radiation therapy have led to a notable improvement of dose conformity in the tumour as well as a reduction of the dose given to organs-at-risk [1]. Spatial modification of the dose distribution, as in spatially fractionated radiation therapy (SFRT), has shown great potential in this context [2,3,4,5,6,7]. In SFRT, the dose profiles are a succession of areas of high dose (peaks) and areas of low dose (valleys). The ratio between the peak and valley doses (peak-to-valley dose ratio, PVDR) is believed to be a biologically relevant parameter: high PVDR with low valleys favours normal tissue sparing [8]. While the exact radiobiological effects underlying SFRT are not yet completely known, possible actors promoting normal tissue sparing might include dose-volume effects [9, 10], cell signalling effects [11] and the so called microscopic prompt tissue-repair effect [7]

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Conclusion