Importance of nuclear interactions in hadrontherapy and space radiation protection

Besides ionisations and excitations of the target molecules due to Coulomb interactions, also nuclear reactions producing secondary hadrons (including ions) can play a significant role in modulating the radiation field characteristics and the dose distributions in tissues and organs with consequences both in hadrontherapy and in radiation protection, in particular with respect to space radiation. The FLUKA Monte Carlo transport code, integrated with radiobiological data from “event-by-event” simulations, was applied to the biophysical characterisation of therapeutic proton beams. Spatial distributions of physical dose and “biological” dose (modelled as the yield of “Complex Lesions”, a clustered DNA damage) were calculated for the proton beam used at PSI for treating ocular tumours. Very good agreement was found between calculations and measurements. Furthermore, the relative contribution of secondary hadrons was found to be higher for the biological dose with respect to the physical dose, mainly due to the higher biological effectiveness of target fragmentation products. Similar results were found with a 160 MeV proton beam. The coupling of FLUKA to two anthropomorphic phantoms allowed us to calculate distributions of organ doses (physical, equivalent and biological) following exposure to the proton component of Solar Particle Events in different shielding conditions. As expected, the skin received higher doses with respect to internal organs, and the doses were found to decrease with increasing the shield thickness. For the October 1989 event, an Aluminium shield of 5 g/cm2 was found to be sufficient to respect the limits indicated by the NCRP for short-term Low Earth Orbit missions, whereas 10 g/cm2 were needed for the August 1972 event. Similarly to the results obtained for hadrontherapy, the relative contributions of nuclear reaction products was found to be higher for the biological dose with respect to the physical dose. Furthermore, such contribution was higher for internal organs than for the skin, mainly due to nuclear interactions occurring in the human body. The recent implementation in FLUKA of nucleus-nucleus interactions below 5 GeV/n made the code suitable also for characterising therapeutic beams of heavier ions (typically Carbon) and investigating the effects of Galactic Cosmic Rays, which are among the main future developments of this work.