A Unified System of Radiation Bio-Effectiveness and its Consequences in Practical Application

Abstract

A model has been formulated as a function of the quality parameter, 'mean free path for linear primary ionisation', denoted here by lambda, for prediction of the risk of inactivation of irradiated mammalian cells. The model is intended to be a unified one applicable to any radiation type. In effect, it determines the yield of double-strand breaks per charged particle fluence in the medium of interest. It includes provision for direct and indirect action with a repair capability that is dependent on the duration of the irradiation, rather than the dose rate, and the known variation in radiosensitivity during progress of the cell cycle. The general validity of the model, derived from probe experiments with heavy charged particles, is tested against the results of Hofer et al in their experiments on synchronised CHO cells, labelled with the Auger electron emitter 125I. Good agreement is obtained for an allocated mean repair time of 2.3 h. Analysis of the predictions of the model leads to new interpretations on the damaging role of heavy particles in therapy and protection, viz: that delta rays around fast heavy ion tracks contribute minimally, possibly negligibly, to the damage; that protons, and neutrons because of the short ranges of the recoils at their most damaging energy, can never reach the saturation level found for the heavier ions at the same lambda and that there is no difficulty in extrapolation of effects in cellular targets for high and low LET radiations to low doses. Maximum RBEs, known to the LET dependent, are found to have a common maximum near 2 nm when the radiation quality is defined as 'the mean free path for linear primary ionisation'. On the basis of this work there seem to be realistic prospects of defining a new unified fluence-based system of dosimetry which, with conceptually new instrumentation designed to have a radiation response simulating that of the nanometre sites, possibly a segment of the DNA, will be more rigorous and meaningful than the currently adopted system. Examples of application of the new quality concept leads to proposals for improved heavy particle therapy and of risk evaluation in radiation protection.