Maximum RBE change in ¹⁹²Ir, ¹²⁵I, and ¹⁶⁹Yb brachytherapy and the corresponding effect on treatment planning

The purpose of this study was to examine RBE variation as a function of distance from the radioactive source, and the potential impact of this variation on a realistic prostate brachytherapy treatment plan.

Three brachytherapy sources (¹²⁵I, ¹⁹²Ir, and ¹⁶⁹Yb) were modelled in Geant4 Monte Carlo code, and the resulting electron energy spectrum in water in 3D space around these sources was scored (voxel size of 2 mm³). With this energy spectrum, microdosimetric techniques were used to calculate the maximum RBE, RBE_M, as a function of distance from the source. RBE_M of ¹²⁵I relative to ¹⁹²Ir was calculated in order to validate simulations against literature; all other RBE_M calculations were done by normalizing electron fluence at various distances to the source position. In order to examine the impact of RBE_M variation in treatment planning, a realistic ¹⁹²Ir prostate plan was re-evaluated in terms of RBE instead of absorbed dose.

The RBE_M of ¹²⁵I, ¹⁹²Ir, and ¹⁶⁹Yb at 8 cm away from the source was 0.994 (+/-0.002), 1.030 (+/-0.003), and 1.066 (+/-0.008), respectively. RBE_M in the HDR prostate treatment plan exhibited several hot (+3.6% in RBE_M) spots.

The large increase RBE_M observed in ¹⁶⁹Yb has not yet been described in the literature. Despite the presence of radiobiological hotspots in the HDR treatment, these variations are likely nominal and clinically insignificant.