Uncertainties in the relative biological effectiveness of therapeutic proton beams associated with bias towards high doses per fraction in radiobiological experiments
Most data supporting the widely accepted relative biological effectiveness (RBE) value of 1.1 for therapeutic proton beams are from radiobiological experiments with relatively high doses per fraction. The purpose of this study was to estimate bias in RBE that differences in dose levels between these experiments and proton radiotherapy treatments may cause. The linear quadratic model was applied to calculate, using prior experimental data, RBE variations with dose and a/b ratio for doses delivered in a standard fractionation regimen. The results suggest that the RBE measured at relatively high doses per fraction typical for a radiobiological experiment underestimates the RBE of proton radiotherapy with a standard fractionation. The bias increases with decreasing radiation dose and decreasing a/b ratio, suggesting that, if differences in dose levels are not accounted for, there may be a large underestimation of biological effects in late-responding tissues exposed to low doses of radiation.
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