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

Abstract

Radioisotopes such as ⁷⁵Se, ¹⁶⁹Yb, and ¹⁵³Gd have photon energy spectra and half-lives that make them excellent candidates as alternatives to ¹⁹²Ir for high-dose-rate brachytherapy. The aim of the present study was to evaluate the relative biological effectiveness (RBE) of current (¹⁹²Ir, ¹²⁵I, ¹⁰³Pd) and alternative (⁷⁵Se, ¹⁶⁹Yb, ¹⁵³Gd) brachytherapy radionuclides using Monte Carlo simulations of lineal energy distributions. Brachytherapy sources (microSelectron v2 [¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb, ¹⁵³Gd], SelectSeed [¹²⁵I], and TheraSeed [¹⁰³Pd]) 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¯_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¯_D ratio for ¹²⁵I to its observed RBE. y¯_D increased with the increasing distance from the source for ¹⁹²Ir, ⁷⁵Se, and ¹⁶⁹Yb, remained constant for ¹⁵³Gd and ¹²⁵I, and decreased for ¹⁰³Pd. The diameter at which the y¯_D ratio coincided with the RBE of 1.15 to 1.20 for ¹²⁵I was ∼25 to 40 nm. The RBE (reference 1 MeV photons) at high doses and dose rates for ¹⁹²Ir, ⁷⁵Se, ¹⁶⁹Yb, ¹⁵³Gd, ¹²⁵I, and ¹⁰³Pd 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. 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.