Comparison of proton interaction physics models and cross section libraries for proton therapy Monte Carlo simulations by MCNP6.2 code

Proton cross-section libraries and interaction models available in the Monte Carlo code MCNP6.2 were tested and compared for energies relevant to proton radiation therapy (100, 150, 226 MeV), and for three different phantom materials. Proton, neutron, and photon characteristics were studied. Primary proton parameters included proton total deposited energy and Bragg peak position. The results varied within +/- 0.5%. Secondary neutron characteristics included the number of originated secondary neutrons, their average initial energy, energy spectrum, depth-dose curve, radiation weighting factor, neutron equivalent dose inside phantoms, and neutron spectral fluence and ambient dose equivalent outside phantoms. Neutron radiation weighting factor inside a phantom varied within +/- 8% and neutron equivalent dose typically within +/- 30%. Large differences were observed in neutron fluence energy spectra both inside and outside a phantom. Secondary photon characteristics consisted of photon equivalent dose inside phantoms. The study demonstrates that the selection of input proton data may have a large impact on the simulated quantity, especially if the intention is to model secondary particles, particularly neutrons.