A Radiobiological Model for the Relative Biological Effectiveness of High-Dose-Rate super(252)Cf Brachytherapy

While there is significant clinical experience using both low- and high-dose-rate super(252)Cf brachytherapy, there are minimal data regarding values for the neutron relative biological effectiveness (RBE) with both modalities. The aim of this research was to derive a radiobiological model for super(252)Cf neutron RBE and to compare these results with neutron RBE values used clinically in Russia. The linear-quadratic (LQ) model was used as the basis to characterize cell survival after irradiation, with identical cell killing rates (S sub(N) = S sub( gamma )) between super(252)Cf neutrons and photons used for derivation of RBE. Using this equality, a relationship among neutron dose and LQ radiobiological parameter (i.e., alpha sub(N), beta sub(N), alpha sub( gamma ), beta sub( gamma )) was obtained without the need to specify the photon dose. These results were used to derive the super(252)Cf neutron RBE, which was then compared with Russian neutron RBE values. The super(252)Cf neutron RBE was determined after incorporating the LQ radiobiological parameters obtained from cell survival studies with fast neutrons and teletherapy photons. For single-fraction high-dose-rate neutron doses of 0.5, 1.0, 1.5 and 2.0 Gy, the total biologically equivalent doses were 1.8, 3.4, 4.7 and 6.0 RBE Gy with super(252)Cf neutron RBE values of 3.2, 2.9, 2.7 and 2.5, respectively. Using clinical data for late-responding reactions from super(252)Cf, Russian investigators created an empirical model that predicted high-dose-rate super(252)Cf neutron RBE values ranging from 3.6 to 2.9 for similar doses and fractionation schemes and observed that super(252)Cf neutron RBE increases with the number of treatment fractions. Using these relationships, our results were in general concordance with high-dose-rate super(252)Cf RBE values obtained from Russian clinical experience.