A critique of Katz's 'high LET constraint on low LET survival'

Katz's interpretation of the connexion between RBE and LET (see ibid., vol.23, p.909 (1978)) is contrasted with a version published previously by Burch (1970). The implications of Katz's model for dose-response relations apply only at ultra-high absorbed doses in Burch's model. In the latter, the shoulder on type-C survival curves for mammalian cells is explained in terms of Haynes' repair model. Under certain conditions the repair model becomes mathematically equivalent to the ' alpha - beta ' model; under some other conditions it becomes equivalent to the 'two-component' model. The formulation of a new repair hypothesis, based on the idea of an inducible repair mechanism, is also set out. It is argued that Katz's 'supralinearity index' is appropriate to the induction of (rare) mutations but inappropriate to cell survival, for which an alternative index is proposed. Certain plausible hypotheses of radiobiological action conflict with Katz's 'logical constraint' which, it is contended, is neither logical nor valid. In conclusion, although experimental findings for some radiobiological systems conform to Katz's 'constraint', the frequently observed violations should not necessarily be regarded as artefacts.