Nanodosimetric descriptors of the radiation quality of carbon ions.

In view of the emerging interest of carbon ions in radiotherapy and of the strong correlation between the track structure and the radiobiological effectiveness of ionising radiations, the track-structure properties of (12)C-ions were studied at particle energies close to the Bragg peak. To perform the investigations, ionisation-cluster-size distributions for nanometre-sized target volumes were measured with the track-nanodosimeter installed at the TANDEM-ALPI accelerator complex at LNL, and calculated using a dedicated Monte Carlo simulation code. The resulting cluster-size distributions are used to derive particular descriptors of particle track structure. Here, the main emphasis is laid on the mean ionisation-cluster size M1 and the cumulative probability Fk of measuring cluster sizes ν ≥ k. From the radiobiological point of view, Fk is of particular interest because an increasing k corresponds to an increase of damages of higher complexity. In addition, Fk saturates with increasing radiation quality like radiobiological cross sections as a function of linear energy transfer. Results will be presented and discussed for (12)C-ions at 96 and 240 MeV.