liu.seSearch for publications in DiVA
Change search
Refine search result
1 - 9 of 9
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • oxford
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Carlsson Tedgren, Åsa
    et al.
    Linköping University, Faculty of Health Sciences. Linköping University, Department of Medicine and Health Sciences, Radiation Physics . Östergötlands Läns Landsting, Centre of Surgery and Oncology, Department of Radiation Physics.
    Bengtsson, Emil
    Karolinska University Hospital.
    Hedtjärn, Håkan
    Linköping University, Faculty of Health Sciences. Linköping University, Department of Medicine and Health Sciences, Radiation Physics .
    Johansson, Asa
    Karolinska University Hospital.
    Karlsson, Leif
    Örebro University Hospital.
    Lamm, Inger-Lena
    Lund University Hospital.
    Lundell, Marie
    Karolinska University Hospital.
    Mejaddem, Younes
    Karolinska University Hospital.
    Munck af Rosenschold, Per
    Lund University Hospital.
    Nilsson, Josef
    Karolinska University Hospital.
    Wieslander, Elinore
    Lund University Hospital.
    Wolke, Jeanette
    Karolinska University Hospital.
    Experience from long-term monitoring of RAKR ratios in Ir-192 brachytherapy2008In: Radiotherapy and Oncology, ISSN 0167-8140, E-ISSN 1879-0887, Vol. 89, no 2, p. 217-221Article in journal (Refereed)
    Abstract [en]

    Background: Ratios of values of brachytherapy source strengths, as measured by hospitals and vendors, comprise constant differences as, e.g., systematic errors in ion chamber calibration factors and measurement setup. Such ratios therefore have the potential to reveal the systematic changes in routines or calibration services at either the hospital or the vendor laboratory, which could otherwise be hidden by the uncertainty in the source strength values.

    Methods: The RAKR of each new source in 13 afterloading units at five hospitals were measured by well-type ion chambers and compared to values for the same source stated on vendor certificates.

    Results: Differences from unity in the ratios of RAKR values determined by hospitals and vendors are most often small and stable around their mean values to within +/- 11.5%. Larger deviations are rare but occur. A decreasing ratio, seen at two hospitals for the same source, was useful in detecting an erroneous pressure gauge at the vendors site.

    Conclusions: Establishing a mean ratio of RAKR values, as measured at the hospital and supplied on the vendor certificate, and monitoring this as a function of time are an easy way for the early detection of problems with equipment or routines at either the hospital or the vendor site.

  • 2.
    Hammersberg, Peter
    et al.
    Linköping University, Department of Mechanical Engineering. Linköping University, The Institute of Technology.
    Stenström, Mats
    Linköping University, Department of Medicine and Care, Radio Physics. Linköping University, Faculty of Health Sciences.
    Hedtjärn, Håkan
    Linköping University, Department of Medicine and Care, Radio Physics. Linköping University, Faculty of Health Sciences.
    Mångård, Måns
    Linköping University, Department of Mechanical Engineering. Linköping University, The Institute of Technology.
    Absolute energy spectra for an industrial micro focal X-ray source under working conditions measured with a Comptonscattering spectrometer: full spectra data1998Report (Other academic)
    Abstract [en]

    Absolute energy spectra [1/(keV·mAs·sr)] for an industrial micro focal X-ray source has been measured under working conditions, using a Compton scattering spectrometer. The energy spectra were measured as a function of tube potential (30 – 190 kV for every 10th kV) at maximum tube charge of 8 W for the minimum focus (~5 μm diameter). Target material was tungsten. The spectra were measured for a highly focused fresh focal spot. Neither focal spot wear (age) nor defocusing of the focal spot was considered.

    The measured spectra were compared to simulated spectra for the same source supplied by the X-ray source manufacturer. It was found that the measured spectra have slightly different energy distributions with a lower mean energy even though their emitted number of photons were similar. The energy calibration was shown to be accurate compared to the energy resolution, Dhu=0.5 keV, used.

  • 3.
    Hedtjärn, Håkan
    Linköping University, Department of Medicine and Care, Radio Physics. Linköping University, Faculty of Health Sciences.
    Dosimetry in brachytherapy: application of the Monte Carlo method to single source dosimetry and use of correlated sampling for accelerated dose calculations2003Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Three issues related to brachytherapy dosimetry are addressed in this thesis. (1) The prospect of using Compton scattering to measure energy spectra from a high dose-rate (HDR) 192Ir source is discussed and a Compton spectrometer evaluated. Promising results are demonstrated but further fine-tuning is needed to resolve problems with background subtraction. (2) Absorbed doses around an interstitial brachytherapy 125I-source (the Symmetra™ seed) are calculated in the TG-43 formalism using Monte Carlo (MC) techniques. A review of the literature on current measurements and MC simulations indicate systematic differences of 3-6% for the model 6702 seed and points to the need for renewed attention to the dosimetry of low-energy photons and careful estimations of uncertainty. The problems concern among other points: the energy response of LiF dosemeters including LET (Linear Energy Transfer) dependence and the influence of high atomic number doping materials, the atomic compositions of Solid Water and other phantom materials used in MC simulations, the proper MC simulation of the new NIST (National Institute of Standards and Technology) air-kerma strength calibration standard based on the wide-angle free air chamber, and the benchmarking of MC codes. (3) In order to promote development of MC based dose planning, correlated sampling as a means of speeding-up MC calculations is evaluated. In this pilot study, simplified physics is used. Only the photoelectric effect (disregarding the emission of characteristic x-rays) and Compton scattering (Klein-Nishina) are considered. Analogue (ANL) and expected value track-length (ETL) estimations are compared. Efficiency gains (relative to uncorrelated ETL estimations) are calculated for simplified geometries with a point isotropic source and a cylindrical heterogeneity of air, AI and W in a water medium. Efficiency gains of 103 - 104 were obtained for modest perturbations (heterogeneity correction factors HCF [0.8 <HCF < 1.2]). At large perturbations [HCF ≈ 0.4-0.5], in volume elements (voxels) behind the heterogeneity, correlated sampling can be even less efficient than uncorrelated sampling. With correlated ETL estimation, an overall gain in efficiency was, however, achieved and relative standard deviations less than 2% were obtained in 90% of the voxels for an 1251- source and 1-hour computing-time. Uncorrelated ETL estimation was 10-100 times more efficient than uncorrelated ANL estimation. Although promising, correlated sampling should be combined with some other variance reduction technique to reduce the variance everywhere in the volume. Analysis of the uncertainties of estimated efficiency gains shows that the use of the Fisher F distribution to derive their confidence intervals is suspect.

    List of papers
    1. Measurements of energy spectra from high dose rate 192Ir sources with a compton scattering spectrometer
    Open this publication in new window or tab >>Measurements of energy spectra from high dose rate 192Ir sources with a compton scattering spectrometer
    1994 (English)In: Measurement assurance in dosimetry: Proceedings of an International Symposium on Measurement Assurance in Dosimetry, 1994, p. 289-297Conference paper, Published paper (Other academic)
    Abstract [en]

    A Compton scattering spectrometer has been used for spectral measurement of high dose rate (HDR) 192Ir sources. On the basis of the Compton formula a measured distribution of scattered photons is used for the calculation of the primary spectrum leaving the source. The two main reasons for measuring the photon energy distribution from such HDR sources are, firstly, to obtain accurate input for Monte Carlo calculations of the dose distribution and, secondly, to calibrate ionization chambers. The lack of spectral information causes calibration laboratories great difficulties in such work. A third possible reason concerns quality assurance with respect to source impurities, etc. The measured spectrum shows good agreement with the spectrum expected from theoretical considerations.

    National Category
    Medical and Health Sciences
    Identifiers
    urn:nbn:se:liu:diva-84494 (URN)92-0100-194-0 (ISBN)
    Conference
    International Symposium on Measurement Assurance in Dosimetry, Vienna, 24-27 May 1993
    Available from: 2012-10-10 Created: 2012-10-10 Last updated: 2012-10-10Bibliographically approved
    2. Monte Carlo-aided dosimetry of the symmetra model I25.S06 125I, interstitial brachytherapy seed
    Open this publication in new window or tab >>Monte Carlo-aided dosimetry of the symmetra model I25.S06 125I, interstitial brachytherapy seed
    2000 (English)In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 27, no 5, p. 1076-1085Article in journal (Refereed) Published
    Abstract [en]

    A dosimetric study of a new 125I seed for permanent prostate implant, the Symmetra 125I Seed model I25.S06, has been undertaken utilizing Monte Carlo photon transport calculations. All dosimetric quantities recommended by the AAPM Task Group 43 (TG-43) report have been calculated. Quantities determined are dose rate constant, radial dose function, anisotropy function, anisotropy factor, and anisotropy constant. The recently (January 1999) revised NIST (National Institute of Standards and Technology) 125I standard for air kerma strength calibration was taken into account as well as updated interaction cross-section data. Calculations were done for the competing model 6702 source for the purpose of comparison. The calculated dose-rate constants for the two seeds are 1.010 and 1.016 cGyh−1U−1 for the Symmetra and model 6702 seeds, respectively. The latter value deviates from the value, 1.039 cGyh−1U−1, recommended in the TG-43 report. The calculated radial dose function for the Symmetra new seed is more penetrating than that of the model 6711 seed (by 20% at 5 cm distance) but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The anisotropy function for the seed is also close to that for the 6702 seed with a tendency of somewhat more pronounced anisotropy (lower values at small angles from the longitudinal axis). Compared to the model 6711 seed, the Symmetra new seed is more isotropic. The anisotropy constants (the anisotropy function averaged with respect to angle and distance) for the three seed models are within 2%.

    Keywords
    dosimetry, radiation therapy, Monte Carlo methods, radioisotopes, iodine, calibration, dosimetry, permanent brachytherapy implant, 125I, Task Group 43, Monte Carlo simulation
    National Category
    Medical and Health Sciences
    Identifiers
    urn:nbn:se:liu:diva-26768 (URN)10.1118/1.598990 (DOI)11369 (Local ID)11369 (Archive number)11369 (OAI)
    Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2017-12-13Bibliographically approved
    3. Accelerated Monte Carlo based dose calculations for brachytherapy planning using correlated sampling
    Open this publication in new window or tab >>Accelerated Monte Carlo based dose calculations for brachytherapy planning using correlated sampling
    2002 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 47, no 3, p. 351-376Article in journal (Refereed) Published
    Abstract [en]

    Current brachytherapy dose calculations ignore applicator attenuation and tissue heterogeneities, assuming isolated sources embedded in unbounded medium. Conventional Monte Carlo (MC) dose calculations, while accurate, are too slow for practical treatment planning. This study evaluates the efficacy of correlated sampling in reducing the variance of MC photon transport simulation in typical brachytherapy geometries. Photon histories were constructed in the homogeneous geometry and weight correction factors applied to account for the perturbing effect of heterogeneities. Two different estimators, expected value track-length (ETL) and analogue (ANL), were used. The method was tested for disc-shaped heterogeneities and point-isotropic sources as well as for a model 6702 125I seed. Uncorrelated ETL estimation was 10–100 times more efficient than its ANL counterpart. Correlated ETL estimation offered efficiency gains as large as 104 in regions where dose perturbations are small (<5%). For perturbations of 40–50%, efficiency gains were in some cases even less than unity. However, correlated ETL was capable of producing less than 2% (1 standard deviation) uncertainty in more than 90% of the voxels in 1 CPU hour. Correlated sampling significantly improves efficiency under selected circumstances and, in combination with other variance reduction strategies, may make MC-based treatment planning a reality for brachytherapy.

    National Category
    Medical and Health Sciences
    Identifiers
    urn:nbn:se:liu:diva-26772 (URN)10.1088/0031-9155/47/3/301 (DOI)11375 (Local ID)11375 (Archive number)11375 (OAI)
    Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2017-12-13Bibliographically approved
    4. Efficiency gain in Monte Carlo simulations using correlated sampling. Application to calculations of absorbed dose distributions in a brachytherapy geometry
    Open this publication in new window or tab >>Efficiency gain in Monte Carlo simulations using correlated sampling. Application to calculations of absorbed dose distributions in a brachytherapy geometry
    (English)Manuscript (preprint) (Other academic)
    Abstract [en]

    The distribution of an estimate of the efficiency gain of the Monte Carlo method based on correlated sampling was simulated, the corresponding 95 % confidence interval was evaluated, and the bias of the estimate was determined. The concept of batches which normalize distributions of scored quantities was used. It was found that the distribution of the estimate of the efficiency gain was sensitive to outliers and could not be described by the F distribution, Two other estimates of an alternative definition of the efficiency gain were tested. They were more robust but their applicability was limited due to their bias when batch averages were not normally distributed.

    National Category
    Medical and Health Sciences
    Identifiers
    urn:nbn:se:liu:diva-84495 (URN)
    Available from: 2012-10-10 Created: 2012-10-10 Last updated: 2013-09-06Bibliographically approved
  • 4.
    Hedtjärn, Håkan
    et al.
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Alm Carlsson, Gudrun
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Williamson, Jeffrey
    Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St Louis, USA .
    Accelerated Monte Carlo based dose calculations for brachytherapy planning using correlated sampling2002In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 47, no 3, p. 351-376Article in journal (Refereed)
    Abstract [en]

    Current brachytherapy dose calculations ignore applicator attenuation and tissue heterogeneities, assuming isolated sources embedded in unbounded medium. Conventional Monte Carlo (MC) dose calculations, while accurate, are too slow for practical treatment planning. This study evaluates the efficacy of correlated sampling in reducing the variance of MC photon transport simulation in typical brachytherapy geometries. Photon histories were constructed in the homogeneous geometry and weight correction factors applied to account for the perturbing effect of heterogeneities. Two different estimators, expected value track-length (ETL) and analogue (ANL), were used. The method was tested for disc-shaped heterogeneities and point-isotropic sources as well as for a model 6702 125I seed. Uncorrelated ETL estimation was 10–100 times more efficient than its ANL counterpart. Correlated ETL estimation offered efficiency gains as large as 104 in regions where dose perturbations are small (<5%). For perturbations of 40–50%, efficiency gains were in some cases even less than unity. However, correlated ETL was capable of producing less than 2% (1 standard deviation) uncertainty in more than 90% of the voxels in 1 CPU hour. Correlated sampling significantly improves efficiency under selected circumstances and, in combination with other variance reduction strategies, may make MC-based treatment planning a reality for brachytherapy.

  • 5.
    Hedtjärn, Håkan
    et al.
    Linköping University, Faculty of Health Sciences. Linköping University, Department of Medicine and Care, Radio Physics. Östergötlands Läns Landsting, Centre of Surgery and Oncology, Department of Radiation Physics.
    Alm Carlsson, Gudrun
    Linköping University, Faculty of Health Sciences. Linköping University, Department of Medicine and Care, Radio Physics. Östergötlands Läns Landsting, Centre of Surgery and Oncology, Department of Radiation Physics.
    Williamson, Jeffrey
    Accelerated Monte Carlo-based dose calculations for brachytherapy planning using correlated sampling.2000In: Digest of papers of the 2000 world congress on medical physics. CD-Rom Chicago July 23-28,2000, IEEE , 2000, p. 372-375 vol. 1Conference paper (Refereed)
    Abstract [en]

    Correlated sampling is evaluated as a strategy for accelerating Monte Carlo photon transport (MCPT) simulation for use as a practical patient-specific treatment-planning tool. Correlated MCPT simulation was shown to offer substantial efficiency gains over conventional MCPT simulation for selected parts of the 3D volume but was not sufficient to improve MCPT simulation everywhere

  • 6.
    Hedtjärn, Håkan
    et al.
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Alm Carlsson, Gudrun
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Williamson, Jeffrey
    Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, USA.
    Monte Carlo-aided dosimetry of the symmetra model I25.S06 125I, interstitial brachytherapy seed2000In: Medical physics (Lancaster), ISSN 0094-2405, Vol. 27, no 5, p. 1076-1085Article in journal (Refereed)
    Abstract [en]

    A dosimetric study of a new 125I seed for permanent prostate implant, the Symmetra 125I Seed model I25.S06, has been undertaken utilizing Monte Carlo photon transport calculations. All dosimetric quantities recommended by the AAPM Task Group 43 (TG-43) report have been calculated. Quantities determined are dose rate constant, radial dose function, anisotropy function, anisotropy factor, and anisotropy constant. The recently (January 1999) revised NIST (National Institute of Standards and Technology) 125I standard for air kerma strength calibration was taken into account as well as updated interaction cross-section data. Calculations were done for the competing model 6702 source for the purpose of comparison. The calculated dose-rate constants for the two seeds are 1.010 and 1.016 cGyh−1U−1 for the Symmetra and model 6702 seeds, respectively. The latter value deviates from the value, 1.039 cGyh−1U−1, recommended in the TG-43 report. The calculated radial dose function for the Symmetra new seed is more penetrating than that of the model 6711 seed (by 20% at 5 cm distance) but agrees closely (within statistical errors) with that of the model 6702 seed up to distances of 10 cm. The anisotropy function for the seed is also close to that for the 6702 seed with a tendency of somewhat more pronounced anisotropy (lower values at small angles from the longitudinal axis). Compared to the model 6711 seed, the Symmetra new seed is more isotropic. The anisotropy constants (the anisotropy function averaged with respect to angle and distance) for the three seed models are within 2%.

  • 7.
    Malusek, Alexandr
    et al.
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Hedtjärn, Håkan
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Williamson, Jeffrey
    Department of Radiation Oncology, Virginia Commonwealth University, Richmond, USA.
    Alm Carlsson, Gudrun
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Efficiency gain in Monte Carlo simulations using correlated sampling. Application to calculations of absorbed dose distributions in a brachytherapy geometryManuscript (preprint) (Other academic)
    Abstract [en]

    The distribution of an estimate of the efficiency gain of the Monte Carlo method based on correlated sampling was simulated, the corresponding 95 % confidence interval was evaluated, and the bias of the estimate was determined. The concept of batches which normalize distributions of scored quantities was used. It was found that the distribution of the estimate of the efficiency gain was sensitive to outliers and could not be described by the F distribution, Two other estimates of an alternative definition of the efficiency gain were tested. They were more robust but their applicability was limited due to their bias when batch averages were not normally distributed.

  • 8.
    Nilsson, Håkan
    et al.
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Matscheko, G.
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Lund, Eva
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Alm Carlsson, G.
    Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
    Measurements of energy spectra from high dose rate 192Ir sources with a compton scattering spectrometer1994In: Measurement assurance in dosimetry: Proceedings of an International Symposium on Measurement Assurance in Dosimetry, 1994, p. 289-297Conference paper (Other academic)
    Abstract [en]

    A Compton scattering spectrometer has been used for spectral measurement of high dose rate (HDR) 192Ir sources. On the basis of the Compton formula a measured distribution of scattered photons is used for the calculation of the primary spectrum leaving the source. The two main reasons for measuring the photon energy distribution from such HDR sources are, firstly, to obtain accurate input for Monte Carlo calculations of the dose distribution and, secondly, to calibrate ionization chambers. The lack of spectral information causes calibration laboratories great difficulties in such work. A third possible reason concerns quality assurance with respect to source impurities, etc. The measured spectrum shows good agreement with the spectrum expected from theoretical considerations.

  • 9. Williamsson, JF
    et al.
    Dempsey, JF
    Kirov, AS
    Monroe, JI
    Binns, WR
    Hedtjärn, Håkan
    Linköping University, Faculty of Health Sciences. Linköping University, Department of Medicine and Care, Radio Physics. Östergötlands Läns Landsting, Centre of Surgery and Oncology, Department of Radiation Physics.
    Plastic scintillator response to low-energy photons1999In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 44, p. 857-871Article in journal (Other (popular science, discussion, etc.))
1 - 9 of 9
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • oxford
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf