A model of biological effects of ionizing particles, especially of protons and other ions, is proposed. The model is based on distinguishing the single-particle and collective effects of the underlying radiobiological mechanism. The probabilities of individual particles causing severe damage to DNA, their synergetic or saturation combinations, and the effect of the cellular repair system are taken into account. The model enables one to describe linear, parabolic and more complex curves, including those exhibiting low-dose hypersensitivity phenomena, in a systematic way. Global shape as well as detailed structure of survival curves might be represented, which is crucial if different fractionation schemes in radiotherapy should be assessed precisely. Experimental cell-survival data for inactivation of V79 cells by low-energy protons have been analysed and corresponding detailed characteristics of the inactivation mechanism have been derived for this case.

Institute of Physics Academy of Sciences of the Czech Republic Na Slovance 2 CZ 182 21 Praha 8 Czech Republic

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Analytical formulas representing track-structure simulations on DNA damage induced by protons and light ions at radiotherapy-relevant energies