Optimization of material in proton-therapy collimators with respect to neutron production

Abstract: In this thesis, a study of neutron production properties for collimator materials is performed. Collimators are used in nuclear physics applications such as within the fields of nuclear energy and radiotherapy. The area of application is primarily reduction of static or unwanted radiation for detectors and treatment beams. This study focuses on a branch of radiotherapy called proton therapy where protons of high energies impinge on the collimator. Proton therapy has advantages compared to common radiotherapy techniques due to the energy deposition characteristics of protons. However, high-energy protons cause nuclear reactions in the dose delivery equipment, especially in the collimators. These reactions produce neutron radiation which is both hazardous and difficult to shield from. The choice of different collimator materials has previously not been thoroughly evaluated with respect to neutron production. In this thesis, the neutron production properties of different materials have been evaluated by running simulations with the Monte Carlo particle transport code MCNPX. In an initial survey, some materials of particular interest could be identified. In a second stage, dose calculations in a patient dummy, i.e., a water phantom, were performed. This was done in order to confirm that the lower neutron production of the interesting materials could reduce the biological effect. It is shown that common collimator materials presently used indeed are suboptimal with respect to neutron production. The common collimator material tungsten should not be recommended for use based on the result of this study. Replacing it with nickel, iron or brass will reduce the neutron dose by approximately 30%, which would lead to a reduction of late effects due to proton treatments.