ESTRO 2022 - Abstract Book

S1525

Abstract book

ESTRO 2022

Materials and Methods The SHIELD-HIT12A Monte Carlo particle transport code is used to simulate experimental setups of in vitro cell survival experiments for proton RBE determination. The inverse of average primary energy as well as track- and dose averaged versions of LET, Q [doi: 10.1080/0284186X.2017.1356468] and z* ² / β² (here called Q eff ) are scored including various secondary particles. These are then used as input variables for a phenomenological RBE model. The Bayesian information criterion (BIC) is utilized to determine the best polynomial degree for relationship between the beam quality quantities and the ratios of the parameters describing the response to protons and photons assuming the LQ model for cell survival, α / α ₓ and β / β ₓ . The goodness of fit is analyzed using a bootstrapping approach. Results A cubic fit for α / α ₓ and a quadratic fit for β / β ₓ were chosen by the BIC-score. Track averaged Q eff including all protons rendered the overall best fit, with a relative RMSE for RBE 2 Gy of 1.45 ± 0.43 (2 σ ). Generally, Q eff outperforms Q, which in turn outperforms LET while the averaging modality (dose or track) and the particles included in the scoring (all, protons or only primaries) have a minor impact on the result as illustrated in Figures 1 and 2 . LETd, however, regardless of the way it is assessed, is still within the 95% confidence interval from the quantity with the best fit. Conclusion While no alternative quantity to LETd could be identified as superior with statistical certainty the results of this study indicate that Q eff outperforms LET, which together with its medium-, and density independence could possibly make it a more suitable quantity to use for RBE calculations. However, as the various beam quality quantities are highly correlated, more experimental data with smaller statistical uncertainties are needed to determine the best input parameter for phenomenological RBE models for protons.

Figure 1: Distribution of RMSE´s for RBE 2 Gy from 10000 bootstrapped samples for all quantities including all protons.