High LET constraints on low LET survival.

Survival curves for biological cells irradiated with gamma rays (and other low LET radiations) cannot be properly interpreted without examining the implications of these interpretations for high LET radiations. The theory of RBE demands that the RBE of any high LET radiation field is at most one when the probability for cell killing as a function of the absorbed dose of gamma rays is linear or sublinear, for homogeneous samples whose radiosensitivity parameters are not altered during the irradiation. A contrary experimental finding strongly suggests that the experimental sample is heterogeneous. Violation of this constraint is not unusual where the survival curve found with low LET radiations has a well defined initial negative slope.