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 at the Tandem–Alpi accelerator facility of Legnaro National Laboratories (LNL), in which the frequency distribution of the number of ionisations produced by proton and carbon ion beams of given energy was measured with the PTB Ion Counter and the LNL StarTrack Counter. Descriptors of the track structure can be derived from these distribution; in particular, the first moment M1, representing the mean number of ionisations produced in the target volume, and the cumulative probabilities Fk of measuring cluster sizes ν ≥ k. Experimental radiobiological data have been compared with nanodosimetric quantities derived from measured ionisation-cluster size distributions. It was found that the cumulative probabilities Fk as a function of M1 behave in the same way as the inactivation cross sections as a function of LET, first increasing with increasing values of M1 and then showing a saturation effect. A comparison of Fk and radiobiological quantities is presented and discussed
