New descriptors of radiation quality based on nanodosimetry, a first approach.

After a short overview on the latest developments in nanodosimetry, measured frequency distributions of ionisation cluster size caused by 4.6 MeV alpha-particles or low-energy electrons in 'nanometric' volumes of nitrogen are compared with cluster-size distributions for liquid water cylinders that are equal in size to segments of DNA of 10 base-pairs length. Such frequency distributions are, to a greater part, governed by the same basic physical interaction data as those to be expected, if charged particles interact with DNA segments. Quantities derived from ionisation cluster-size distributions should, therefore, behave as a function of radiation quality similarly to the yields of single or double strand breaks in the DNA. To test this assumption, extensive Monte Carlo simulations were performed for electrons in the energy range between 12.5 eV and 100 keV for protons at energies between 0.7 MeV and 250 MeV and for alpha-particles in the energy range between 2 MeV and 100 MeV. The results are then compared with the yields of single- or double-strand breaks in the DNA, taken from the literature.