Anna Bianchi

Environmental radiation fields are characterized by very low dose rates and sparse energy-deposition events, conditions under which absorbed dose alone provides an incomplete description of irradiation at cellular and subcellular scales. This study assessed the feasibility of environmental microdosimetry in the external laboratory of the Laboratori Nazionali del Gran Sasso (LNGS) using a refurbished 5 cm spherical tissue-equivalent proportional counter (EuTEPC). The detector was converted into a stand-alone system, equipped for remote control and timed spectral acquisition, and installed at LNGS for long-duration measurements. Initial LNGS measurements were dominated by electromagnetic interference and did not yield physically interpretable spectra. Practical troubleshooting indicated that the dominant disturbance was associated with sporadic electromagnetic pickup, although the coupling mechanism was not investigated systematically. Extending the shielding to the detector and the complete cabling chain substantially reduced the noise and enabled the acquisition of stable lineal-energy distributions. Spectra acquired several months apart preserved the same overall shape, indicating consistent measurement performance under very low event-rate conditions. Comparison with measurements at Legnaro National Laboratories (LNLs) and with published environmental microdosimetric data from Ottawa and the high-altitude site of Gulmarg showed physically consistent spectral features, while also highlighting differences consistent with variations in radiation-field composition, detector characteristics and acquisition conditions. These results establish an operational platform for environmental microdosimetry at LNGS and identify electromagnetic compatibility as a critical requirement for measurements in very-low-background environments.

Objective. This work presents the first experimental characterization of the DIODE detector, a novel detection system based on single-crystal diamond, designed for simultaneous dosimetric and microdosimetric measurements in clinical proton therapy. The aim is to evaluate its capability to measure absorbed dose, and estimate LET and RBE variations along a clinical proton beam. Approach. The detector was tested under a 70 MeV monoenergetic proton beam at the Trento Proton Therapy Centre. Depth-dose and lateral profiles were compared with EBT3 radiochromic films. Microdosimetric spectra were simultaneously acquired and benchmarked against a mini tissue-equivalent proportional counter (TEPC). Monte Carlo (MC) simulations were also performed to model the experimental setup and the DIODE geometry. Main results. The detector showed linearity with dose with a sensitivity of 0.60 ± 0.01 nC Gy⁻¹, with a dark current below 0.1 pA, ensuring a good signal-to-noise ratio. Depth-dose profiles matched EBT3 film data and MC simulations, with differences below 2% in peak-to-plateau ratios, indicating limited LET dependence. The dose-mean lineal energy y ¯ D closely agrees with simulated dose-averaged LET values, except in the beam entrance region. In this area, y ¯ D values were lower than those obtained with the mini-TEPC due to electronic saturation limiting the detection of rare high-LET secondary fragments. Variation in RBE was assessed from the microdosimetric data, based on Loncol’s weighting function, which refers to clonogenic cell survival. RBE values ranged from ∼1.1 at the entrance to ∼1.8 in the distal region, consistent with mini-TEPC data and literature RBE₁₀ values for V79 and U89 cells. Lateral dose and microdosimetric profiles confirmed high spatial resolution and revealed proton energy variations near the Bragg peak. Significance. The DIODE detector demonstrated reliable performance for simultaneous dosimetric and microdosimetric characterization of clinical proton beams. Its ability to measure dose, and to support LET and RBE assessment through microdosimetric modelling, in a single device, highlights its potential as an advanced tool for beam quality assessment and biologically optimized treatment planning in proton therapy.

Different types of radiation cause varying levels of biological damage, even when the physical dose is the same, due to the unique way energy is distributed at the subcellular level. The MUSICA project is focused on creating an innovative detector that not only provides quantitative dosimetric data but also qualitative insights. These qualitative insights are derived from measuring physical parameters that are linked to biological effectiveness. This approach, particularly in proton therapy, aims to enhance treatment precision and improve clinical outcomes. Radiation damage occurs through processes at multiple scales, from the DNA level (2 nm) to the entire cell nucleus (10 μm). The stochastic spatial distribution of energy deposition can be studied using microdosimetric techniques with tissue-equivalent gas proportional counters (TEPC). While many studies employing TEPCs have examined sites smaller than 2 μm (such as chromosomes), characterization at the 10 μm scale has typically been performed useing solid-state detectors. However, gas microdosimeters provide benefits in terms of sensitivity, geometry, and tissue equivalence, making it highly advantageous to conduct multi-site characterization using a single detector. The project has developed a TEPC with two charge collection zones, allowing for the definition of two different volume sizes (e.g., 1 and 10 μm) without changing the gas pressure. This will enable dual-dimension microdosimetric characterization in a single measurement session without switching detectors. The resulting bi-dimensional data will be integrated into new radiobiological models that link physical measurements to biological outcomes, enhancing our understanding of how ionizing radiation affects living tissue.

This paper explores the role of microdosimetry in boron neutron capture therapy (BNCT), a cancer treatment involving the selective accumulation of boron-containing compounds in cancer cells, followed by neutron irradiation. Neutron interactions with ¹⁰B induces a nuclear reaction, releasing densely ionizing particles, specifically alpha particles and recoiling lithium-7 nuclei. These particles deposit their energy within a small tissue volume, potentially targeting cancer cells while sparing healthy tissue. The microscopic energy distribution, subject to significant fluctuations due to the short particle range, influences treatment efficacy. Microdosimetry, by studying this distribution, plays a crucial role in optimizing BNCT treatment planning. The methodology employs paired tissue equivalent proportional counters (TEPCs), one with cathode walls enriched with boron and the other without. Precise assessment of boron concentration is essential, as well as the ability to extrapolate results to the actual ¹⁰B concentration within the treatment region. The effective ¹⁰B concentrations within four boronated TEPCs, containing 10, 25, 70, and 100 ppm of ¹⁰B, have been determined. Results show variations of less than 3% from nominal values. Additionally, dose enhancement due to BNC interactions was measured and found to be proportional to the ¹⁰B concentration, with a proportionality factor of 7.7 × 10⁻³ per ppm of boron. Based on these findings, a robust procedure is presented for assessing the impact of BNCT in the treatment region, considering potential variations in boron content relative to the TEPC used.

In the framework of the MUNES project, a prototype accelerator-based thermal neutron source was developed and installed at the Legnaro National Laboratories of INFN. The microdosimetric characterization of this radiation field was performed with a Tissue-Equivalent Proportional Counter with interchangeable cathode walls, either doped with 100 ppm of ¹⁰B or without boron doping. A suitable subtraction procedure allowed to discriminate the gamma, neutron and BNC dose components (Selva et al., 2022, Appl. Radiat. Isot. 182, 110144). The measured microdosimetric spectra can be weighted with a biological weighting function to estimate the Relative Biological Effectiveness of the radiation field, for the purpose of intercomparison between different thermal neutron sources. This work compares, therefore, the biological doses resulting from four different weighting functions applied to the same initial microdosimetric spectrum, discussing strengths and limitations of each of them.