Figure 1.

Illustration of experimental RBE uncertainties. Top: “Measured” RBEs have been generated by sampling values for photon and proton radiosensitivity based on a “true” RBE of 1.1 with normally distributed uncertainties of different magnitudes (points). Even small experimental uncertainties can lead to large uncertainties in derived RBEs (95% CI, dotted line). Bottom: Number of independent experiments needed to achieve a 95% CI of ± 2% on RBE in this system. Even with modest experimental errors of 10%, over 250 experiments studying identical conditions are needed to achieve this target. CI,confidence interval; RBE, relative biologicaleffectiveness.