Abstract
Boron carbide is a material proposed as an alternative to graphite for use as an energy degrader in proton therapy facilities, and is favoured due to its mechanical robustness and promise to give lower lateral scattering for a given energy loss. However, the mean excitation energy of boron carbide has not yet been directly measured. Here we present a simple method to determine the mean excitation energy by comparison with the relative stopping power in a water phantom, and from a comparison between experimental data and simulations we derive a value for it of 83.1±2.8eV suitable for use in Monte-Carlo simulation. This is consistent with the existing ICRU estimate (84.7eV with 10-15% uncertainty) that is based on indirect Bragg additivity calculation, but it has a substantially smaller uncertainty. The method described can be readily applied to predict the ionisation loss of other boron carbide materials in which the atomic constituent ratio may vary, and allows this material to be reliably used as an alternative to graphite, diamond or beryllium.
Highlights
Proton therapy is a form of radiation therapy whose benefit in particular is the fact that protons release most of their energy at a spe cific depth to ensure maximum conformity of the deposited dose distribution to the tumour volume
For proton multiple scattering simulations, it has been shown that the models available in the GEANT4 physics lists show different degrees of agreement with experimental data; Fuchs et al [30] have shown that the WentzelVI model for multiple Coulomb scattering (MCS) results in a better agreement with data published in the literature; the WentzelVI model has been the default MCS model used in the EMZ option since version 10.02 of GEANT4, and is used by us in the present work
The additional option ‘HP’ used with this physics list ensures that high-precision neutron models and cross sections are used in the simulation
Summary
Proton therapy is a form of radiation therapy whose benefit in particular is the fact that protons release most of their energy at a spe cific depth (the so-called Bragg peak) to ensure maximum conformity of the deposited dose distribution to the tumour volume. To adjust the proton range to that required for a given treatment layer depth, the proton beam passes through an energy modulation system, which consists of material placed on the beam path either immediately before the patient or upstream of that soon after the cyclotron extraction. For upstream energy variations, such a system is called a degrader. Proton energies down to 70 MeV are routinely used in clinical treatments; the required degrader thicknesses for such an energy reduction are several centimetres, and induce significant lateral scattering and momentum spread, which is reduced using sets of collimators and a so-called energy selection system (ESS) [4]
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