Definitions of physical and biological low dose

The concept of 'low dose' may be understood to refer to an average frequency of microdosimetric events (charged-particle traversals) that is substantially less than unity in cells or their nuclei. An important consequence is that in this case the probability of any effect on autonomous cells must be proportional to the absorbed dose and independent of dose rate. However, this definition may be unnecessarily restrictive because--especially in the case of low-LET radiation--only a small fraction of events may cause the effects under consideration (e.g. cell lethality). This results in larger 'biological' rather than 'physical' 'small doses'. From a pragmatic viewpoint, one may consider the fact that in the linear-quadratic model deviations from proportionality between effect probability and absorbed dose are attributed to a term that depends on the square of the absorbed dose. This permits the selection of a criterion which establishes as 'small doses' those in which such deviations are less than a chosen value which in the examples given here is 10%. Different applications of this criterion to the inactivation of V 79 hamster cells are considered.