Variation of neutron RBE as a function of energy for different biological systems: a review

RBE determinations have been performed in all neutron therapy centres prior to any clinical application. In some centres, the minimum checks were done in order to start the treatments in safe conditions. In other centres, extensive investigations were performed comparing several biological systems. A large amount of data was thus accumulated. However, only a few investigations were performed in order to systematically assess the RBE variation as a function of neutron energy for the same biological system, which could allow an accurate exchange of information between centres. Form the available radiobiological data, it can be concluded that RBE increases with decreasing neutron energy, but the slope of the RBE/neutron energy relationship depends on the biological system and endpoint. RBE values as high as 1.53 were observed for d(20) + Be relative to p(65) +Be neutrons. Even when comparing the modern neutron therapy facilities (neutrons produced by protons with energy higher than 40 MeV), significant RBE variations are still observed, which needs to be taken into account when designing multicentre therapeutic protocols. (author)