Monte Carlo simulation of a CPO beam line: modelling the nuclear interactions

Abstract
The Proton Therapy Center in Orsay (CPO) and CEA/DAPNIA launched the joint project on Monte Carlo modeling of a CPO beam line with the aim to achieve a prediction of dose distribution in all the calibration configurations (depth and the shape of the tumor) better than 2%. The calculation module is intended to be used for the absolute dosimetry of the clinical beam (patient Quality Assurance - QA), and in a second stage for predicting the dose distribution on a voxelized phantom constructed from the Computer Tomography (CT) patient's data. The MCNPX code was used as a basic Monte Carlo simulation tool in this study. All the elements of a CPO beam line were modelled with sub-millimetric precision. To speed up the calculations of modulated dose profiles in water phantom, a routine simulating rotation of modulator wheel was created to save phase space of particle tracks crossing the modulator. The modeling of nuclear interactions, which contribute noticeably into the total absorbed dose in water phantom at the prescribed proton energy (average raw beam energy 201 MeV), was improved by means of creation and implementation of the two new evaluated proton-induced nuclear data files up to 200 MeV for ¹H and ¹⁶O. Thus, the simulations of 3D dose profiles in water show excellent agreement with measured data allowing to move forward to the absolute dose calibrations.