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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
Linköping University, Department of Medicine and Care, Radio Physics. Linköping University, Faculty of Health Sciences.
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: urn:nbn:se:liu:diva-27489Local ID: 12143ISBN: 91-7373-549-3 (print)OAI: oai:DiVA.org:liu-27489DiVA: diva2:248041
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
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, 289-297 p.Conference 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, 1076-1085 p.Article 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%.

Keyword
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, 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.

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

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