Target size in radiobiology

In the region of thindown, target size plays a larger role in the calculation of action cross-sections than elsewhere in the track-width regime. If the target is small compared to the ''delta-ray track width'' this width may determine the cross-section, while if the target is large the target size may determine the cross-section. As a consistency test of track theory the authors examine the implications of target size estimates drawn from survival data in the grain-count regime for action cross-sections in the thindown region. From cellular radiosensitivity parameters obtained earlier (for similar but different cells) the authors extract the theoretical target size. They then calculate the ion-kill cross-sections for these cells and the HZE ions used in GSI-Darmstadt experiments, and join these results to our earlier findings. The cross-section calculations are limited by lack of knowledge of the radial distribution of dose for these particles, but simply extrapolating formulas developed earlier (somewhat beyond their limit of validity), their cross-section calculations yield qualitative agreement with much of the experimental data. Target size implications from track theory seem to have physical reality