Experimental characterization of a wall-less TEPC for nanodosimetric measurements

The deep understanding of the interaction between ionizing radiation and soft tissue is of primary importance for radiation therapy and in particular, hadron therapy. These cancer treatments exploit beams of intense energy to cause severe damage in the DNA of malignant cells and inactivate them. It is recognized that the observable radiobiological effects of ionizing radiations are strongly correlated to the clustering of damages in micrometer- and nanometer-sized subcellular structures, hence to the particle track structure. The characteristic properties of track structure are directly measurable nowadays with bulky experimental apparata, which cannot be easily operated in a clinical environment. With this in mind, an avalanche-confinement Tissue Equivalent Proportional Counter (TEPC) was constructed to simulate biological target sites of size down to 25 nm. This range of operation offers the means to perform a direct comparison between microdosimetric spectra and track-nanodosimetric distributions. A study on experimental gains of the filling gas of the detector has been conducted aiming at verifying the agreement with theoretical gains evaluated through different models. The TEPC was irradiated with an isotopic ²⁴⁴Cm alpha source at different impact parameters (i.e. distances between its center and the trajectory of the primary particle) showing that, once calibrated, it is capable of assessing a nanodosimetric cluster size distribution for some tenths of nanometers in simulated size. Several Monte Carlo simulations have been performed, showing a good agreement with experimental results. Moreover, the behavior of nanodosimetric distributions as certain parameters and conditions vary was studied. Finally, cluster size distributions produced by ²⁴⁴Cm alpha particles were simultaneously measured with the Startrack nanodosimeter (INFN-Laboratori Nazionali di Legnaro, Italy) and with the avalanche confinement TEPC. The comparison of pairwise measurements shows an excellent agreement when the signals from the TEPC are processed in order to include the reduced detection efficiency and the dead time of the Startrack detector. This result suggests a minor influence of the gas-avalanche stochastics on the measured distributions and encourages the use of proportional counters for the experimental characterization of therapeutic hadron beams at the nanometer level. Further measurements with different light ions are ongoing to confirm this finding. A direct and systematic comparison between nanometric-domain microdosimetric distributions and track-nanodosimetric measurements will allow investigating the feasibility of using the nano-microdosimetric spectrum to derive track-structure quantities at the nanometer level (for instance the mean cluster size M1 and the cumulative distributions F1, F2 and F3) which have been observed to correlate strongly with the radiation damage. (author)