First characterization of a multi-site microdosimeter

Abstract

Background: It is well established that different types of radiation cause varying degrees of biological damage, even when delivered at the same physical dose. This variability arises from differences in the spatial distribution of energy deposition at the subcellular level. Understanding this is critical for improving radiation therapy techniques, particularly hadron therapy, where precise beam characterization is essential for optimizing treatment plans. Traditional dosimetric approaches focus primarily on quantitative dose measurements, but a deeper understanding of the biological effectiveness of radiation requires additional information.

Purpose: The MUSICA project, funded by the 5^th scientific commission of the Italian National Institute for Nuclear Physics, aims to develop an innovative microdosimeter that will enhance conventional dosimetry by incorporating qualitative insights into radiation interactions at the cellular and subcellular levels. By measuring physical parameters correlated with biological effectiveness, this detector provides a more comprehensive characterization of radiation quality. A specific application in hadron therapy, particularly proton therapy, is expected to improve treatment accuracy and patient outcomes by refining dose distributions and radiobiological effectiveness models.

Methods: Radiation-induced damage occurs across multiple scales, from DNA (≈2 nm) to the entire cell nucleus (≈10 µm). The stochastic spatial distribution of energy deposition can be experimentally investigated using microdosimetric techniques. A widely used approach relies on tissue-equivalent gas proportional counters (TEPCs), which enable measurement of microscopic energy deposition in a manner representative of human tissue. While previous studies with TEPCs primarily focused on chromosomal-scale sites (<2 µm), characterizing energy deposition at the scale of entire nuclei (≈10 µm) has often relied on solid-state detectors. However, gas microdosimeters offer advantages in sensitivity, geometric flexibility, and tissue equivalence. This project introduces a novel multi-site TEPC capable of performing microdosimetric measurements at two distinct site sizes (e.g., 1 and 10 µm) within a single measurement session, without requiring gas pressure adjustments. The detector achieves this by incorporating two charge collection regions, enabling simultaneous characterization at both microscopic scales. This unique capability facilitates the direct comparison of radiation interactions at different spatial scales, potentially leading to a more refined understanding of how ionizing radiation damages living matter.

Results: The performance of the multi-site TEPC was evaluated by characterizing its response to different radiation qualities and site sizes. Preliminary results indicate that the detector effectively captures variations in energy deposition patterns, providing valuable data for assessing radiation quality. The two-dimensional microdosimetric information obtained with this detector offers insights into the relationship between physical parameters of the ionizing radiation field and biological effectiveness, which could contribute to the development of improved radiobiological models.

Conclusions: The innovative microdosimetric detector represents a significant advancement in radiation quality assessment. By enabling simultaneous characterization at multiple spatial scales, this technology bridges the gap between conventional dosimetry and radiobiological modeling. The ability to integrate qualitative and quantitative dosimetric data could lead to more accurate treatment planning in proton therapy and other applications in radiation medicine. Future studies will further explore the potential of this detector to refine current radiobiological models and enhance clinical outcomes in hadron therapy.

Keywords: TEPC; hadron therapy; microdosimetry; multi‐site microdosimetry; proton therapy; radiation quality.