Comparative study of a microdosimetric biological weighting function for RBE₁₀ modeling in particle therapy with a solid state SOI microdosimeter

Objective: the recently developed V79-RBE₁₀ biological weighting function (BWF) model is a simple and robust tool for a fast relative biological effectiveness (RBE) assessment for comparing different exposure conditions in particle therapy. In this study, the RBE₁₀ derived by this model (through the particle and heavy ion transport code system (PHITS) simulated d(y) spectra) is compared with values of RBE₁₀ using experimentally derived d(y) spectra from a silicon-on-insulator (SOI) microdosimeter. Approach: experimentally measured d(y) spectra are used to calculate an RBE₁₀ value utilizing the V79-RBE₁₀ BWF model as well as the modified microdosimetric kinetic model (MKM) to produce an RBE₁₀-vs-y_D trend for a wide range of ions. In addition, a beamline specific PHITS simulation was conducted which replicated the exact experimental conditions that were used with the SOI microdosimeter at the heavy ion medical accelerator in Chiba biological beamline with ¹²C ions. Main Results: the RBE₁₀-vs-y_D trend for ¹H, ⁴He, ⁷Li, ¹²C, ¹⁴N, ¹⁶O, ²⁰Ne, ²⁸Si, ⁵⁶Fe, and ¹²⁴Xe ions is examined with good agreement found between the SOI microdosimeter derived RBE₁₀ values with the V79-RBE₁₀ BWF model and MKM, as well as the PHITS simulations for ¹H, ⁴He, ⁷Li, ¹²C, ¹⁶O, and ⁵⁶Fe ions while some discrepancies were seen for ¹⁴N, ²⁰Ne, and ²⁸Si ions. Deviations have been attributed to the difference in the derivation of the d(y) spectra based on the different methods utilized. Good agreement was found between y_D values and an over estimation was observed for RBE₁₀ values for the beamline specific simulation of the ¹²C ion beam. Significance: overall, this study shows that the SOI microdosimeter is a valuable tool that can be utilized for quick and accurate experimental derivation of the d(y) spectra, which can then be convoluted with the weighting function of the V79-RBE₁₀ BWF model to derive RBE₁₀. The SOI microdosimeter is able to derive experimental values of y_D and RBE₁₀ for various ions in any irradiation condition utilizing other radiobiological models.