1,721,072 research outputs found
EPR/alanine dosimetry in Stereotactic Radiosurgery treatments through Helical Tomotherapy
No full textIntracranial stereotactic radiosurgery (SRS) is a technique to deliver an ablative radiation dose with an extremely sharp dose gradient to small brain tumors. This tecnique allows to deliver high doses of radiation to the tumor sparing the surrounding healthy tissue.
In this study the accuracy of the dose delivered in a SRS session by a non conventional radiotherapy machine, the TomoTherapy Hi-Art System, was investigated using an "end-to-end" test. This is perfomed by means of alanine Electron Paramagnetic Resonance (EPR) pellets. The response of these dosimeters is compared to that of gafchromic films which are particularly suitable for twodimensional dose verification providing accurate estimate of the distribution and of the gradients of the absorbed dose.
Dose verifications were made using alanine dosimeters placed in an antropomorphic head phantom (Alderson Rando Phantom) under different treatment conditions in case of both single and multiple brain tumors. 1.25mm slice kVCT scan of the phantom was used to generate SRS plans on the TomoTherapy Planning Station platform, prescribing dose at the 95% isodose level of the "PTV" using different combinations of pitch and Modulation Factor. Before each session a MVCT was performed for setup verification.
Commercial alanine dosimeters (Synergy Health, Germany) were irradiated in various positions of the phantom. EPR measurements were carried out through Bruker ECS106 spectrometer working at about 9.7 GHz.
The dose values measured through alanine dosimeters and gafchromic films show a good agreement with the dose values calculated by the TomoTherapy Treatment Planning System, for both organs at risk and tumors.
Alanine absolute dose measurements showed to be useful for the dosimetric validation of HT SRS treatments
Phenol compounds for EPR dosimetry in radiation therapy
No full textAmong the various dosimetric techniques used for characterizing the radiation beams used in radiation therapy, the electron paramagnetic resonance (EPR) arouses increasing interest for applications in various therapy procedures [1]. Free radicals are known to be produced when a compound is irradiated with ionizing radiations. The concentration of radiation-induced free radicals is proportional to the absorbed dose and this allows for dosimetric measurements through EPR technique which able to quantitatively determine the radical concentration [2]. Our research group has started an investigation of the EPR response of some phenols compounds for possible EPR dosimetric applications suitable features, such as high efficiency of radiation-matter energy transfer and radical stability at room temperature [3-5].
In this work we report the EPR investigation of IRGANOX 1076 pellets and thin films exposed to various type of radiation beams (clinical photon and electron beams, neutron-photon mixed field).
Phenols are compounds possessing a benzene ring attached to a OH group. After irradiation the final product is a stable phenoxy radical. The stability of such radical can be improved by adding other alkyl chains which can be attached to the benzene ring. In particular, the phenol octadecyl-3-(3,5-ditert. butyl-4-hydroxyphenyl)-propionate (IRGANOX 1076) gave interesting results. Moreover, its high molecular weight, the low volatility and the compatibility with the dosimeter binding material (wax) are advantages with respect to lower molecular weight phenols. Photon and electron irradiations at various energies were performed with clinical LINAC. Thermal neutron irradiations were performed at the thermal neutron column at the Triga Mark II reactor of Pavia (Italy). EPR dosimeters were readout by means of a Bruker ECS106 spectrometer equipped with a TE102 rectangular cavity at room temperature.
The dosimetric features of these EPR dosimeters were investigated and the results regarding the dependence on microwave power and modulation amplitude are reported. The dependence on beam type and energy, the detection limits for various beam typologies, signal stability after irradiation were analyzed. The dose response was found to be linear for all beams used in the dose range analyzed.
The possibility of obtaining depth dose profile was investigated.
In conclusion, the phenols show radiometric features that designate it as a new material for EPR
Analysis of the spatial distribution of free radicals in ammonium tartrate by pulse EPR techniques
No full textUsing pulse electron paramagnetic resonance (EPR) on a series of L(+)-ammonium tartrate (AT) dosimeters exposed to radiations with different linear energy transfer (LET), we assessed the ability of pulse EPR spectroscopy to discriminate the quality of various radiation beams such as (60)Co gamma-ray photons,, protons and thermal neutrons at various doses by analyzing the local radical distributions produced by the different beams. We performed two types of pulse EPR investigations: two-pulse electron spin echo decay obtained by varying the microwave power, and a double electron-electron resonance (DEER) study. Both methods provide information about the dipolar interactions among the free radicals and about their spatial distributions. The first method provided information on the instantaneous diffusion and hence the microscopic concentration of the radicals that is compared with the macroscopic one obtained by CW-EPR. The DEER spectra yielded the distributions of distances between pairs of radicals two to five crystal cells apart produced by the same radiation event, a result reported here for the first time. The inter-radical distributions given by the DEER results have been simulated by modeling the radical distributions according to the details of the matter-radiation interactions for the various beams. The results of both types of pulse experiments are strongly dependent: on the radiation quality. This was also observed for samples giving indistinguishable CW-EPR spectral profiles. We conclude that the pulse EPR measurements can be valuable tools for distinguishing the LET of the radiation beams, an important parameter for radiobiological considerations
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Monte Carlo simulation of energy absorbed in phenolic ESR dosimeters added with gadolinium exposed to thermal, epithermal and fast neutrons
No full textIn this work analyses of the energy released per unit mass in phenolic compound exposed to neutron
beams were performed with the aim of predicting the increase in dose achievable by addition of gadolinium
(Gd) inside the pellets. In particular, Monte Carlo (MC) simulations were carried out for IRGANOX
1076 phenolic compound irradiated with neutron beams with different energy spectra at various depths
inside a water phantom. The addition of gadolinium increases sensitivity of phenolic ESR (electron spin
resonance) dosimeters to neutrons thanks to the high gadolinium cross section for neutron capture and to
the large number of secondary particles (mainly Auger and internal conversion electrons) which are able
to release energy inside the sensitive material layers. For small depths in water phantom and low energy
neutron spectra the increase in dose due to gadolinium is large (more than a factor 50). The enhancement
is smaller in case of epithermal neutron beam, whereas the increase in dose for fast neutrons is less than
50%. In order to have a comparison with other ESR dosimeters the energy released per unit mass in phenolic
compound was compared with that calculated in alanine pellets. For thermal neutron beams the
energy released in phenolic compound with gadolinium is comparable to that released in alanine for
small depths in phantom, whereas it is larger than in alanine for large depths. In case of epithermal
and fast neutron beams the energy released in phenolic compound is larger than in alanine samples
because the elastic scattering with hydrogen nuclei is more probable for high neutron energies and this
phenolic compound is characterized by an higher number of 1H nuclei than alanine. All results here found
suggest that these phenolic pellets could be fruitfully used for dosimetric applications in Neutron Capture
Therapy
NEW FRICKE GEL WITH HIGH SENSITIVITY AND LOW DIFFUSION FOR3D-MRI DOSIMETRY
Full text linkFricke gel (FG) dosimeters are good candidates for 3D dose assessment in biological materials. Their effective atomic number and density are similar to those of soft tissue. In view of their chemical and morphological characteristics, FG serve as dosimeters and as phantoms at the same time. FG dosimeters are obtained by incorporating an acidic aqueous solution of ferrous ions Fe2+ into a gel matrix.
After exposure to ionizing radiations ferrous ions are oxided into ferric ions (Fe3+) which modify the relaxation times and, therefore, the 3D spatial distribution of radiation dose could be obtained through MRI. In order to address the limitations of gels based on natural matrices, we have studied FG produced with a matrix of poly-vinyl alcohol (PVA) cross-linked with glutaraldehyde (GTA). The proposed gel contains 10% w/v of PVA and GTA of 1%w/v. A common formulation agarose gel was also prepared and studied for comparison. PVA-GTA gel samples were irradiated using 6 MV x-ray clinical beams. The PVA-GTA FG was read out with magnetic resonance imaging.
MR images were recorded with a 1.5T clinical scanner in order to optimize the acquisition parameters and obtain high contrast between irradiated and non-irradiated samples. The PVA-GTA gels were found to offer good linearity in the range of 0-15 Gy and a stable signal for several hours after irradiation.
The sensitivity was about 40% higher compared to gels produced with agarose as gelling agent.
The analysis of the Fe3+ ions diffusion carried out through a 7T preclinical MRI scanner for small animals showed that the diffusion process is much slower (more than five times) for PVA-GTA gels than for agarose ones. The dosimetric accuracy of the 3D gels was investigated by comparing their response to percentage depth dose and off-axis ratio measurements made with an ionization chamber in a water phantom
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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