| 1. |
- Hedtjärn, Håkan, 1964-, et al.
(author)
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Accelerated Monte Carlo-based dose calculations for brachytherapy planning using correlated sampling.
- 2000
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In: Digest of papers of the 2000 world congress on medical physics. CD-Rom Chicago July 23-28,2000. - : IEEE. - 0780364651 ; , s. 372-375 vol. 1
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Conference paper (peer-reviewed)abstract
- 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
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| 2. |
- Hedtjärn, Håkan, 1964-, et al.
(author)
-
Accelerated Monte Carlo based dose calculations for brachytherapy planning using correlated sampling
- 2002
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In: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 47:3, s. 351-376
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Journal article (peer-reviewed)abstract
- 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.
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| 3. |
- Hedtjärn, Håkan, 1964-
(author)
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Dosimetry in brachytherapy : application of the Monte Carlo method to single source dosimetry and use of correlated sampling for accelerated dose calculations
- 2003
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Doctoral thesis (other academic/artistic)abstract
- 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.
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| 4. |
- Hedtjärn, Håkan, 1964-, et al.
(author)
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Monte Carlo-aided dosimetry of the symmetra model I25.S06 125I, interstitial brachytherapy seed
- 2000
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In: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 27:5, s. 1076-1085
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Journal article (peer-reviewed)abstract
- 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%.
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