A mini-tissue-equivalent proportional-counter (TEPC) has been constructed to study the possibility to manufacture mini-counters without field-shaping tubes for radiation therapy. The mini-TEPC can be assembled with and without field-shaping tubes. It can be equipped with a mini-alpha source for a precise lineal energy calibration. After the positive conclusions of this study, a slim TEPC has been designed and constructed. The slim TEPC has an external diameter of only 2.7 mm. It has been tested with therapeutic proton beams and gamma ray sources.

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 F_k 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.