A new DNA target model for track structure calculations and its first application to I-125 Auger electrons.

A DNA target model has been developed, based on the geometrical co-ordinates of individual atoms. This model is used to analyse DNA damage produced by Auger electron tracks from the decay of 125I. The high resolution of this target model enables the distinction between direct and indirect electron hits, i.e. hits inside the atomic volumes of the DNA molecule and those hitting the water molecules assumed in the space between the atomic volumes. Three types of calculations have been performed: (1) the evaluation of the energy deposition in the surroundings of the decaying 125I nuclide demonstrating different fractions of direct to indirect hits at different parts of the DNA molecule (phosphate/sugar strand or bases), (2) a detailed energy deposition pattern in the radiolabelled base, indicating that this most burdened molecule is not necessarily destroyed by direct hits, and (3) a calculation of single- and double- strand breaks by using different threshold values for effective direct and indirect hits, resulting in a good correlation with experimental data on strand break efficiency.