| 1. |
- Holm, Åsa, et al.
(författare)
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A linear programming model for optimizing HDR brachytherapy dose distributions with respect to mean dose in the DVH-tail
- 2013
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Ingår i: Medical physics (Lancaster). - : American Association of Physicists in Medicine. - 0094-2405. ; 40:8
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Recent research has shown that the optimization model hitherto used in high-dose-rate (HDR) brachytherapy corresponds weakly to the dosimetric indices used to evaluate the quality of a dose distribution. Although alternative models that explicitly include such dosimetric indices have been presented, the inclusion of the dosimetric indices explicitly yields intractable models. The purpose of this paper is to develop a model for optimizing dosimetric indices that is easier to solve than those proposed earlier. less thanbrgreater than less thanbrgreater thanMethods: In this paper, the authors present an alternative approach for optimizing dose distributions for HDR brachytherapy where dosimetric indices are taken into account through surrogates based on the conditional value-at-risk concept. This yields a linear optimization model that is easy to solve, and has the advantage that the constraints are easy to interpret and modify to obtain satisfactory dose distributions. less thanbrgreater than less thanbrgreater thanResults: The authors show by experimental comparisons, carried out retrospectively for a set of prostate cancer patients, that their proposed model corresponds well with constraining dosimetric indices. All modifications of the parameters in the authors model yield the expected result. The dose distributions generated are also comparable to those generated by the standard model with respect to the dosimetric indices that are used for evaluating quality. less thanbrgreater than less thanbrgreater thanConclusions: The authors new model is a viable surrogate to optimizing dosimetric indices and quickly and easily yields high quality dose distributions.
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| 2. |
- Holm, Åsa
(författare)
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Dose Plan Optimization in HDR Brachytherapy using Penalties : Properties and Extensions
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- High dose-rate (HDR) brachytherapy is a specific type of radiotherapy used to treat tumours of for example the cervix, prostate, and breasts. In HDR brachytherapy applicators are implanted into or close to the tumour volume. A radioactive source is moved through these applicators and stops at certain positions, known as dwell points. For each patient an anatomy-based dose plan is created that decides for example where to place the applicators, which dwell points to use, and for how long. The aim when creating a dose plan is to deliver an as high dose as possible to the tumour while simultaneously keeping the dose to the surrounding healthy organs as low as possible.In order to improve the quality of dose plans mathematical optimization methods are today used in clinical practice. Usually one solves a linear penalty model that minimizes a weighted deviation from dose intervals provided by a physician. In this thesis we study certain properties and alterations of this model.One interesting property of the model that we study is the distribution of the basic variables. We show that due to the distribution of these variables only a limited number of dwell positions can be used. Since relatively few dwell positions are used some of the corresponding dwell times have to be long in order for the desired overall dose level to be reached. These long dwell times have been observed in clinical practice and are considered to be a problem.Another property that we study is the correlation between the objective value of the linear penalty model and dose-volume parameters used for evaluation of dose plans. We show that the correlation is weak, which implies that optimizing the linear penalty model does not give a solution to the correct problem.Some alternative models are also considered. One that includes into the optimization the decision of where to place the applicators, when HDR brachytherapy is applied for prostate cancer, and one that reduces the long dwell times by using piecewise linear penalties. The solutions to both models show significant improvements.
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| 3. |
- Holm, Åsa, et al.
(författare)
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Heuristics for Integrated Optimization of Catheter Positioning and Dwell Time Distribution in Prostate HDR Brachytherapy
- 2016
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Ingår i: Annals of Operations Research. - : Springer. - 0254-5330 .- 1572-9338. ; 236:2, s. 319-339
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Tidskriftsartikel (refereegranskat)abstract
- High dose-rate (HDR) brachytherapy is a kind of radiotherapy used to treat, among others, prostate cancer. When applied to prostate cancer a radioactive source is moved through catheters implanted into the prostate. For each patient a treatment plan is constructed that decide for example catheter placement and dwell time distribution, that is where to stop the radioactive source and for how long.Mathematical optimization methods has been used to find quality plans with respect to dwell time distribution, however few optimization approaches regarding catheter placement have been studied. In this article we present an integrated optimization model that optimize catheter placement and dwell time distribution simultaneously. Our results show that integrating the two decisions yields greatly improved plans, from 15% to 94% improvement.Since the presented model is computationally demanding to solve we also present three heuristics: tabu search, variable neighbourhood search and genetic algorithm. Of these variable neighbourhood search is clearly the best, outperforming a state-of-the-art optimization software (CPLEX) and the two other heuristics.
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| 4. |
- Holm, Åsa, et al.
(författare)
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Impact of Using Linear Optimization Models in Dose Planning for HDR Brachytherapy
- 2012
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Ingår i: Medical physics (Lancaster). - : American Association of Physicists in Medicine. - 0094-2405. ; 39:2, s. 1021-1028
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Dose plans generated with optimization models hitherto used in HDR brachytherapy have shown a tendency to yield longer dwell times than manually optimized plans. Concern has been raised for the corresponding undesired hot spots and various methods to mitigate these have been developed. The hypotheses of this work are a) that one cause for the long dwell times is the use of objective functions comprising simple linear penalties and b) that alternative penalties, being piecewise linear, would lead to reduced length of individual dwell times.Methods: The characteristics of the linear penalties and the piecewise linear penalties are analysed mathematically. Experimental comparisons between the two types of penalties are carried out retrospectively for a set of prostate cancer patients.Results: While most dose-volume parameters do not differ significantly between the two types of penalties significant changes can be seen in the dwell times. On the average, total dwell times were reduced by 4.2%, with a reduction of maximum dwell times by 30%, using the alternative penalties.Conclusion: The use of linear penalties in optimization models for HDR brachytherapy is one cause for undesired longer dwell times appearing in mathematically optimized plans. By introducing alternative penalties significant reduction in dwell times can be achieved for HDR brachytherapy dose plans. Although various constraints as to reduce the long dwell times have been developed our finding is of fundamental interest in showing the shape of the objective function to be one reason for their appearance.
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| 5. |
- Holm, Åsa
(författare)
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Mathematical Optimization of HDR Brachytherapy
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One out of eight deaths throughout the world is due to cancer. Developing new treatments and improving existing treatments is hence of major importance. In this thesis we have studied how mathematical optimization can be used to improve an existing treatment method: high-dose-rate (HDR) brachytherapy.HDR brachytherapy is a radiation modality used to treat tumours of for example the cervix, prostate, breasts, and skin. In HDR brachytherapy catheters are implanted into or close to the tumour volume. A radioactive source is moved through the catheters, and by adjusting where the catheters are placed, called catheter positioning, and how the source is moved through the catheters, called the dwelling time pattern, the dose distribution can be controlled.By constructing an individualized catheter positioning and dwelling time pattern, called dose plan, based on each patient's anatomy, it is possible to improve the treatment result. Mathematical optimization has during the last decade been used to aid in creating individualized dose plans. The dominating optimization model for this purpose is a linear penalty model. This model only considers the dwelling time pattern within already implanted catheters, and minimizes a weighted deviation from dose intervals prescribed by a physician.In this thesis we show that the distribution of the basic variables in the linear penalty model implies that only dwelling time patterns that have certain characteristics can be optimal. These characteristics cause troublesome inhomogeneities in the plans, and although various measures for mitigating these are already available, it is of fundamental interest to understand their cause.We have also shown that the relationship between the objective function of the linear penalty model and the measures commonly used for evaluating the quality of the dose distribution is weak. This implies that even if the model is solved to optimality there is no guarantee that the generated plan is optimal with respect to clinically relevant objectives, or even near-optimal. We have therefore constructed a new model for optimizing the dwelling time pattern. This model approximates the quality measures by the concept conditional value-at-risk, and we show that the relationship between our new model and the quality measures is strong. Furthermore, the new model generates dwelling time patterns that yield high-quality dose distributions.Combining optimization of the dwelling time pattern with optimization of the catheter positioning yields a problem for which it is rarely possible to find a proven optimal solution within a reasonable time frame. We have therefore developed a variable neighbourhood search heuristic that outperforms a state-of-the-art optimization software (CPLEX). We have also developed a tailored branch-and-bound algorithm that is better at improving the dual bound than a general branch-and-bound algorithm. This is a step towards the development of a method that can find proven optimal solutions to the combined problem within a reasonable time frame.
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| 6. |
- Holm, Åsa, et al.
(författare)
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On the Correlation Between DVH Parameters and Linear Penalties in Optimization of HDR Prostate Brachytherapy Dose Plans
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- When optimizing dwell times for HDR brachytherapy it is common to use a model comprising an objective of linear penalties. However whether a planis considered good or not depends on other measures such as DVH-based parameters. We show through experiments that the correlation between the value of the objective function and the values of DVH-based parameters, such as D90, is weak in some cases. It seems that the objective function can only classify solutions into better or worse, however it can not distinguish the best with respect to DVH-based parameters.
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| 7. |
- Holm, Åsa, et al.
(författare)
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Study of the Relationship Between Dosimetric Indices and Linear Penalties in Dose Distribution Optimization for HDR Prostate Brachytherapy
- 2013
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Purpose: Most clinical software for optimizing dwelling time patterns is based on a linear penalty model. The quality of a dose distribution generated by the dwelling time pattern is, however, evaluated through a number of dosimetric indices. The purpose of this article is to investigate the relationship between the linear penalty model and the dosimetric indices.Method and Materials: Data sets from three patients, previously treated for prostate cancer with HDR brachytherapy as a boost to external beam therapy, were used for this study, and for each of them 300 random dwelling time patterns were generated. The relationship between the linear penalty model and the dosimetric indices were studied both by the Pearson’s product moment correlation coefficient between the objective function value of the linear penalty model and the values of the dosimetric indices, and by scatter-grams.Results: For one of the three patients we found a clear connection between the linear penalty model and the values of the dosimetric indices, but not for the other two. For the two patients without a clear connection there where some dosimetric indices that actually improved with deteriorating objective function value.Conclusion: The dwelling time pattern found by using the linear penalty model does not correspond to the optimal dose distribution with respect to dosimetric indices.
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| 8. |
- Strand, Åsa, et al.
(författare)
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Impact of an icy winter on the Pacific oyster (Crassostrea gigas Thunberg, 1793) populations in Scandinavia
- 2012
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Ingår i: Aquatic Invasions. - : Regional Euro-Asian Biological Invasions Centre Oy (REABIC). - 1798-6540 .- 1818-5487. ; 7:3, s. 433-440
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Tidskriftsartikel (refereegranskat)abstract
- The Pacific oyster (Crassostrea gigas) is an invasive species that has dispersed into Scandinavia during the last few decades. The objective of this study was to evaluate the effects of extreme winter conditions on the mortality of the Pacific oyster in Scandinavia. The study was done by compiling mortality data from independent surveys in Denmark, Sweden and Norway. Winter mortality of the oysters increased with latitude, which can be explained by the colder climate experienced at higher latitudes. Mortality was also found to be affected by site specific conditions such as water depth at the sampling sites of oyster populations. Despite the severe winter conditions of 2009/2010 causing high mortality, the Pacific oyster still exists in large numbers in Scandinavia. The present investigation indicates that extreme winter conditions may result in a temporary reduction of the density of the Pacific oyster, but that the species can be expected to continue its invasion of Scandinavian coastal areas.
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| 9. |
- Adler, Mats, et al.
(författare)
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Diagnosdiskussion
- 2016
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Ingår i: Tidskriften för Svensk Psykiatri. - 1653-8579. ; 2016:1, s. 77-78
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Senaste halvåret har en arbetsgrupp på Svenska Psykiatriska Föreningens uppdrag arbetat med problem som finns runt psykiatriska diagnoser. En viktig del av arbetet har varit att ta fram ett diskussionsunderlag främst riktat till dem som i sitt arbete sätter psykiatriska diagnoser. Detta underlag syftar till att problematisera, inte till att polemisera. Självklart har vi i gruppen uppfattningar, men vi har inte alltid samma uppfattningar inom detta område. En sak är vi dock helt överens om – psykiatrisk diagnostik är svårt! samt att det idag finns uppenbara problem.
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| 10. |
- Adler, Mats, et al.
(författare)
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Psykiatrisk diagnostik är svårt.
- 2015
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Ingår i: Tidskriften för Svensk Psykiatri. - 1653-8579. ; 2015:4, s. 36-37
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)
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