The high steepness of dose-response curves for late-responding normal tissues

Values are calculated for the parameters α, β. and the number of tissue-rescuing units (TRU), which together describe the location and the steepness of dose-incidence curves for functional injury in various normal tissues. The analysis is based on a Poisson model of the distribution of surviving TRUs. The steepness of the curves for early-responding tissues has been shown previously to be compatible with values of sensitivity for the target colony-forming cells in these tissues. We now show that the steepness of curves for late responses in spinal cord, lung, and kidney is higher by a factor of up to 3 than the steepness of curves for early responses in other tissues. Although the interpretation of this higher steepness is not fully understood, this observation is likely to be of importance for radiotherapy.