Monte Carlo modelling of the treatment line of the Proton Therapy Center in Orsay

This paper presents the main results of a Monte Carlo simulation describing the Orsay Proton Therapy Center (CPO) beam line. The project aimed to obtain a prediction of the dose distribution in a water phantom within 2% accuracy in the dose value and a 2 mm of range. The simulation tool used was MCNPX, version 2.5.0, and included all the elements of the CPO beam line. A new algorithm of multiple Coulomb scattering has been incorporated in MCNPX, resulting in a better prediction of the spatial dose distribution and absolute values of the deposited energy. The simulations of 3D dose profiles in water show a very good agreement with measured data to within 2%. We first performed a comparative analysis of the dosimetry in heterogeneous phantoms between the pencil beam algorithm and MCNPX. The simulations give a better agreement with experimental data compared to the pencil beam approach. In a second phase, we simulated the patient-dependent fields along with the spatial dose distributions in a water phantom. The simulated response of a Pixel chamber located 2 m upstream of the water phantom revealed a good agreement with the measured data to within 1%. The results presented herein support the applicability of Monte Carlo models for absolute dosimetry and for design purposes regarding existing and new beam lines at CPO. This work completes a series of publications reporting the progress in the development of a Monte Carlo simulation tool for the CPO beam line dedicated for the treatment of head and neck tumours.