A Monte Carlo modeling tool was applied at the Institut-Curie Centre de Protonthérapie d'Orsay, France, to simulate the passively scattered beam line used for treatment of ocular melanoma. The primary aim of this study is to validate the model for subsequent calculation of patient doses due to secondary neutrons.

The Monte Carlo code MCNPX is used here to model the geometry of the beam line. The beam parameters at the entrance of the ophthalmologic beam line are not well known (beam emittance, lateral distribution, and energy spread). Hence, to accurately implement the beam source in the model, we need to calculate and measure these parameters in the first step of this study. Then, we perform comparisons between calculated and measured proton absorbed dose profiles under various scattering conditions.

Comparisons between calculated and measured depth versus dose profiles show discrepancies <0.6 mm (range) and <1.1 mm (beam size and penumbra) for the lateral dose profiles. Hence, calculated relative dose profiles are considered to be correctly described by the Monte Carlo model. Some improvements are still needed to reproduce absolute dose profiles. This study should lead to the use of the numerical model for radiation protection applications.