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Candela-Juan C.,Polytechnic University of Valencia | Karlsson M.,Linkoping University | Lundell M.,Karolinska University Hospital | Ballester F.,University of Valencia | Tedgren A.C.,Swedish Radiation Safety Authority
Medical Physics | Year: 2015

Purpose: During the first part of the 20th century, 226Ra was the most used radionuclide for brachytherapy. Retrospective accurate dosimetry, coupled with patient follow up, is important for advancing knowledge on long-term radiation effects. The purpose of this work was to dosimetrically characterize two 226Ra sources, commonly used in Sweden during the first half of the 20th century, for retrospective doseeffect studies. Methods: An 8 mg 226Ra tube and a 10 mg 226Ra needle, used at Radiumhemmet (Karolinska University Hospital, Stockholm, Sweden), from 1925 to the 1960s, were modeled in two independent Monte Carlo (MC) radiation transport codes: GEANT4 and GEANT 5. Absorbed dose and collision kerma around the two sources were obtained, from which the TG-43 parameters were derived for the secular equilibrium state. Furthermore, results from this dosimetric formalism were compared with results from a MC simulation with a superficial mould constituted by five needles inside a glass casing, placed over a water phantom, trying to mimic a typical clinical setup. Calculated absorbed doses using the TG-43 formalism were also compared with previously reported measurements and calculations based on the Sievert integral. Finally, the dose rate at large distances from a 226Ra point-like-source placed in the center of 1 m radius water sphere was calculated with GEANT 4. Results: TG-43 parameters [including gL(r), F(r,θ), λ, and sK] have been uploaded in spreadsheets as additional material, and the fitting parameters of a mathematical curve that provides the dose rate between 10 and 60 cm from the source have been provided. Results from TG-43 formalism are consistent within the treatment volume with those of a MC simulation of a typical clinical scenario. Comparisons with reported measurements made with thermoluminescent dosimeters show differences up to 13% along the transverse axis of the radium needle. It has been estimated that the uncertainty associated to the absorbed dose within the treatment volume is 10%15%, whereas uncertainty of absorbed dose to distant organs is roughly 20%25%. Conclusions: The results provided here facilitate retrospective dosimetry studies of 226Ra using modern treatment planning systems, which may be used to improve knowledge on long term radiation effects. It is surely important for the epidemiologic studies to be aware of the estimated uncertainty provided here before extracting their conclusions. © 2015 American Association of Physicists in Medicine.

Carlsson Tedgren A.,Linkoping University | Elia R.,Linkoping University | Hedtjarn H.,Swedish Radiation Safety Authority | Olsson S.,Linkoping University | Alm Carlsson G.,Linkoping University
Medical Physics | Year: 2012

Purpose: Experimental radiation dosimetry with thermoluminescent dosimeters (TLDs), calibrated in a 60Co or megavoltage (MV) photon beam, is recommended by AAPM TG-43U1for verification of Monte Carlo calculated absorbed doses around brachytherapy sources. However, it has been shown by Carlsson Tedgren Med. Phys. 38, 5539-5550 (2011) that for TLDs of LiF:Mg,Ti, detector response was 4 higher in a 137Cs beam than in a 60Co one. The aim of this work was to investigate if similar over-response exists when measuring absorbed dose to water around 192Ir sources, using LiF:Mg,Ti dosimeters calibrated in a 6 MV photon beam. Methods: LiF dosimeters were calibrated to measure absorbed dose to water in a 6 MV photon beam and used to measure absorbed dose to water at distances of 3, 5, and 7 cm from a clinical high dose rate (HDR) 192Ir source in a polymethylmethacrylate (PMMA) phantom. Measured values were compared to values of absorbed dose to water calculated using a treatment planning system (TPS) including corrections for the difference in energy absorption properties between calibration quality and the quality in the users' 192Ir beam and for the use of a PMMA phantom instead of the water phantom underlying dose calculations in the TPS. Results: Measured absorbed doses to water around the 192Ir source were overestimated by 5 compared to those calculated by the TPS. Corresponding absorbed doses to water measured in a previous work with lithium formate electron paramagnetic resonance (EPR) dosimeters by Antonovic Med. Phys. 36, 2236-2247 (2009), using the same irradiation setup and calibration procedure as in this work, were 2 lower than those calculated by the TPS. The results obtained in the measurements in this work and those obtained using the EPR lithium formate dosimeters were, within the expanded (k 2) uncertainty, in agreement with the values derived by the TPS. The discrepancy between the results using LiF:Mg,Ti TLDs and the EPR lithium formate dosimeters was, however, statistically significant and in agreement with the difference in relative detector responses found for the two detector systems by Carlsson Tedgren Med. Phys. 38, 5539-5550 (2011) and by Adolfsson Med. Phys. 37, 4946-4959 (2010). Conclusions: When calibrated in 60Co or MV photon beams, correction for the linear energy transfer (LET) dependence of LiF:Mg,Ti detector response will be needed as to measure absorbed doses to water in a 192Ir beam with highest accuracy. Such corrections will depend on the manufacturing process (MTS-N Poland or Harshaw TLD-100) and details of the annealing and read-out schemes used. © 2012 American Association of Physicists in Medicine.

Savage D.,Savage Earth Associates Ltd | Liu J.,Swedish Radiation Safety Authority
Applied Clay Science | Year: 2015

The performance of bentonite used in geological repositories for radioactive waste may be impaired by long-term clay transformations to non-swelling minerals. Intrinsic to alteration processes is the role of water/clay ratio, defined in a bentonite-pore fluid system by (the inverse of) porosity. Water/(water+clay) mass ratios are low for both 'total' (≤0.25) and 'free' (≤0.05) porosities in compacted bentonite at the dry density envisaged for waste package buffers (≥1500kgm-3). A survey of laboratory experimental studies of clay alteration has shown that they have tended to focus on systems with dispersed clays at high water/(water+clay) mass ratios (≥0.75) because of experimental practicalities and a desire to accelerate reactions.New thermodynamic calculations have illustrated that the fluid/clay ratio can have an important impact not only upon the magnitude of alteration, but also upon the nature of the reaction path. Reaction of a pure Na-montmorillonite with cement pore fluids, a Fe-rich fluid and a KCl solution to attempt to simulate reaction of clay with cement/concrete, iron/steel, and potassium-rich fluids (to investigate the smectite to illite reaction path), respectively has shown that under fluid-dominated conditions (high water/clay ratio), clay alteration consisted of C-S-H solids, low-Si zeolites, and chlorite. Under clay-dominated conditions (low water/clay ratios), alteration typically consisted of high-Si zeolites, feldspar and Mg-corrensite. Consequently, it is of key importance that the most relevant water/clay ratio ('porosity') is used not only in geochemical calculations, but also in experimental systems. © 2015 Elsevier B.V.

Hellstrom P.,Swedish Radiation Safety Authority
International Topical Meeting on Probabilistic Safety Assessment and Analysis, PSA 2015 | Year: 2015

The Swedish Radiation Safety Authority (SSM) is responsible for radiation safety. SSM's mission is to protect people and the environment from unwanted radiation impact, now and in the future. SSM is currently in the process of enhancing the use of risk information in its supervision activities. This paper presents the background and requirements in this area. Further, descriptions are provided of the rather complex scope of SSMs radiation safety responsibility covering all sources of both ionizing and non-ionizing radiation in Sweden that makes it very challenging to implement a risk management process where a structured and full scope risk analysis is a major necessity. The outline of the risk analysis approach, including the consequence criteria and frequency classes, and the first results including some lessons learned are presented together with a proposed risk management process. Finally, the outlook and requirements for further development and to realize benefits of this development are presented.

Tedgren A.C.,Linkoping University | Hedman A.,Linkoping University | Grindborg J.-E.,Swedish Radiation Safety Authority | Carlsson G.A.,Linkoping University
Medical Physics | Year: 2011

Purpose: High energy photon beams are used in calibrating dosimeters for use in brachytherapy since absorbed dose to water can be determined accurately and with traceability to primary standards in such beams, using calibrated ion chambers and standard dosimetry protocols. For use in brachytherapy, beam quality correction factors are needed, which include corrections for differences in mass energy absorption properties between water and detector as well as variations in detector response (intrinsic efficiency) with radiation quality, caused by variations in the density of ionization (linear energy transfer (LET) -distributions) along the secondary electron tracks. The aim of this work was to investigate experimentally the detector response of LiF:Mg,Ti thermoluminescent dosimeters (TLD) for photon energies below 1 MeV relative to 60Co and to address discrepancies between the results found in recent publications of detector response. Methods: LiF:Mg,Ti dosimeters of formulation MTS-N Poland were irradiated to known values of air kerma free-in-air in x-ray beams at tube voltages 25-250 kV, in 137Cs- and 60Co-beams at the Swedish Secondary Standards Dosimetry Laboratory. Conversions from air kerma free-in-air into values of mean absorbed dose in the dosimeters in the actual irradiation geometries were made using EGSnrc Monte Carlo simulations. X-ray energy spectra were measured or calculated for the actual beams. Detector response relative to that for 60Co was determined at each beam quality. Results: An increase in relative response was seen for all beam qualities ranging from 8 at tube voltage 25 kV (effective energy 13 keV) to 3%-4% at 250 kV (122 keV effective energy) and 137Cs with a minimum at 80 keV effective energy (tube voltage 180 kV). The variation with effective energy was similar to that reported by Davis Radiat. Prot. Dosim. 106, 33-43 (2003) with our values being systematically lower by 2%-4%. Compared to the results by Nunn Med. Phys. 35, 1861-1869 (2008), the relative detector response as a function of effective energy differed in both shape and magnitude. This could be explained by the higher maximum read-out temperature (350 °C) used by Nunn Med. Phys. 35, 1861-1869 (2008), allowing light emitted from high-temperature peaks with a strong LET dependence to be registered. Use of TLD-100 by Davis Radiat. Prot. Dosim. 106, 33-43 (2003) with a stronger super-linear dose response compared to MTS-N was identified as causing the lower relative detector response in this work. Conclusions: Both careful dosimetry and strict protocols for handling the TLDs are required to reach solid experimental data on relative detector response. This work confirms older findings that an over-response relative to 60Co exists for photon energies below 200-300 keV. Comparison with the results from the literature indicates that using similar protocols for annealing and read-out, dosimeters of different makes (TLD-100, MTS-N) differ in relative detector response. Though universality of the results has not been proven and further investigation is needed, it is anticipated that with the use of strict protocols for annealing and read-out, it will be possible to determine correction factors that can be used to reduce uncertainties in dose measurements around brachytherapy sources at photon energies where primary standards for absorbed dose to water are not available. © 2011 American Association of Physicists in Medicine.

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