The physical basis for the biological action of heavy ions

Abstract

The physical properties of heavy ions determine to some extent their biological action. The spatial pattern of energy deposition is different from that by sparsely ionizing radiations like x- or γ-rays. The interaction with atomic electrons may lead to large local energy depositions in sensitive targets. Dose is essentially determined by particle fluence and the linear energy transfer (LET) of the projectile. The biological action depends on the number of targets hit by the particle and the probability to cause an effect by a single traversal which is called the ‘action cross section’. Action cross-section increases with LET approaching the geometrical cross section of sensitive targets. A detailed analysis shows that ion tracks have a certain structure produced by the action of more energetic far-reaching electrons liberated by the initial interactions of the incoming ion. Track structure is analysed using a simple model based on classical collision dynamics describing at least the essential features. It is shown that very high energies are deposited in the track centre which may lead to large local concentrations of initial lesions. The radial extension of tracks can lead to damage in sensitive targets even if they are not directly traversed. Track structure analysis can account for this in a qualitative way but the quantitative agreement is still poor. The models are not able to explain the higher relative biological effectiveness (RBE) of heavy ions compared to x- or γ-rays. Biological factors such as lesion clustering and repair processes have additionally been taken into account. This can be done by `phenomenological models' incorporating track structure analysis and assumptions on the dependence of biological effectiveness on local energy density.