Comparison of basic features of proton and helium ion pencil beams in water using GATE

Purpose: The aim of this study was to investigate the basic features of helium ions for their possible application in advanced radiotherapy and to benchmark them against protons, the current particle of choice in the low linear energy transfer (LET) range. Material and methods: Geant4 Application for Emission Tomography (GATE) simulations were performed with beams of 1 x x107 monodirectional particles traversing a water phantom. Initial energies ranged from 50 to 250 MeV per nucleon (MeV/A). The following parameters were evaluated: particle range at the distal 80% of maximum energy deposition (Emax), width of the Bragg peak (BP) at 60% of Emax, and dose fall-off width between 80% and 20% of Emax for longitudinal spectra. In addition the fragmentation tail was quantified in terms of length, percental energy deposition, and contributing particles. For each energy lateral profiles were registered along the beam axis and the FWHM at four different depths was extracted. Besides the comparison of parameters between the two particle types, results were also compared to data in the literature. Results: As expected, the position of the BP as a function of initial kinetic energy showed similar values for protons and helium ions, with deviations smaller than 1.3%. The quantitative results of the Monte Carlo (MC) study showed less range straggling effects and smaller lateral deflections for helium ions compared to protons for the investigated energy range. On average, an about 56% reduction of the width of the BP and a 48% reduction of the dose fall-off was observed for helium ions compared to protons. Both the width of the BP and the dose fall-off width as a function of particle range or energy showed an almost linear increase with increasing energy. The tail length increased from 55.9 mm to 592.7 mm and the deposited energy increased from 0.5% to 7.3% for energies between 90 and 250 MeV/A. Lateral profiles of helium ions were about 52% narrower than those of protons. Conclusions: Due to their mass and charge helium ions distinguish themselves from protons in reduced range straggling effects, smaller lateral deflections, and a fragmentation tail. The MC based comprehensive data set for 21 clinically relevant energies can be used to create look-up tables for semi-analytical pencil beam model for helium ions. (orig.)