The biological action of ionizing charged particles is initiated at the DNA level, and the effectiveness with which the initial physical effect changes into measurable biological damage is likely ruled by the stochastics of ionizations produced by the incident ions in subcellular nanometric volumes. Based on this hypothesis, experimental nanodosimetry aims at establishing a new concept of radiation quality that builds on measurable characteristics of the particle track structure at the nanometer scale. Three different nanodosimetric detection systems have been developed to date that allow measurements of the number of ionizations produced by the passage of a primary particle in a nanometer-size gas volume (in unit density scale). Within the Italian project MITRA (MIcrodosimetry and TRAck structure), funded by the Italian Istituto Nazionale di Fisica Nucleare (INFN) and the EMRP Joint Research Project 'BioQuaRT' (Biologically Weighted Quantities in Radiotherapy), experiments have been carried out, in which the frequency distribution of ionizations produced by proton and carbon ion beams of given energy was measured with the three nanodosimetric detectors. Descriptors of the track structure can be derived from these distributions. In particular, the first moment M1, representing the mean number of ionizations produced in the target volume, and the cumulative probability Fk of measuring a number ν ≥ k of ionizations. The correlation between measured nanodosimetric quantities and experimental radiobiological data available in the literature is here presented and discussed.

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.

Nanodosimetry is a branch of dosimetry for investigation and modeling of the interaction pattern of ionizing radiation in nanometre site-sizes (at unit density), which dates back to the 1970's (Pszona S. A track ion counter. Proceedings of Fifth Symposium on Microdosimetry EUR 5452 d-e-f, Published by the Commission of the European Communities, Luxemburg, pp. 1107-1122 (1976)). To date, the different experimental approaches have lead to developing of three fully functional nanodosimeters: the Jet Counter operated at NCBJ, the Ion Counter operated at PTB and Startrack Counter operated at INFN-LNL. Descriptions of each nanodosimeter as well as of the techniques used to investigate the track structure of ionizing particles are presented.

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The first experimental data on the distribution of ionisation cluster size produced by low energy electrons in a target cylinder of nitrogen, 3.5 nm in diameter at unit density, which is equivalent to approximately 2 nm in liquid water are presented. In the experiment, nanometric targets were simulated in the so-called Jet Counter. It consists of a pulse-operated valve which injects an expanding jet of nitrogen into an interaction chamber where a cylindrical sensitive volume is created. This sensitive volume was irradiated by electrons at 300 eV, 500 eV and 1 keV, emitted by an electron gun. The distribution of ionisation cluster size was measured using the single-ion-counting method and compared with the results of Monte Carlo simulation.