Proton beams are widely used worldwide to treat localized tumours, the lower entrance dose and no exit dose, thus sparing surrounding normal tissues, being the main advantage of this treatment ...modality compared to conventional photon techniques. Clinical proton beam therapy treatment planning is based on the use of a general relative biological effectiveness (RBE) of 1.1 along the whole beam penetration depth, without taking into account the documented increase in RBE at the end of the depth dose profile, in the Bragg peak and beyond. However, an inaccurate estimation of the RBE can cause both underdose or overdose, in particular it can cause the unfavourable situation of underdosing the tumour and overdosing the normal tissue just beyond the tumour, which limits the treatment success and increases the risk of complications. In view of a more precise dose delivery that takes into account the variation of RBE, experimental microdosimetry offers valuable tools for the quality assurance of LET or RBE-based treatment planning systems. The purpose of this work is to compare the response of two different microdosimetry systems: the mini-TEPC and the MicroPlus-Bridge detector. Microdosimetric spectra were measured across the 62 MeV spread out Bragg peak of CATANA with the mini-TEPC and with the Bridge microdosimeter. The frequency and dose distributions of lineal energy were compared and the different contributions to the spectra were analysed, discussing the effects of different site sizes and chord length distributions. The shape of the lineal energy distributions measured with the two detectors are markedly different, due to the different water-equivalent sizes of the sensitive volumes: 0.85 μm for the TEPC and 17.3 μm for the silicon detector. When the Loncol's biological weighting function is applied to calculate the microdosimetric assessment of the RBE, both detectors lead to results that are consistent with biological survival data for glioma U87 cells. Both the mini-TEPC and the MicroPlus-Bridge detector can be used to assess the RBE variation of a 62 MeV modulated proton beam along its penetration depth. The microdosimetric assessment of the RBE based on the Loncol's weighting function is in good agreement with radiobiological results when the 10% biological uncertainty is taken into account.
A sealed miniaturized Tissue Equivalent Proportional Counter (mini-TEPC) able to work in gas-steady modality was developed at the Legnaro National Laboratories of the Italian National Institute of ...Nuclear Physics (LNL – INFN, Legnaro, Italy).
The aim of the present work is to compare the response of this mini-TEPC with that of a silicon microdosimeter based on a monolithic telescope.
Pairwise measurements were performed at the 62 MeV proton clinical Spread Out Bragg Peak (SOBP) of CATANA at the Southern National Laboratories of INFN (LNS – INFN, Catania, Italy). The dose mean lineal energy values were derived from the spectra measured with the two detectors and compared with the total dose-averaged LET calculated by means of Geant4 Monte Carlo simulations. Finally, the possibility to apply the Microdosimetric Kinetic Model (MKM) to reproduce RBE variations with depth along the Spread Out Bragg Peak was investigated.
•A new sealed mini-TEPC was tested as clinical monitor of radiation quality.•Silicon telescope was used to enhance resolution in the distal part of SOBP.•Dose mean lineal energy was evaluated as microdosimetric estimation of LETd.•RBE was estimated using the MKM model, differences and similarities are discussed.
The deep understanding of the interaction between ionizing radiation and soft tissue is of fundamental for radiotherapy and hadron therapy. The complexity of the damage caused by radiation and its ...biological effectiveness depends on the structure of particles track at the scale of subcellular targets, of the order of some nanometers. Microdosimetry and track-nanodosimetry provide the methodologies and tools to describe and comprehend this phenomenon and optimize the therapy.
A novel wall-less avalanche-confinement Tissue Equivalent Proportional Counter (TEPC) designed for simulating nanometric sites down to 25 nm was constructed. This range of operation provides the means to perform the direct comparison between microdosimetric spectra and track-nanodosimetric distributions acquired by STARTRACK nanodosimeter at the INFN-LNL. The novel TEPC has been characterized employing an 241Am alpha source for both determining the best operation conditions in terms of voltages applied to the electrodes in the range 100 – 25 nm and analyzing its response down to the nanometric region. The nano-microdosimeter was able to measure alpha spectra down to 25 nm. The gas gain for different pressures has been calculated adopting analytical track structure calculations. The propane gas gain results always higher than the dimethyl ether (DME) one for each investigated pressure.
Monte Carlo simulations carried out with an ad hoc track structure code show that the avalanche statistics has a minor role in the acquired distribution and suggest that this TEPC is able in reproducing nanodosimetric distributions of alpha particles for simulated site sizes of some tenths of nanometers.
•Avalanche-confinement TEPC able to simulate site sizes down to nanometric region.•Direct comparison between track-nanodosimetry and microdosimetry.•The TEPC reproduces track-nanodosimetry Monte Carlo simulations.
It is recognized today 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 apparatuses, which cannot be easily operated in a clinical environment. It is therefore interesting to investigate the feasibility of new portable detectors able to characterize the real therapeutic beams. With this in mind, a novel avalanche-confinement Tissue Equivalent Proportional Counter (TEPC) was constructed for simulating nanometric sites down to 25 nm. Experimental cluster size distributions measured with this TEPC were compared with Monte Carlo simulations of the same experiment and with cluster size distributions measured with the Startrack nanodosimeter.
A nano-microdosimetric tissue-equivalent proportional counter (TEPC) capable of measuring microdosimetric spectra of ionizing radiation in the range 500–25 nm was designed, constructed and deeply ...characterized in order to fill the gap between nanodosimetry and experimental microdosimetry. This work describes the first microdosimetric characterization at nanometric level of a 195.2 MeV/u carbon ion beam available at CNAO (National Centre for Oncological Hadron Therapy). The detector was properly placed at different depths in PMMA phantom across the depth-dose profile of the primary beam for measuring microdosimetric distributions for different simulated site sizes down to 25 nm at different depths.
The acquired spectra show that this TEPC is capable of reproducing the beam slowing down, showing a shift towards higher lineal energies as the primary particles slow-down. Moreover, the distributions at different simulated site sizes for the same depth are influenced by secondary electrons: smaller site size spectra exhibit a shift towards higher lineal energies as the site decreases, while this is not the case for more distal positions, where the edge of the spectra is almost independent of the simulated site size. Monte Carlo simulations performed with the FLUKA code show a good agreement with the experimental results obtained in the present paper.
•A nano-microdosimetric avalanche-confinement TEPC for the simulation of nanometric sites.•Microdosimetric characterization at nanometric level of 195.2 MeV/u carbon ion beam at CNAO.•Comparison between experimental data and Monte Carlo simulations performed with FLUKA.
Abstract
In proton therapy, most treatment planning systems (TPS) use a fixed relative biological effectiveness (RBE) of 1.1 all along the depth-dose profile. Innovative TPS are now investigated ...considering the variability of RBE with radiation quality. New TPS need an experimental verification in the quality assurance (QA) routine in clinics, but RBE data are usually obtained with radiobiological measurements that are time consuming and not suitable for daily QA. Microdosimetry is a useful tool based on physical measurements which can monitor the radiation quality. Several microdosimeters are available in different research institutions, which could potentially be used for the QA in TPS. In this study, the response functions of five detectors in the same 62-MeV proton Spread Out Bragg Peak is compared in terms of spectral distributions and their average values and microdosimetric RBE. Their different response function has been commented and must be considered in the clinical practice.
The Italian National Centre for Oncological Hadrontherapy (CNAO) has been treating patients since 2011 with carbon-ion beams using the active-scanning modality. In such irradiation modality, the beam ...spot, which scans the treatment area, is characterised by very high particle-fluence rates (more than 105 s-1 mm-2). Moreover, the Bragg-peak is only ~1 mm-FWHM. Commercial tissue-equivalent proportional counters (TEPC), like the Far West Technologies LET-½, are large, hence they have limited capability to measure at high counting fluence rates. In this study we have used two home-made detectors, a mini-TEPC 0.81 mm2 in sensitive area and a silicon telescope 0.125 mm2 in sensitive area, to perform microdosimetric measurements in the therapeutic carbon-ion beam of CNAO. A monoenergetic carbon-ion beam of 189.5 ± 0.3 MeV/u scanning a 3 × 3 cm2 area has been used. Spectral differences are visible in the low y-value region, but the mean microdosimetric values, measured with the two detectors, result to be pretty consistent, as well as the microdosimetric spectra in the high y-value region.
Tissue equivalent proportional counter (TEPC) is the most accurate device for measuring the microdosimetric properties of a particle beam, nevertheless no detailed information on the track structure ...of the impinging particles can be obtained, since the lower operation limit of common TEPCs is ~0.3 μm. On the other hand, the pattern of particle interactions at the nanometer level is measured by only three different nanodosimeters worldwide: practical instruments are not yet available. In order to partially fill the gap between microdosimetry and track-nanodosimetry, a low-pressure avalanche-confinement TEPC was recently designed and constructed for simulating tissue-equivalent sites down to the nanometric region. The present article aims at describing the response of this newly developed TEPC in the range 0.3 μm-25 nm against a fast neutron field from a 241Am-Be source and a quasi-monoenergetic neutron beam. The experimental results are in good agreement with Monte Carlo simulations carried out with the FLUKA code.
A Bonner sphere spectrometer was designed for the measurement of cosmic neutrons at high elevation within the INFN-based project SAMADHA. The spectrometer consists of 8 moderating spheres (6 ...polyethylene and 2 polyethylene plus high atomic number inserts), each embedding a cylindrical
3
He proportional counter. The response matrix was calculated with MCNP6. In view of the very low counting rates expected in the environment, specific design criteria were adopted to prevent non-neutron signals. The spectrometer was exposed in a reference
241
Am–Be neutron field at
Politecnico di Milano
, which allowed the estimation of the overall uncertainty of the simulated response matrix of about ± 2%.
The tissue-equivalent proportional counter (TEPC) is the most accurate device for measuring the microdosimetric properties of a particle beam but, since the lower operation limit of common TEPCs is ...~0.3 μm, no detailed information on the track structure of the impinging particles can be obtained. The pattern of particle interactions at the nanometric level is measured directly by only three different nanodosimeters worldwide: practical instruments are not yet available. In order to partially fill the gap between microdosimetry and track-nanodosimetry, a low-pressure avalanche-confinement TEPC was designed and constructed for simulating tissue-equivalent sites down to the nanometric region. The present paper aims at describing the response of this TEPC in the range 0.3 μm-25 nm to a 62 MeV/n 4He ion beam. The experimental results, for depths near the Bragg peak, show good agreement with FLUKA simulations and suggest that, for smaller depths, the distribution is highly influenced by secondary electrons.
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