1,721,086 research outputs found
A Feasibility Study for BNCT of Skin Melanoma with an Accelerator - Based Neutron Source
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Numerical modeling of the gas gain of low-pressure Tissue-Equivalent Proportional Counters
No full textProportional counters are radiation detectors widely used in many applications. The design of the counter, to best fit each application, needs an accurate knowledge and physical modeling of the electron avalanche process. A particular proportional counter is the tissue-equivalent proportional counter (TEPC), the reference detector for experimental microdosimetry, which consists of a spherical or cylindrical chamber filled with low-density tissue-equivalent gas to simulate the energy deposition in tissue sites of micrometric size. The lower operation limit of standard TEPCs operated in the pulse-height analysis mode is about 0.3μm. In order to overcome this technological limit, different avalanche-confinement nano-microdosimetric TEPCs capable of measuring microdosimetric spectra in the nanometric domain were designed and constructed. In this work, a novel numerical tool developed for the Monte Carlo simulation of the electron avalanche process inside a low-pressure TEPC is described. The Monte Carlo code allows to simulate complex 3D electric field configurations exploiting COMSOL finite elements analysis. Several models for the electron interactions (i.e. scattering and ionization) are included in the code. The code has been benchmarked with the experimental results of a wall-less avalanche-confinement TEPC in terms of absolute gas gain for different operating conditions (i.e. gas pressures and electrode voltages). The results show that the code is capable of reproducing the absolute value of the gas gain for the avalanche-confinement TEPC simulating some tenths of nanometers in site size. Moreover, the code can reproduce both the extension and the shape of the proportional counter working windows. The code was also applied for simulating the probability of absorption of electrons by the central third electrode: the helix. The results show a non-negligible probability of absorption in the common range of operation. This code will be further applied for optimizing the TEPC design, capable of simulating site sizes closer to the nanometer region
Microdosimetry at nanometric sites of charged Helium, Carbon and Oxygen beams with an advanced Tissue Equivalent Proportional Counter
No full textThe effectiveness of radiotherapy, particularly hadron therapy, is closely tied to the interactions of radiation at the cellular and sub-cellular levels. Understanding the local energy deposition of charged particles is essential for accurately predicting their biological effects. Traditional dosimetric approaches, based on absorbed dose, fail to describe the stochastic nature of energy deposition at micrometric and nanometric scales. This study investigates the micro- and nano-dosimetric properties of Helium, Carbon, and Oxygen ion beams at 62 MeV/u using an advanced Tissue Equivalent Proportional Counter (TEPC). This TEPC, designed to simulate site sizes ranging from 0.5 μm to 25 nm, was placed at various depths across the Bragg peaks of the ion beams at the INFN-LNS facility in Catania (Italy). Results show a clear dependence of microdosimetric distributions on both simulated site size and position across the depth-dose profile. Smaller site sizes shift the distribution toward higher lineal energies, especially at proximal depths, suggesting that microdosimetric spectra at nanometric scale can offer different insights on the radiation interaction with tissue. This study also underlines the role of secondary electrons and fragmentation effects, which vary with the atomic number of the ion, producing different effects for Helium, Carbon and Oxygen ions. These findings may have significant implications for improving relative biological effectiveness (RBE) models in hadron therapy. By extending microdosimetric analysis to the nanometric scale, this research provides new data for a possible improvement of the predictive accuracy of radiation-induced biological effects. The novel TEPC used in this study bridges the gap between microdosimetry and nanodosimetry, offering a more refined assessment of radiation quality
Solid state microdosimetry of a 148 MeV proton spread-out Bragg peak with a pixelated silicon telescope
Full text linkA constant value of the Relative Biological Effectiveness (RBE), equal to 1.1, to weight the physical dose of proton therapy treatment planning collides with the experimental evidence of an increase of effectiveness along the depth dose profile, especially at the end of the particle range. In this context, it is desirable to develop new optimized treatment planning systems that account for a variable RBE when weighting the physical dose. In particular, due to the increasing interest on microdosimetry as a possible methodology for measuring physical quantities correlated with the biological effectiveness of the therapeutic beam, the development of new Tissue-Equivalent Proportional Counters (TEPCs) specifically designed for the clinical environment are in progress. In this framework, the silicon technology allows to produce solid state detectors of real micrometric dimensions. This is a valid alternative to the TEPC from a practical point of view, being simple, easy-of-use and more versatile. The feasibility of a solid state microdosimeter based on a monolithic double stage silicon telescope has been previously proposed and deeply investigated by comparing its response to the one obtained by reference TEPCs in various radiation fields. The device is constituted by a matrix of cylindrical elements, 2 μm in thickness and 9 μm in diameter, coupled to a single E stage, 500 μm in thickness. Each segmented ΔE stage acts as a solid state microdosimeter, while the E stage gives information on the energy of the impinging proton up to about 8 MeV. This work is dedicated to the description of the microdosimetric characterization of the 148 MeV energy-modulated proton beam at the radiobiological research line of the Trento Proton Therapy Centre by means of a pixelated silicon microdosimeter. All measurements were carried out at different positions across the spread-out Bragg peak (SOBP) and the corresponding microdosimetric distributions were derived by applying a novel extrapolation algorithm. Finally, microdosimetric assessment of Relative Biological Effectiveness was carried out by weighting the dose distribution of the lineal energy with the Loncol's biological weighting function. Benefits and possible limitations of this approach are discussed
An Accelerator-Based Neutron Source for BNCT of Skin Melanoma
No full textBNCT of skin malignant melanoma bas been developed in Japan since 1972. Primary lesions and unobservable satellite metastasis can be selectively treated with this technique. The clinical results related to 18 patients treated in Japan up to January 1996 are analysed in ref. 1. The survival rate after two or more years resulted to be 78%, regardless the level of melanoma progression. 10B-paraboronphenylanine (BPA) was used as the boron carrier to the tumour. The patiens were irradiated at a research reactor facility for 1-2 hours with thermal neutrons and the related maximum fluences at the tumour sites were in the range (1.0-2.0)x1013 cm-2.The design of the accelerator-based neutron source for BNCT of skin melanoma discussed in the present work is described in ref. 3. Neutrons are produced by bombarding a thick beryllium target with 7 MeV deuterons. The target is contained in a heavy water moderator in turn enclosed in a graphite structure. Lead filters are placed on the Faraday cup containing the beryllium target and on the heavy water container to reduce the prompt gamma ray dose. The experimental verification of the accelerator-based source was performed at the CN Van De Graaff accelerator of the Laboratori Nazionali di Legnaro (LNL, Italy). Measurements of thermal fluence uniformity and prompt gamma ray dose at the irradiation position, together with estimates of the neutron fluences inside an Alderson phantom are discussed in this paper
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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