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Accelerated Monte Carlo based dose calculations for brachytherapy planning using correlated sampling
Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.
Linköping University, Department of Medicine and Care, Radiation Physics. Linköping University, Faculty of Health Sciences.ORCID iD: 0000-0003-0209-498X
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St Louis, USA .
2002 (English)In: Physics in Medicine and Biology, ISSN 0031-9155, E-ISSN 1361-6560, Vol. 47, no 3, 351-376 p.Article 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.

Place, publisher, year, edition, pages
2002. Vol. 47, no 3, 351-376 p.
National Category
Medical and Health Sciences
Identifiers
URN: urn:nbn:se:liu:diva-26772DOI: 10.1088/0031-9155/47/3/301Local ID: 11375OAI: oai:DiVA.org:liu-26772DiVA: diva2:247322
Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2015-03-20Bibliographically approved
In thesis
1. Dosimetry in brachytherapy: application of the Monte Carlo method to single source dosimetry and use of correlated sampling for accelerated dose calculations
Open this publication in new window or tab >>Dosimetry in brachytherapy: application of the Monte Carlo method to single source dosimetry and use of correlated sampling for accelerated dose calculations
2003 (English)Doctoral 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.

Place, publisher, year, edition, pages
Linköping: Linköpings universitet, 2003. 59 p.
Series
Linköping University Medical Dissertations, ISSN 0345-0082 ; 790
National Category
Medical and Health Sciences
Identifiers
urn:nbn:se:liu:diva-27489 (URN)12143 (Local ID)91-7373-549-3 (ISBN)12143 (Archive number)12143 (OAI)
Public defence
2003-05-27, Berzeliussalen, Hälsouniversitet, Linköping, 09:00 (Swedish)
Opponent
Available from: 2009-10-08 Created: 2009-10-08 Last updated: 2012-10-10Bibliographically approved

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Hedtjärn, HåkanAlm Carlsson, Gudrun

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