MO-D-T-6E-08: Derivation of the Relative Biological Effectiveness of High Dose Rate 252Cf Brachytherapy

Abstract

Purpose: While there is significant clinical experience using both low- and high-dose rate (HDR) brachytherapy, there is 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 neutron RBE and to compare these results with neutron RBE values used clinically in Russia. Method and Materials: The linear-quadratic (L-Q) model was used as the basis to characterize cell survival following irradiation, with identical cell killing rates (SN = Sγ) between neutrons and photons used for derivation of RBE. Using this equality, a relationship among neutron dose and L-Q radiobiological parameter (i.e., αN, βN, αγ, βγ) was obtained without need to specify the photon dose. These results were used to derive the neutron RBE which was then compared with Russian neutron RBE values. The neutron RBE was determined after incorporating the L-Q radiobiological parameters obtained from cell survival studies with fast neutrons and teletherapy photons. Results: For single-fraction HDR neutron doses of 0.5, 1.0, and 1.5 Gy, the total (neutron plus photon) doses were 3.5, 6.0, and 8.1 Gy, with neutron RBE values of 6.4, 5.5, and 4.9, respectively. Russian clinicians obtained HDR neutron RBE values ranging from 7 to 3 for similar doses and fractionation schemes, and observed that neutron RBE increases with the number of fractions and is dose rate dependent. A value of 5 was obtained for HDR neutron RBE. Conclusion: The methodology presented herein presents a reasonable technique to utilize well-characterized radiobiology parameters from radiation sources having similar radiobiological properties towards obtaining calculated values for RBE. Using these relationships, results were in general concordance with RBE values obtained from Russian clinical experience.