ESTRO 2021 Abstract Book

S620

ESTRO 2021

variable for these models but the way the LETd was defined, calculated or determined was not consistent, which may impact the corresponding RBE value. This study compares consistently calculated LETd with other quantities as input variables for a phenomenological RBE model and attempts to determine which quantity that can best predict the RBE value for protons. Materials and Methods Experimental set-ups of in vitro cell survival experiments for proton RBE determination are simulated using the SHIELD-HIT12A Monte Carlo particle transport code. In addition to LETd, several other beam quality quantities are determined, such as track averaged LET, z*²/β², Q and average energy. The averaging methods applied and which secondary particles are included were also varied for each included quantity. A phenomenological RBE model is finally applied to the in vitro data with the various beam quality quantities used as input variables and the goodness of fit is determined and compared. Results Assuming a linear relationship between the beam quality quantities and the α/α ₓ and β/β ₓ -terms, track averaged Q (Qt) including only primary protons has the overall best fit, with a RMSE of 0.74 ± 0.11 and 1.14 ± 0.17, respectively. Also for a quadratic relationship, Qt, including only primary protons has the best fit with an RMSE of 0.70 ± 0.11 and 0.88 ± 0.14 for α/α ₓ and β/β ₓ , respectively. While these quantities yield slightly better average results than LETd, LETd with any variation of included secondary particles is still within one standard error from the quantity with the best fit (Figure 1 and Figure 2). This indicates that for protons, the choice of input parameters considered as alternatives to LETd in the RBE model has a smaller impact than the statistical uncertainties in the cell survival experiments.