[en] The proton transport in matter is described by the Boltzmann kinetic equation for the proton flux density. This equation, however, does not have a general analytical solution. Some approximate analytical solutions have been developed within a number of significant simplifications. Alternatively, the Monte Carlo simulations are widely used. Current work is devoted to the discussion of the proton energy spectra obtained by simulation with SRIM2006, GEANT4 and MCNPX packages. The simulations have been performed considering some further applications of the obtained results in computed tomography with proton beam (pCT). Thus the initial and outgoing proton energies (3 / 300 MeV) as well as the thickness of irradiated target (water and aluminum phantoms within 90% of the full range for a given proton beam energy) were considered in the interval of values typical for pCT applications. One from the most interesting results of this comparison is that while the MCNPX spectra are in a good agreement with analytical description within Fokker-Plank approximation and the GEANT4 simulated spectra are slightly shifted from them the SRIM2006 simulations predict a notably higher mean energy loss for protons. (author)