Monte Carlo simulation of fast neutron spectra: Mean lineal energy estimation with an effectiveness function and correlation to RBE

Abstract

Purpose: Intercomparisons of radiotherapy trials conducted at different fast neutron facilities are complicated by the dependence of the relative biologic effectiveness (RBE) of the different beams on the fast neutrons spectra. To obtain a better understanding of the influence of neutron energy on radiation quality, Monte Carlo simulations were performed to calculate fast neutron (FN) spectra at different irradiation positions. To allow for comparisons with experimental data, the positions were chosen to be the same as that used by other investigators to obtain microdosimetry readings and radiobiological data. Methods and Materials: The primary neutron yield for beryllium targets bombarded with protons at the National Accelerator Center, Louvain, Nice, and Orleans facilities were calculated using the FLUKA code. Neutron transport simulations were performed with MCNP-4A, giving FN spectra for various phantom depths, hardening filter thickness, and field sizes. Using an effectiveness function, FN energy groups were correlated with mean lineal energies (y∗-values) obtained experimentally by other workers. Results: Calculations confirm earlier measurements that a decrease in beam quality by a hardening filter is the result of a reduction in the low-energy neutron component, i.e., neutrons below 3 MeV. Variations in RBE due to changes in field size and different phantom depths could also be explained by variations of neutrons with energies between 3–15 MeV. The effectiveness function allows one to calculate changes in y∗ observed for the NAC beam with great accuracy (R 2 = 0.99, p < 0.0001). Also, when this function is applied to beams with different neutron energies, y∗ calculated values show a very significant correlation with measured RBE values (R 2 = 0.98, p < 0.0001). Conclusion: The effectiveness function appears to be suitable to predict changes in y∗-values and variations in RBE, using FN spectra simulated for various neutron therapy facilities.