Current biophysical approaches to the understanding of biological effects of radiation in terms of local energy deposition

Relevant biological effects of radiation in mammals are due mostly to radiation damage to individuals cells. It is also well established that the spatial and temporal distribution of radiation interactions within the cell or its nucleus (diameter approximately 10 microns) has an important influence on biological effectiveness. Therefore these distributions must be considered if we are seeking either practical quantities for comparison of different radiations in radiation protection or therapy, or if we are seeking a more fundamental understanding of the mechanisms of action. Local energy deposition can readily be measured experimentally over the larger subcellular dimensions (down to 1 micron). Monte-Carlo track structure techniques allow calculation of local energy deposition, and many other properties, down to 1 nm. The structure of Monte-Carlo simulated tracks of alpha-particles and protons (such as are produced by neutron irradiation) are discussed in relation to the dimensions of possible subcellular genetic targets. The biologically relevant properties of the radiations must be sought amongst a vast back-ground of biologically 'irrelevant' atomic damage. This search may be guided by radiobiological experimental data for diverses radiations and by biophysical models of radiation action. These highlight the apparent importance of properties of radiation tracks over very small distances of less than 100 nm. Consequently computations are being undertaken of distributions of local energy deposition in small geometric volumes (including simple models of DNA and chromatin) of dimensions down to 1 nm, for a wide variety of radiations. It is shown that with appropriate assumptions it is possible to predict the absolute probabilities of cell death (per unit absorbed dose) for a variety of alpha-particle energies