Benchmarking Monte Carlo simulations against experimental data in clinically relevant passive scattering proton therapy beamline configurations

Monte Carlo simulations are widely used to calculate secondary neutron doses in proton therapy. When using the passive scattering technique, a small variation in the tumor size or location requires a full adjustment of the beamline configuration. This work thus focuses on modeling all possible beamline elements of the local facility with the MCNPX code while covering the entire clinical range of proton beam energies. This paper documents the experimental validation of realistic beamline configurations dedicated to intracranial tumor treatments, with different sources and beamline elements. Simulations were compared against measurements and treatment planning system (TPS) data considering percentage depth dose distributions (PDD) and relative lateral dose profiles. The results show that simulated and measured PDD distributions differ by no more than 1.6 mm on the proton beam range (tolerance set at 2 mm/2%) for the 162 MeV configuration. Meanwhile, for the modulated lateral dose profiles, the 219 MeV configuration presented the largest difference for the in-plane field width at 90% with 2 mm difference in measurements. The modeled proton sources and beamline elements were validated, and the new realistic geometries can be used to calculate secondary neutron doses to healthy tissues.