Introduction: Accurate prediction of Relative Biological Effectiveness (RBE) is critical for carbon ion radiotherapy due to the strong variation of biological response along the beam path. Microdosimetric modeling provides a physics-based approach to estimate RBE by characterizing microscopic energy deposition. This study aims to evaluate the RBE at a 10% survival fraction (RBE₁₀) for a mono-energetic 330 MeV/u carbon ion beam using Monte Carlo-based microdosimetric analysis.

Materials and Methods: Monte Carlo calculations were conducted using the TOol for PArticle Simulation (TOPAS) incorporating microdosimetric extension. A mono-energetic 330 MeV/u carbon ion beam traversing a water phantom was modeled, with a three-dimensional silicon-on-insulator (SOI) Bridge microdosimeter embedded in the geometry to record microdosimetric energy deposition. Microdosimetric spectra were obtained at various depths, and the dose-mean lineal energy (yD) was calculated. The modified Microdosimetric Kinetic Model (MKM) with HSG cell parameters was applied to estimate RBE₁₀ along the beam path.

Results: The simulation predicted the Bragg peak at 201 mm, closely matching the experimental result with a deviation of approximately 2.8 mm. The (yD) increased markedly toward the Bragg peak region, reflecting enhanced linear energy transfer. The calculated RBE₁₀ increased from approximately 1.17 at the beam entrance to a maximum of 3.02 ± 0.38 at a depth of 201 mm, corresponding to the Bragg peak, followed by a gradual decrease beyond this region.

Conclusion: Monte Carlo-based microdosimetric modeling using an SOI microdosimeter and modified MKM provides a feasible approach for estimating RBE₁₀ distributions in carbon ion therapy. This approach is valuable for biological dose assessment and optimization of treatment planning.