Development of a new microdosimetric biological weighting function for the RBE₁₀ assessment in case of the V79 cell line exposed to ions from ¹H to ²³⁸U

<jats:title>Abstract</jats:title> <jats:p>An improved biological weighting function (IBWF) is proposed to phenomenologically relate microdosimetric lineal energy probability density distributions with the relative biological effectiveness (RBE) for the <jats:italic>in vitro</jats:italic> clonogenic cell survival (surviving fraction = 10%) of the most commonly used mammalian cell line, i.e. the Chinese hamster lung fibroblasts (V79). The IBWF, intended as a simple and robust tool for a fast RBE assessment to compare different exposure conditions in particle therapy beams, was determined through an iterative global-fitting process aimed to minimize the average relative deviation between RBE calculations and literature <jats:italic>in vitro</jats:italic> data in case of exposure to various types of ions from <jats:sup>1</jats:sup>H to <jats:sup>238</jats:sup>U. By using a single particle- and energy- independent function, it was possible to establish an univocal correlation between lineal energy and clonogenic cell survival for particles spanning over an unrestricted linear energy transfer range of almost five orders of magnitude (0.2 keV <jats:italic>µ</jats:italic>m<jats:sup>−1</jats:sup> to 15 000 keV <jats:italic>µ</jats:italic>m<jats:sup>−1</jats:sup> in liquid water). The average deviation between IBWF-derived RBE values and the published <jats:italic>in vitro</jats:italic> data was ∼14%. The IBWF results were also compared with corresponding calculations (<jats:italic>in vitro</jats:italic> RBE<jats:sub>10</jats:sub> for the V79 cell line) performed using the modified microdosimetric kinetic model (modified MKM). Furthermore, RBE values computed with the reference biological weighting function (BWF) for the <jats:italic>in vivo</jats:italic> early intestine tolerance in mice were included for comparison and to further explore potential correlations between the BWF results and the <jats:italic>in vitro</jats:italic> RBE as reported in previous studies. The results suggest that the modified MKM possess limitations in reproducing the experimental <jats:italic>in vitro</jats:italic> RBE<jats:sub>10</jats:sub> for the V79 cell line in case of ions heavier than <jats:sup>20</jats:sup>Ne. Furthermore, due to the different modelled endpoint, marked deviations were found between the RBE values assessed using the reference BWF and the IBWF for ions heavier than <jats:sup>2</jats:sup>H. Finally, the IBWF was unchangingly applied to calculate RBE values by processing lineal energy density distributions experimentally measured with eight different microdosimeters in 19 <jats:sup>1</jats:sup>H and <jats:sup>12</jats:sup>C beams at ten different facilities (eight clinical and two research ones). Despite the differences between the detectors, irradiation facilities, beam profiles (pristine or spread out Bragg peak), maximum beam energy, beam delivery (passive or active scanning), energy degradation system (water, PMMA, polyamide or low-density polyethylene), the obtained IBWF-based RBE trends were found to be in good agreement with the corresponding ones in case of computer-simulated microdosimetric spectra (average relative deviation equal to 0.8% and 5.7% for <jats:sup>1</jats:sup>H and <jats:sup>12</jats:sup>C ions respectively).</jats:p>