It is recommended for obvious reasons to determine the reference absorbed dose to water in a clinical proton beam in a water phantom. However, in situations where the position accuracy is very critical, it can be more convenient to measure in a plastic phantom. For absorbed dose to water determination in electron beams, fluence correction factors are introduced to account for the differences in electron fluence distributions at equivalent depths in the plastic phantom and a water phantom. In proton beams a similar approach can be applied. In the present report, fluence correction factors as a function of depth are determined for proton beams with different energies using the Monte Carlo code PTRAN for PMMA and polystyrene with reference to water. The influence of the non-elastic nuclear interaction cross sections is studied. It is found that the differences in proton fluence distributions is entirely due to the differences of the non-elastic nuclear interaction cross sections between the plastic materials and water. For proton beams with energies lower than 100 MeV, for which the contributions from non-elastic interactions become small compared to the total dose, the corrections are smaller than 1%. For beams with energies above 200 MeV, depending on the cross sections data-set for non-elastic nuclear interactions, fluence corrections of 2-5% are found at the largest depths.
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