In this study, Monte Carlo simulations based on the Geant4-DNA code were employed to investigate the microdosimetric characteristics of an Iridium-192 brachytherapy source at a distance of 1 cm. The realistic photon spectrum was extracted according to the TG-186 model and used as input for the simulations. Photon interactions and secondary electron spectra in liquid water were simulated using the G4EmDNAPhysics_option2 physics model. Microdosimetric quantities, including dose-mean lineal energy (y ̅_D) and frequency-mean lineal energy (y ̅_F), were calculated for spherical targets with diameters ranging from 2 to 40 nm. The DNA double-strand break yield (Y_DSB) was determined using an energy-threshold model based on track-structure data. Results showed that as target diameter increased, y ̅_D and y ̅_F decreased, indicating a volumetric averaging effect. In contrast, Y_DSB increased with larger target sizes, attributable to the higher number of sensitive biological targets within the larger volume. Distributions of lineal energy and specific energy revealed significant inhomogeneity in energy deposition at the nanometric scale, particularly pronounced in smaller targets. These findings underscore that assessing radiation quality in brachytherapy based solely on macroscopic dose metrics is insufficient; microdosimetric and nanodosimetric approaches are essential for a deeper understanding of radiobiological mechanisms and for optimizing treatment planning. Unlike macroscopic dosimetry approaches such as MIRD, which rely on organ-level absorbed doses and energy averaging over large volumes, our nanoscale track-structure approach quantifies stochastic energy depositions at the DNA level, enabling the calculation of biologically relevant quantities such as double-strand break yield that are not accessible with conventional methods.