Purpose: 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. Methods: Three brachytherapy sources ( 125 I, 192 Ir, and 169 Yb) 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 mm 3 ). 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 125 I relative to 192 Ir 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 192 Ir prostate plan was re-evaluated in terms of RBE instead of absorbed dose. Results: The RBE M of 125 I, 192 Ir, and 169 Yb 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. Conclusions: The large increase RBE M observed in 169 Yb 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. 103Pd. These sources have different energy spectra and therefore have different mean energies (table 1).Since it is known that lower energy particles are more damaging, the question of whether RBE changes and if so by how much has been posed many times before [6][7][8][9][10]. In this section, the previous works on RECEIVED