Microdosimetric Evaluation of Current and Alternative Brachytherapy Sources-A Geant4-DNA Simulation Study

Purpose: Radioisotopes such as Se-75, Yb-169, and Gd-153 have photon energy spectra and half-lives that make them excellent candidates as alternatives to Ir-192 for high-dose-rate brachytherapy. The aim of the present study was to evaluate the relative biological effectiveness (RBE) of current (Ir-192, I-125, Pd-103) and alternative (Se-75, Yb-169, Gd-153) brachytherapy radionuclides using Monte Carlo simulations of lineal energy distributions. Methods and Materials: Brachytherapy sources (microSelectron v2 [Ir-192, Se-75, Yb-169, Gd-153], SelectSeed [I-125], and TheraSeed [Pd-103]) were placed in the center of a spherical water phantom with a radius of 40 cm using the Geant4 Monte Carlo simulation toolkit. The kinetic energy of all primary, scattered, and fluorescence photons interacting in a scoring volume were tallied at various depths from the source. Electron tracks were generated by sampling the photon interaction spectrum and tracking all the interactions down to 10 eV using the event-by-event capabilities of the Geant4-DNA models. The dose mean lineal energy ((y) over bar (D)) values were obtained through random sampling of transfer points and overlaying spherical scoring volumes within the associated volume of the tracks. The scoring volume diameter was determined by fitting the (y) over bar (D) ratio for I-125 to its observed RBE. Results: (y) over bar (D) increased with the increasing distance from the source for Ir-192, Se-75, and Yb-169, remained constant for Gd-153 and I-125, and decreased for Pd-103. The diameter at which the (y) over bar (D) ratio coincided with the RBE of 1.15 to 1.20 for I-125 was similar to 25 to 40 nm. The RBE (reference 1 MeV photons) at high doses and dose rates for Ir-192, Se-75, Yb-169, Gd-153, I-125, and Pd-103 was 1.028 to 1.034, 1.05 to 1.07, 1.12 to 1.15, 1.16 to 1.21, 1.15 to 1.20, and 1.17 to 1.22, respectively. Conclusions: The radiation quality of the radionuclides under investigation was greater than that of high-energy photons. The present study has provided a set of values to modify the prescription doses for brachytherapy to account for the variation in radiation quality among radionuclides. (C) 2017 Elsevier Inc. All rights reserved.