Rivard, M. J., Melhus, C. S., Zinkin, H. D., Stapleford, L. J., Evans, K. E., Wazer, D. E. and Odlozilíková, A. A Radiobiological Model for the Relative Biological Effectiveness of High-Dose-Rate ²⁵²Cf Brachytherapy. Radiat. Res. 164, 319– 323 (2005).

While there is significant clinical experience using both low- and high-dose-rate ²⁵²Cf 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 ²⁵²Cf 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_N = S_γ) between ²⁵²Cf neutrons and photons used for derivation of RBE. Using this equality, a relationship among neutron dose and LQ radiobiological parameter (i.e., α_N, β_N, α_γ, β_γ) was obtained without the need to specify the photon dose. These results were used to derive the ²⁵²Cf neutron RBE, which was then compared with Russian neutron RBE values. The ²⁵²Cf 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 ²⁵²Cf neutron RBE values of 3.2, 2.9, 2.7 and 2.5, respectively. Using clinical data for late-responding reactions from ²⁵²Cf, Russian investigators created an empirical model that predicted high-dose-rate ²⁵²Cf neutron RBE values ranging from 3.6 to 2.9 for similar doses and fractionation schemes and observed that ²⁵²Cf neutron RBE increases with the number of treatment fractions. Using these relationships, our results were in general concordance with high-dose-rate ²⁵²Cf RBE values obtained from Russian clinical experience.