Abstract
PurposeThe dose core of a proton pencil beam (PB) is enveloped by a low dose area reaching several centimeters off the central axis and containing a considerable amount of the dose. Adequate modeling of the different components of the PB profile is, therefore, required for accurate dose calculation. In this study, we experimentally validated one electromagnetic and two nuclear scattering models in GATE/Geant4 for dose calculation of proton beams in the therapeutic energy window (62–252 MeV) with and without range shifter (RaShi).MethodsThe multiple Coulomb scattering (MCS) model was validated by lateral dose core profiles measured for five energies at up to four depths from beam plateau to Bragg peak region. Nuclear halo profiles of single PBs were evaluated for three (62.4, 148.2, and 252.7 MeV) and two (97.4 and 124.7 MeV) energies, without and with RaShi, respectively. The influence of the dose core and nuclear halo on field sizes varying from 2–20 cm was evaluated by means of output factors (OFs), namely frame factors (FFs) and field size factors (FSFs), to quantify the relative increase of dose when increasing the field size.ResultsThe relative increase in the dose core width in the simulations deviated negligibly from measurements for depths until 80% of the beam range, but was overestimated by up to 0.2 mm in σ toward the end of range for all energies. The dose halo region of the lateral dose profile agreed well with measurements in the open beam configuration, but was notably overestimated in the deepest measurement plane of the highest energy or when the beam passed through the RaShi. The root‐mean‐square deviations (RMSDs) between the simulated and the measured FSFs were less than 1% at all depths, but were higher in the second half of the beam range as compared to the first half or when traversing the RaShi. The deviations in one of the two tested hadron physics lists originated mostly in elastic scattering. The RMSDs could be reduced by approximately a factor of two by exchanging the default elastic scattering cross sections for protons.ConclusionsGATE/Geant4 agreed satisfyingly with most measured quantities. MCS was systematically overestimated toward the end of the beam range. Contributions from nuclear scattering were overestimated when the beam traversed the RaShi or at the depths close to the end of the beam range without RaShi. Both, field size effects and calculation uncertainties, increased when the beam traversed the RaShi. Measured field size effects were almost negligible for beams up to medium energy and were highest for the highest energy beam without RaShi, but vice versa when traversing the RaShi.
Highlights
In this work, we follow the terminology of Gottschalk et al.[1] to describe a pencil beam (PB) in proton therapy with a brief summary provided below
The halo is enveloped by the aura, which contains a negligible fraction of the dose of the PB and is transferred by indirectly ionizing particles
This study aims at validating electromagnetic and nuclear scattering models implemented in GATE/Geant[4] in the clinical relevant energy range (62.4–252.7 MeV) and relies on a beam model taking into account all nozzle elements.[19]
Summary
We follow the terminology of Gottschalk et al.[1] to describe a pencil beam (PB) in proton therapy with a brief summary provided below. The Gaussian shaped central high dose region, the dose core, is dominated by multiple Coulomb scattering (MCS) of the primary protons in the target material. This central high dose region is enveloped by a non-Gaussian low-dose region, the dose halo, constituting a considerable fraction of the laterally integrated dose. It is deposited by secondary charged particles resulting from nuclear scattering and single large-angle scattered primary particles.[1,2,3,4,5] The beam halo extends in lateral direction up to approximately one third of the beam range and reaches its maximum contribution around midrange (“midrange bump”). In this study core and halo refer to the dose inside and outside of the central Gaussianshaped dose region, respectively
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