Effects of nuclear interactions on energy and stopping power in proton beam dosimetry

Most experimental methods for proton beam dosimetry require stopping power values and proton energy distributions in the irradiated materials. At proton energies of interest in radiotherapy, nuclear interactions in biological tissue or in tissue-equivalent materials are not negligible. As a consequence of nuclear interactions the primary proton fluence is attenuated and lower energy secondary protons and other charged particles are generated. Whenever the proton energy has to be determined along the beam path also this low-energy component should be accounted for. The energy spectra due to primary and secondary protons in a tissue-equivalent material were obtained by a Monte Carlo simulation based on the FLUKA code. The results obtained for various incident energies from 60 to 300 MeV show that the proton energy spectra along the depth of the irradiated material can appreciably depend on the amount of proton nuclear interactions. If the effect of nuclear interactions is not accounted for in determining the proton energy distribution, deviations even greater than 10% can occur in the corresponding stopping powers. The consequences of this effect on absorbed dose determination are discussed and quantitatively assessed in the energy range of interest.