  This study investigates the influence of several Monte Carlo radiation
transport codes and nuclear models on the simulation of secondary neutron
spectra and its impact on calculating and measuring neutron doses in proton
therapy. Three different multi-purpose Monte Carlo radiation transport codes
(FLUKA, MCNPX, Geant4) were used with different available nuclear models to
calculate secondary neutron energy spectra at various points inside a water
tank phantom with PMMA walls using a 10 cm x 10 cm rectangular, mono-energetic
proton beam (110, 150, 180, and 210 MeV). Using Kerma approximation secondary
neutron doses were calculated applying fluence-to-dose equivalent conversion
coefficients in water. Moreover, the impact of varying spectra for
electrochemically etched CR39 detector calibration was analyzed for different
codes and models. In distal positions beyond the Bragg peak, results show
largest variations between the codes, up to 53% for the high energy neutron
fluence at 16 cm from the Bragg peak of the 110 MeV proton beam. In lateral
positions, the variation between the codes is smaller and for the total neutron
fluence within 20%. Variation in the nuclear models in MCNPX was only visible
for the proton beam energies of 180 and 210 MeV and modeling the high energy
neutron fluence which reached up to 23% for 210 MeV at 11 cm lateral from the
beam axis. Impact on neutron dose equivalent was limited for the different
models used (<8%) while it was pronounced for the different codes (45% at 16 cm
from the Bragg peak of the 110 MeV proton beam). CR39 calibration factors in
lateral positions were on average varying 10% between codes and 5 % between
nuclear models.This study demonstrated a large impact on the neutron fluence
spectra calculated by different codes while the impact of different models in
MCNPX proved to be less prominent for the neutron modeling in proton therapy.
