RBE of ion beams in hypofractionated radiotherapy (SBRT)

An important advantage for the application of carbon ion beams in tumour therapy is their increased relative biological effectiveness (RBE) as compared to conventional photon radiation. Since RBE among other factors depends on the dose level, the precise knowledge of the RBE dependence on the dose is of particular importance for the comparison of different fractionation schemes, involving different doses per fraction. Here we describe some general properties of the RBE vs. dose dependence, which are determined using a simple modelling approach based on the linear-quadratic model as well as a more sophisticated predictive model for the description of RBE. We show that for both approaches the systematic dependence of RBE on the cell or tissue type as characterized by the α/β-ratio of the photon linear quadratic parameters is expected to be inverted at high doses as compared to low doses. We demonstrate that this inversion is not a model specific feature, but a rather generic feature resulting from the linear-quadratic shape of dose response curves and the correlation between RBE and the photon α/β-ratio. The results are discussed with respect to other modelling approaches and to their implications for clinical applications of carbon ion beams using hypofractionated treatment schedules.