  The Monte Carlo (MC) simulation method is a powerful tool for radiation
physicists, and several general-purpose software packages are commonly applied
in a myriad of different radiation physics fields today. In medical physics,
charged particle detectors for proton Computed Tomography are under
development, a modality introduced in order to increase the accuracy of proton
radiation therapy. MC simulations are helpful during the development and
optimization phase of such detector systems. In order to justify the usage of
MC for such purposes, the simulation output must be validated against
experimental or theoretical data, or even cross-checked between different MC
software packages. In this study, we compare three general-purpose MC software
packages (GATE/Geant4, MCNP6 and FLUKA) with respect to how they predict the
spatial distribution of the stopping position of protons. They are compared to
each other and to semi-empirical data, using the mean proton range, the
longitudinal and lateral variation of individual proton ranges, and the
fraction of primary protons lost to nuclear interactions. This comparison is
performed in two homogeneous materials and in a detector geometry designed for
proton Computed Tomography. The three MC software packages agree well, and
sufficiently reproduce the semi-empirical data. Some discrepancies are
observed, such as less lateral beam spreading in GATE/Geant4, and a small
deficiency in the MCNP6 proton range in water: This is consistent with
previously published data. Due to the general agreement, the choice of
simulation framework may be made on personal preferences. It is important to
note that the choice of physics packages, simulation parameter settings and
material definitions are important aspects when performing MC simulations, both
during the preparation, execution and interpretation of the simulation results.
