EFFECTS OF NUCLEAR-INTERACTIONS ON ENERGY AND STOPPING POWER IN PROTON-BEAM DOSIMETRY

Most experimental methods for proton beam dosimetry require stopping p ower values and proton energy distributions in the irradiated material s. At proton energies of interest in radiotherapy, nuclear interaction s in biological tissue or in tissue-equivalent materials are not negli gible. As a consequence of nuclear interactions the primary proton flu ence is attenuated and lower energy secondary protons and other charge d particles are generated. Whenever the proton energy has to be determ ined along the beam path also this low-energy component should be acco unted 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 en ergies from 60 to 300 MeV show that the proton energy spectra along th e depth of the irradiated material can appreciably depend on the amoun t of proton nuclear interactions. If the effect of nuclear interaction s is not accounted, for in determining the proton energy distribution, deviations even greater than 10% can occur in the corresponding stopp ing powers. The consequences of this effect on absorbed dose determina tion are discussed and quantitatively assessed in the energy range of interest.