Quantifying the Spatial and Angular Distribution of Lethal Neutrons for Treating Planning

It is known that high energy protons in proton therapy generate secondary particles. Of which, secondary neutrons are a main concern as they deposit out-of-field doses and can have long-term health effects on cancer patients. In this paper, the energy, 3-D spatial and angular distribution of the production yield of neutrons is scored along the proton beam path in different types of tissue medium. The degree of biological damage is then quantified through factoring in the relative biological effectiveness of neutrons. This systematic study involved simulating 70, 150 and 200 MeV proton beam transport in various tissue compositions with the GEANT4 code. System specifications of the Hitachi proton therapy system were used in this study. Simulation results showed that the neutrons are forward facing and are generally emitted at a preferential angle. With considerations on the RBE variation with neutron's energy, the spatial and angular distribution of the production of lethal neutrons were identified along the proton track. Non-trivial relations between biological damage in different tissue medium were observed. Such comprehensive simulation studies have not been reported and this input information can be useful for treatment planning in reducing out-of-field neutron dose in sensitive organs.