| 1. |
- Anderson, Leif G, 1951, et al.
(författare)
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Variability in river runoff distribution in the Eurasian Basin of the Arctic Ocean
- 2004
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Ingår i: Journal of Geophysical Research-Oceans. - 0148-0227. ; 109:C1
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Tidskriftsartikel (refereegranskat)abstract
- The distribution of freshwater within the Arctic Ocean and its export from it are intimately involved in climate and climate change processes both within and outside the Arctic Ocean. River runoff in the Arctic Ocean constitutes a major part of the Arctic Ocean freshwater budget. Within the Arctic Ocean, variability in the distribution of river runoff will be reflected in the location of the cold halocline that isolates the sea ice from the warm Atlantic Layer. Outside the Arctic Ocean, such variability will impact on the salinity of North Atlantic waters (Great Salinity Anomaly) and on deep convection areas of the North Atlantic Ocean, and thereby potentially on global thermohaline circulation. Rivers entering the Arctic Ocean have high levels of total alkalinity that contribute significantly to the total alkalinity of the surface Polar Mixed Layer. We exploit total alkalinity data to trace river runoff in the surface Polar Mixed Layer and to observe variability in the river runoff distribution in the Eurasian Basin over the period 1987-2001. The river runoff front changed from a position over the Gakkel Ridge in 1987 and 1991 to over the Lomonosov Ridge in 1996, and returned to a midpoint between the two ridges in 2001. Wind field changes as characterized by the Arctic Oscillation index are considered to be a major factor in determining ice and surface water flow. We note a correlation with 4-6 years delay between changes in river runoff distribution and the Arctic Oscillation index. We show that the delay can be inferred from a geostrophic flow calculation.
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| 2. |
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| 3. |
- Björk, Peter
(författare)
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Absorbed Dose Determination and Characteristics of Degraded Electron Beams: Application to Intraoperative Radiation Therapy
- 2003
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this work was to quantify limitations of and uncertainties in commonly used dosimetric techniques for relative absorbed dose determination in degraded electron beams, such as those encountered in intraoperative radiation therapy (IORT) and small-electron-field radiotherapy. Three different detector types were investigated with regard to measurements of output factors and relative absorbed dose distributions: (1) a parallel-plate ionization chamber, (2) a p-type silicon diode detector and (3) a diamond detector. The Monte Carlo method was used to obtain a better understanding of the beam characteristics in the complex treatment situations. Comparisons were made with the beam characteristics in the reference geometry where the techniques for determining the absorbed dose are well described in international dosimetry protocols. The following results were obtained: (1) the IORT fields contained a considerably larger amount of scattered electrons than the reference field; (2) the IORT beams exhibited a broader energy spectrum and a wider angular distribution of the electrons at the phantom surface; and (3) the smaller the field size, the higher the mean energy at a certain depth in the phantom. These characteristics will change the radiation conditions at the measurement point, which can influence the detector signal as well as the mass collision stopping-power ratio between water and the detector material. This latter parameter is used in the general cavity equation to convert the mean absorbed dose in the detector to the absorbed dose in water. The diamond detector was shown to exhibit excellent properties for relative absorbed dose measurements in degraded electron beams and was therefore used as a reference. The diode detector was found to be well suited for practical measurements of both output factors and relative absorbed dose distributions, although the water-to-silicon stopping-power ratio was shown to vary slightly with treatment set-up and irradiation depth, especially for nominal electron energies below 6 MeV. Application of ionization-chamber-based dosimetry will introduce uncertainties smaller than 0.3% in the output factor determination for conventional IORT beams. The IORT system at our department includes a 0.3 cm thick plastic scatterer inside the therapeutic beam, which furthermore increases the energy degradation of the electrons. By ignoring the change in the water-to-air stopping-power ratio due to this scatterer, the output factor could be underestimated by up to 1.3%. The dosimetry protocols can be applied in a direct manner to obtain depth-dose distributions in degraded electron beams as the relative variation of the water-to-air stopping-power ratio is very similar for the reference field and the other investigated complex treatment situations. For practical reasons, the use of ion chambers for relative dosimetry in degraded beams is discouraged, due to the numerous corrections needed (e.g., for ion recombination, the polarity effect and stopping-power ratio variation). Instead, the readily available p-type silicon diode detector is recommended for relative absorbed dose measurements in complex electron fields.
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| 4. |
- Björk, Peter, et al.
(författare)
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Comparative dosimetry of diode and diamond detectors in electron beams for intraoperative radiation therapy
- 2000
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 27:11, s. 2580-2588
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Tidskriftsartikel (refereegranskat)abstract
- The aim of the present study is to examine the validity of using silicon semiconductor detectors in degraded electron beams with a broad energy spectrum and a wide angular distribution. A comparison is made with diamond detector measurements, which is the dosimeter considered to give the best results provided that dose rate effects are corrected for. Two-dimensional relative absorbed dose distributions in electron beams (6-20 MeV) for intraoperative radiation therapy (IORT) are measured in a water phantom. To quantify deviations between the detectors, a dose comparison tool that simultaneously examines the dose difference and distance to agreement (DTA) is used to evaluate the results in low- and high-dose gradient regions, respectively. Uncertainties of the experimental measurement setup (+/- 1% and +/- 0.5 mm) are taken into account by calculating a composite distribution that fails this dose-difference and DTA acceptance limit. Thus, the resulting area of disagreement should be related to differences in detector performance. The dose distributions obtained with the diode are generally in very good agreement with diamond detector measurements. The buildup region and the dose falloff region show good agreement with increasing electron energy, while the region outside the radiation field close to the water surface shows an increased difference with energy. The small discrepancies in the composite distributions are due to several factors: (a) variation of the silicon-to-water collision stopping-power ratio with electron energy, (b) a more pronounced directional dependence for diodes than for diamonds, and (c) variation of the electron fluence perturbation correction factor with depth. For all investigated treatment cones and energies, the deviation is within dose-difference and DTA acceptance criteria of +/- 3% and +/- 1 mm, respectively. Therefore, p-type silicon diodes are well suited, in the sense that they give results in close agreement with diamond detectors, for practical measurements of relative absorbed dose distributions in degraded electron beams used for IORT.
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| 5. |
- Björk, Peter, et al.
(författare)
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Design and dosimetry characteristics of a soft-docking system for intraoperative radiation therapy
- 2000
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Ingår i: International Journal of Radiation Oncology, Biology, Physics. - 0360-3016. ; 47:2, s. 527-533
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: The design concept and the dosimetric characteristics of an applicator system for intraoperative radiation therapy (IORT) with special emphasis on alignment methods, the effect of a plastic scatterer in the beam, radiation leakage, and misalignment dosimetry, are presented in this paper. MATERIALS AND METHODS: A soft-docking system for a linear accelerator, which enables collimation of electron beams (4-22 MeV) for IORT has been developed. The system includes twenty-one circular polymethylmethacrylate (PMMA) treatment cones of different lengths, diameters and end angles. All in-water measurements are made using p-type silicon diode detectors. RESULTS: The effect of introducing a PMMA scatterer in the therapeutic beam includes increased surface dose values (above 83% for all nominal electron energies and for all cones) and improved dose homogeneity within the therapeutic range. Electrons scattered from the inside wall of the cone result in dose profile horns at depth of dose maximum always lower than 109%. The radiation leakage outside the cone is less than 13%. Large changes in the dose profiles occur if the intraoperative cone is misaligned more than 0.5. CONCLUSION: The alignment procedure of the soft-docking system is easy to handle and the applicator design provides adequate collimation of electron beams for IORT.
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| 6. |
- Björk, Peter, et al.
(författare)
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Dosimetry characteristics of degraded electron beams investigated by Monte Carlo calculations in a setup for intraoperative radiation therapy.
- 2002
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 47:2, s. 239-256
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Tidskriftsartikel (refereegranskat)abstract
- Degraded electron beams, as used for intraoperative radiation therapy (IORT) or similar complicated dosimetric situations, have different characteristics compared to conventional electron therapy beams. If international dosimetry protocols are applied in a direct manner to such degraded beams, uncertainties will be introduced in the absorbed dose determination. The Monte Carlo method has been used to verify experimentally determined relative absorbed dose distributions and output factors in an IORT geometry. Monte Carlo generated dose distributions are mostly within +/-2% or +/-2 mm of measured data. The simulated output variation between the IORT cones (relative output factors) are mostly within 2% of measured values. By comparing IORT and conventional electron beam characteristics (e.g. energy spectra, angular distributions and the contributions of different system components to these quantities) limitations and uncertainties of commonly used dosimetric techniques in IORT electron fields are quantified. The intraoperative treatment field contains a larger amount of scattered electrons, which leads to a broader energy spectrum as well as a wider angular distribution of electrons at the phantom surface. The dose from the scattered electrons can contribute up to 40% of the total dose at a depth of dose maximum, compared to approximately 10% for standard beams. A study of the energy spectra at the reference depth reveals that an uncertainty of the order of 1% can be introduced if ionization chamber based dosimetry is used to determine output factors for the investigated IORT system. We recommend that relative absorbed dose distributions and output factors in IORT electron beams and for similar complicated dosimetric situations should be determined with detectors having a small energy and angular dependence (e.g. diamond detectors or p-Si diodes).
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| 9. |
- Björk, Staffan, et al.
(författare)
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Designing Ubiquitous Computing Games - A Report from a Workshop Exploring Ubiquitous Computing Entertainment
- 2002. - 2
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Ingår i: Personal and Ubiquitous Computing. - 1617-4909 .- 1617-4917. ; 6, s. 443-458
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Tidskriftsartikel (refereegranskat)abstract
- We report from a Research Atelier that explored how ubiquitous computing could be applied to fun and entertainment. The Atelier lasted for five days, starting with two days of scenario development and brainstorming activities. This led to three fairly concrete – though very different – game ideas. The background and motivation for the Atelier is described, as well as the method used and the games developed.
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| 10. |
- Bruun, Laila, et al.
(författare)
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Rapid elimination by glomerular filtration of free prostate specific antigen and human kallikrein 2 after renal transplantation.
- 2004
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Ingår i: Journal of Urology. - : Ovid Technologies (Wolters Kluwer Health). - 1527-3792 .- 0022-5347. ; 171:4, s. 1432-1435
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: The low molecular mass and short half-life of free (f) prostate specific antigen (PSA) implies elimination from blood by glomerular filtration. In addition, patients with terminal renal failure have increased fPSA in serum but there have been sparse data reported on the rates and pathways of elimination of PSA complexes and human kallikrein 2 (hK2). We studied glomerular filtration dependent elimination of fPSA and hK2 in patients with renal insufficiency undergoing successful renal transplantation.MATERIALS AND METHODS: We studied 14 patients with immediate onset of renal function after renal transplantation. Blood samples were obtained before and at regular intervals up to 160 hours after transplanted kidney reperfusion. Measurements of fPSA, total PSA and hK2 were performed with immunofluorometric assays and complexed PSA was determined by a chemiluminiscence assay. Glomerular filtration rates were monitored by analyzing serum creatinine and cystatin C. NONMEM, a multivariate pharmacokinetic approach, was used to determine the elimination rates of fPSA and hK2 after renal transplantation.RESULTS: Serum fPSA and hK2 but not PSA complexes, decreased rapidly after renal transplantation. Significant reductions in fPSA and hK2 were observed after only 16 and 8 hours, respectively. fPSA and hK2 showed similar elimination patterns, decreasing to 42% and 44% of their original levels compared to cystatin C, which was at 44% after 160 hours. The median half-lives of fPSA and hK2 were 17.4 and 11.5 hours, respectively.CONCLUSIONS: These results verify the hypothesis that fPSA and hK2 are eliminated from the blood circulation by glomerular filtration and severe renal failure influences the levels of the 2 proteins in serum.
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