Particle species dependence of cell survival RBE: Evident and not negligible

With great interest we noticed the work of Singers S o rensen, Overgaard and Bassler [1], where the authors question if the linear energy transfer (LET) of ion beams can serve as a unique parameter determining the relative biological effectiveness (RBE) of ion irradiation, neglecting the particular particle species. For this purpose the authors collected a set of in vitro cell survival data from the published literature and came to the conclusion that ‘ the infl uence of varying the particle type on the RBE was far from obvious, compared to the general experimental noise ’ . They further state in the abstract that ‘ ...a dependence of particle type cannot be concluded, and LET alone in fact does seem to be an adequate parameter for describing RBE at 10% survival ’ . We agree that collecting in vitro cell survival data is the most convenient way to elucidate general systematics of the RBE. We also agree that fl uctuations of RBE values as observed in the data are large. Moreover, it is obvious that for all ions for a given LET only a certain range of RBE values is assumed. However, the conclusions S o rensen et al. draw seem to be misleading and not consistent to us. By reanalyzing the data they used, a dependence of RBE on LET and particle species rather than LET alone is clearly evident, although it is partially shaded by larger RBE fl uctuations. Using a purely LET dependent RBE estimate would in turn mean a systematic error in the RBE estimates. We show that such a simplifi cation hence suggests implications for particle therapy, which are not acceptable with respect to clinical endpoints. We propose that neglecting the particle type dependence of RBE is only appropriate for having a fi rst guess of possible RBE values instead of a reliable prediction. The use of treating LET as the only parameter determining the RBE is thus very limited. Figure 1 shows RBE at 10% survival of V79 cells plotted against the LET for He, C and Ne ions. Data points were reproduced from fi gure 1d of [1], hence the present discussion is based on the same data as used by S o rensen et al. As the authors nicely demonstrated in this LET range the RBE-LET characteristics are in good agreement with a linear relationship (although at the high LET end of the plot this can be questioned for He, as in this region the RBE-LET maximum occurs). The solid lines are linear fi ts to each data set of He, C and Ne ions. A visual impression suggests: 1) that the data points Acta Oncologica, 2013; 52: 589–603