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An improved biological weighting function (IBWF) is proposed to phenomenologically relate microdosimetric lineal energy probability density distributions with the relative biological effectiveness (RBE) for the in vitro clonogenic cell survival (surviving fraction = 10%) of the most commonly used mammalian cell line, i.e. the Chinese hamster lung fibroblasts (V79). The IBWF, intended as a simple and robust tool for a fast RBE assessment to compare different exposure conditions in particle therapy beams, was determined through an iterative global-fitting process aimed to minimize the average relative deviation between RBE calculations and literature in vitro data in case of exposure to various types of ions from 1H to 238U. By using a single particle- and energy- independent function, it was possible to establish an univocal correlation between lineal energy and clonogenic cell survival for particles spanning over an unrestricted linear energy transfer range of almost five orders of magnitude (0.2 keV µm−1 to 15 000 keV µm−1 in liquid water). The average deviation between IBWF-derived RBE values and the published in vitro data was ∼14%. The IBWF results were also compared with corresponding calculations (in vitro RBE10 for the V79 cell line) performed using the modified microdosimetric kinetic model (modified MKM). Furthermore, RBE values computed with the reference biological weighting function (BWF) for the in vivo early intestine tolerance in mice were included for comparison and to further explore potential correlations between the BWF results and the in vitro RBE as reported in previous studies. The results suggest that the modified MKM possess limitations in reproducing the experimental in vitro RBE10 for the V79 cell line in case of ions heavier than 20Ne. Furthermore, due to the different modelled endpoint, marked deviations were found between the RBE values assessed using the reference BWF and the IBWF for ions heavier than 2H. Finally, the IBWF was unchangingly applied to calculate RBE values by processing lineal energy density distributions experimentally measured with eight different microdosimeters in 19 1H and 12C beams at ten different facilities (eight clinical and two research ones). Despite the differences between the detectors, irradiation facilities, beam profiles (pristine or spread out Bragg peak), maximum beam energy, beam delivery (passive or active scanning), energy degradation system (water, PMMA, polyamide or low-density polyethylene), the obtained IBWF-based RBE trends were found to be in good agreement with the corresponding ones in case of computer-simulated microdosimetric spectra (average relative deviation equal to 0.8% and 5.7% for 1H and 12C ions respectively).

In this work, a detailed analysis of the properties of a novel microdosimeter based on a synthetic single crystal diamond is reported. Focused ion microbeams were used to investigate the device spectropscopic properties as well as the induced radiation damage effects. A diamond based Schottky diode was fabricated by chemical vapor deposition with a very thin detecting region, about 400 nm thick (approximately 1.4 μm water equivalent thickness), corresponding to the typical size in microdosimetric measurements. A 200 × 200 μm2 square metallic contact was patterned on the diamond surface by standard photolithography to define the sensitive area. Experimental measurements were carried out at the Ruder Boškovic′ Institute microbeam facility using 4 MeV carbon and 5 MeV silicon ions. Ion beam induced charge maps were employed to characterize the microdosimeter response in terms of its charge collection properties. A stable response with no evidence of polarization or memory effects was observed up to the maximum investigated ion beam flux of about 1.7 × 109 ions·cm−2·s−1. A homogeneity of the response about 6% was found over the sensitive region with a well-defined confinement of the response within the active area. Tests of the radiation damage effect were performed by selectively irradiating small areas of the device with different ion fluences, up to about 1012 ions/cm2. An exponential decrease of the charge collection efficiency was observed with a characteristic decay constant of about 4.8 MGy and 1 MGy for C and Si ions, respectively. The experimental data were analyzed by means of GEANT4 Monte Carlo simulations. A direct correlation between the diamond damaging effect and the Non Ionizing Energy Loss (NIEL) fraction was found. In particular, an exponential decay of the charge collection efficiency with an exponential decay as a function of NIEL is observed, with a characteristic constant of about 9.3 kGy-NIEL for both carbon and silicon ions.

Microdosimetry is a well-established field of scientific research, and in the past 60 years numerous publications and monographs described procedures and fundamental parameters within the field. Here, all definitions of the microdosimetric quantities, their correlations, and the way in which the spectral representations are obtained, are omitted. The recent publication by Lindborg and Waker on experimental methods and application of microdosimetry (Lindborg and Waker 2017) and ICRU reports on 'Fundamental quantities and units in ionizing radiation' (ICRU 2011) and on 'Microdosimetry' (ICRU 1983) are the references for the reader interested in more details on concepts as energy deposited by a single event 1 (in the text it will simply indicated as ), the density and cumulative distributions of , f( ) and F( ), the lineal energy, y, the frequency and dose density distributions of lineal energy, f( y ) and d( y ), the respective cumulative distributions of lineal energy, F( y ) and D( y ), the mean values of the frequency distribution, y F , and of the dose distribution, y D .Radiation quality is defined as the full spectrum of particle types and their energies. The term 'specification' of radiation quality has been used since the funding paper on microdosimetry of H H Rossi to indicate that microdosimeters specify the fraction of dose in each LET interval and this can be considered as an expression of the radiation quality (Rossi 1959). The term specification is also used in ICRU report on 'The quality factor in radiation protection' (ICRU 1986) proposing two options for the specification of radiation quality, in terms of LET 1 or in terms of lineal energy.

In this work, the fabrication and characterization of a microdosimeter based on a synthetic single crystal diamond is reported. The microdosimeter is realized by means of both standard photolithography and selective chemical vapor deposition techniques to accurately define its micrometric sensitive volume. Experimental measurements were carried out at the Ruđer Bošković Institute microbeam facility using different particles such as proton, helium, lithium, carbon and oxygen. Ion beam induced charge (IBIC) technique was performed to characterize the microdosimeter response in terms of its charge collection properties. The experimental data were also analyzed by means of Geant4 Monte Carlo simulations. Diamond based microdosimeter shows a well-defined active volume. Homogeneity of the response was estimated of about 7% and linked to structural defects of diamond surface as deduced by AFM inspection. The detector response shows a good linear behaviors for different kind and energy ions indicating that the detector is suitable for measuring a wide range of particles and LET i.e. 100÷3000 keV/µm. Finally, microdosimetric capabilities of the diamond based microdosimeter were preliminarily tested in low LET radiation fields (i.e. protons beam).

All the four microdosimeters are able to monitor the dose-mean LET with the 11% precision up to the distal edge. In the distal edge region, the ratio of y- to LET¯ changes. Such variability is possibly due to a dependence of the detector response on depth, since the particle mean-path length inside the detectors can vary, especially in the distal edge region.

Purpose To investigate for the first time the potentialities of obtaining microdosimetric measurements in scanned clinical carbon‐ion beams using synthetic single crystal diamond detector and to verify the spectral conversion methods. Methods Microdosimetric measurements were performed at different depths in a water phantom at the therapeutic scanned carbon‐ion beam of the National Center of Oncological Hadrontherapy (CNAO) in Pavia, using waterproof encapsulated diamond microdosimeter developed at “Tor Vergata” University. A monoenergetic carbon‐ion beam of 195 MeV/μ scanned over a square field of 2 × 2 cm2 was used. Experimental microdosimetric spectra were compared with those obtained with a propane‐filled Tissue Equivalent Proportional Counters (TEPCs) microdosimeter in the same facility at the same conditions. To this purpose, the spectra in diamond were converted to the spectra that would have been collected with a propane‐filled cylindrical sensitive volume by means of a novel analytic methodology, recently developed at MedAustron. Results The microdosimetric spectra acquired by the diamond microdosimeter show different shapes in the 10 keV µm−1 ÷ 103 keV µm−1 lineal‐energy range at different water depths. In spite of the high counting rate, no spectral distortion, due to pile‐up events and polarization effects, were observed. The experimental spectra have a low detection threshold of about 6 keV µm−1 due to the electronic noise in the irradiation room. The comparison between the spectra converted to propane from diamond detector and the spectra collected directly with propane‐filled TEPC shows a good agreement in the whole lineal‐energy range. Furthermore this comparison confirms that diamond detector response is LET independent. The frequency‐ and dose‐mean lineal energy values were also assessed for all spectra. The frequency‐mean values obtained with diamond microdosimeter at different depths scales rather well with the absorbed dose values. Conclusions Microdosimetric characterization of a synthetic single crystal diamond detector in high‐energy scanned carbon‐ion beams was performed. The results of the present study showed that this detector is suitable for microdosimetry of clinical carbon ion beams. In addition, the good agreement between the converted diamond spectra and those obtained with TEPC provides the first experimental validation of the spectra conversion methodologies as valuable tools for the comparison of spectra collected with different detectors.

Purpose The purpose of this study was to investigate for the first time the performance of a synthetic single crystal diamond detector for the microdosimetric characterization of clinical 62 MeV ocular therapy proton beams. Methods A novel diamond microdosimeter with a well‐defined sensitive volume was fabricated and tested with a monoenergetic and spread‐out Bragg peak (SOBP) of the CATANA therapeutic proton beam in Catania, Italy. The whole sensitive volume of the detector has an active planar‐sectional area of 100 µm × 100 µm and a thickness of approximately 6.3 um. Microdosimetric measurements were performed at several water equivalent depths, corresponding to positions of clinical relevance. From the measured spectra, microdosimetric quantities such as the frequency mean lineal energy (y¯F), dose mean lineal energy (y¯D) as well as microdosimetric relative biological effectiveness (RBEµ) values were derived for each depth along both a pristine Bragg curve and SOBP. Finally, Geant4 Monte Carlo simulations were performed modeling the detector geometry and CATANA beamline in order to calculate the average linear energy transfer (LET) values in the diamond active layer and water. Results The microdosimetric spectra acquired by the diamond microdosimeter show different shapes as a function of the water equivalent depths. No spectral distortion, due to pile‐up events and polarization effects, was observed. The experimental spectra have a very low detection threshold due to the electronic noise during the irradiation of about 1 keV/μm. The y¯F and y¯D values were in agreement with expected trends, showing a sharp increase in mean lineal energy at the distal edge of the Bragg peak. In addition, a good agreement between the mean lineal energy values and the calculated average LET ones was also observed. Finally, the RBE values evaluated with the diamond microdosimeter were in excellent agreement with those obtained with a mini tissue equivalent proportional counter as well as with radiobiological measurements in the same proton beam field. Conclusions The microdosimetric performance of the tested synthetic single crystal diamond microdosimeter clearly indicates its suitability for quality assurance in clinical proton therapy beam.