Осложнения желчекаменной болезни и причины летальности

ЖЕЛЧНЫХ ПУТЕЙ ХИРУРГИЧЕСКИЕ ОПЕРАЦИИ /ТЕНДЕНЦИИЖЕЛЧНОКАМЕННАЯ БОЛЕЗНЬ /ОСЛ /СМЕРТН /ХИРХОЛЕЦИСТИТ КАЛЬКУЛЕЗНЫЙ /ХИРЛАПАРОСКОПИЯВОЗРАСТНЫЕ ФАКТОР

AAPM WGDCAB Report 372: A joint AAPM, ESTRO, ABG, and ABS report on commissioning of model-based dose calculation algorithms in brachytherapy

The introduction of model-based dose calculation algorithms (MBDCAs) in brachytherapy provides an opportunity for a more accurate dose calculation and opens the possibility for novel, innovative treatment modalities. The joint AAPM, ESTRO, and ABG Task Group 186 (TG-186) report provided guidance to early adopters. However, the commissioning aspect of these algorithms was described only in general terms with no quantitative goals. This report, from the Working Group on Model-Based Dose Calculation Algorithms in Brachytherapy, introduced a field-tested approach to MBDCA commissioning. It is based on a set of well-characterized test cases for which reference Monte Carlo (MC) and vendor-specific MBDCA dose distributions are available in a Digital Imaging and Communications in Medicine—Radiotherapy (DICOM-RT) format to the clinical users. The key elements of the TG-186 commissioning workflow are now described in detail, and quantitative goals are provided. This approach leverages the well-known Brachytherapy Source Registry jointly managed by the AAPM and the Imaging and Radiation Oncology Core (IROC) Houston Quality Assurance Center (with associated links at ESTRO) to provide open access to test cases as well as step-by-step user guides. While the current report is limited to the two most widely commercially available MBDCAs and only for Ir-based afterloading brachytherapy at this time, this report establishes a general framework that can easily be extended to other brachytherapy MBDCAs and brachytherapy sources. The AAPM, ESTRO, ABG, and ABS recommend that clinical medical physicists implement the workflow presented in this report to validate both the basic and the advanced dose calculation features of their commercial MBDCAs. Recommendations are also given to vendors to integrate advanced analysis tools into their brachytherapy treatment planning system to facilitate extensive dose comparisons. The use of the test cases for research and educational purposes is further encouraged

The relationship between management accounting, profitability and operations in an uncertain world : evidence from literature and practice

At the heart of many core Management Accounting (MA) practices there is a potential mismatch between the assumption of a materially predictable future operating environment, and the reality of an uncertain and unpredictable world. Practices such as budgets, product costing, investment appraisal and financial projections, aimed at facilitating the achievement of profitability goals, are based on the assumption that the future is sufficiently stable and predictable to benefit from analytical calculation. However, we live in a world where the future can be uncertain, unstable and unpredictable. Does this mean that when operating conditions become unstable, unpredictable and uncertain many MA practices lose their core modus operandi? This thesis addresses this issue through an interwoven mix of a longitudinal case study and literature reviews spread over three projects. The case study was longitudinal and based on in depth participant observation. The firm involved was a £38m UK logistics company. The study benefited from totally unrestricted access to all strategic, financial and operational activities and data, because of the author’s senior role in the firm. The literature review was conducted using a targeted systematic review (Tranfield and Denyer, 2003) supported by additional narrative reviews. This synoptic paper provides a reflective synthesis of the findings and the contribution of the three projects which together constitute the research. Four core interlinked findings emerged from the study, based on the assumption that the achievement of profitability goals is the primary goal of the organisation. First, building on the proposals of (Otley, 1999) a framework showing the relationship between MA, profitability, operations and uncertainty is proposed. It demonstrates how MA financialises operations by creating a parallel financial space to the operational space; how profitability outcomes result from the financial consequences of operational actions; how the role of MA is to inform and control operational actions in a manner that achieves profitability goals; and how uncertainty has a critical impact on MA functionality. Second, the differing dimensions and implications of uncertainty are distinguished. The principal distinction is between external and internal uncertainty. External uncertainties arise from unanticipated changes from customers, suppliers and the market and thus affect the predictability of the future on which plans and targets are based. The data gathered during the course of this research suggests that external uncertainty tends to be typified by pockets of instability oscillating with periods of relative stability. Internal uncertainties occur in relation to management effectiveness, reporting validity and choice of appropriate accounting perspective (five are identified - Product, Customer, Throughout, Process, Financial Accounting). The external uncertainties magnify the impact of the internal uncertainties by potentially changing and thus de-stabilising the requirements of management, the validity of reporting and the appropriateness of the accounting perspective used. Third, Management Accounting Systems (MAS) respond to external uncertainties, and the aspirations of external financial stakeholders for increased profitability, by operating in two differing modes – the first is fixed/control (Fixed), the second is inform/flex (Flex). Fixed is the default mode and assumes conditions of relative certainty; the role is to control the achievement of agreed plans and targets. Flex is intermittently initiated when, signalled by feedback, the impact of external uncertainties or profit pressures trigger the need to change original plans and targets. Calculative analysis informs revised operational plans aimed at maintaining the achievement of profitability goals; targets are flexed to reflect the changes. The intent is to develop a revised position of relative stability in which the achievement of profitability plans and targets can be controlled via reverting back to Fixed. The process is therefore continual, but appears to be typified by an uneven series of oscillations between the two modes. Four, the Financial Accounting (FA) profitability measure, with the goal derived from external financial stakeholders, provides partial responses to the three internal uncertainties by introducing for each an element of certainty. For management effectiveness uncertainty, the profitability goal provides a relatively certain external referent which can be cascaded down the organisational structure, and against which performance can be evaluated. For reporting validity uncertainty, FA standards provide an authoritatively accepted definition of profitability, so that reported profitability is treated as if it were ‘true and fair’. For multiple accounting perspectives uncertainty, four perspectives (Product, Customer, Throughout, Process) make up a range of MA tools for developing actions to achieve target profitability levels, and the fifth (FA) provides the definition of profitability; all five are complementary and compatible as their differing aggregations are composed of the same underlying financial transactions. These responses, however, are only partial as the aspirations of external financial stakeholders are in themselves substantially self referential and liable to change, and the underlying uncertainty of FA reporting validity still exists, even if treated as if it does not. The study contributes to the further development of MA theory. It extends the Otley (1999) framework towards linking operations and profitability through parallel operational and financial spaces, and incorporating the central role of uncertainty. It adds to the debate in MA research on uncertainty by providing a classification of its dimensions, and its impact on triggering a requirement for differing MA modes. It highlights the central role of profitability in providing a stable certainty of purpose as a counterbalance to inherent internal and external uncertainties. It provides a clear identification of the differences and complementarities between MA and FA, FA defining the quantum of profitability achieved, MA facilitating the achievement of profitability goal. Finally the study inputs to a wide range of issues addressed by MA research which at their heart reflect the impact of uncertainty (Budgeting, Accounting Representation, Costing Perspectives). The study contributes to practice by proposing a set of ten tenets designed to provide guidelines for MAS development, implementation and evaluation. These are drawn from a cross sectional deconstruction of the four findings, viewed as a whole, aimed at identifying the specific factors that have direct implications for practice. The intent is that these tenets provide a bridge between theory and practice, based on the premise that, since MA theory was drawn from practice, the test of MA theory development is its applicability and relevance to practice.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Impact of Using Linear Optimization Models in Dose Planning for HDR Brachytherapy

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

Impact of Using Linear Optimization Models in Dose Planning for HDR Brachytherapy

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

A mathematical optimization model for spatial adjustments of dose distributions in high dose-rate brachytherapy

High dose-rate brachytherapy is a modality of radiation therapy used for cancer treatment, in which the radiation source is placed within the body. The treatment goal is to give a high enough dose to the tumour while sparing nearby healthy tissue and organs (organs-at-risk). The most common criteria for evaluating dose distributions are dosimetric indices. For the tumour, such an index is the portion of the volume that receives at least a specified dose level (e.g. the prescription dose), while for organs-at-risk it is instead the portion of the volume that receives at most a specified dose level. Dosimetric indices are aggregate criteria and do not consider spatial properties of the dose distribution. Further, there are neither any established evaluation criteria for characterizing spatial properties, nor have such properties been studied in the context of mathematical optimization of brachytherapy. Spatial properties are however of clinical relevance and therefore dose plans are sometimes adjusted manually to improve them. We propose an optimization model for reducing the prevalence of contiguous volumes with a too high dose (hot spots) or a too low dose (cold spots) in a tentative dose plan. This model is independent of the process of constructing the tentative plan. We conduct computational experiments with tentative plans obtained both from optimization models and from clinical practice. The objective function considers pairs of dose points and each pair is given a distance-based penalty if the dose is either too high or too low at both dose points. Constraints are included to retain dosimetric indices at acceptable levels. Our model is designed to automate the manual adjustment step in the planning process. In the automatic adjustment step large-scale optimization models are solved. We show reductions of the volumes of the largest hot and cold spots, and the computing times are feasible in clinical practice.Funding Agencies|Swedish Research CouncilSwedish Research Council [VR-NT 2015-04543]; Swedish Cancer SocietySwedish Cancer Society [CAN 2015/618, CAN 2018/622]</p

Mathematical optimization of high dose-rate brachytherapy-derivation of a linear penalty model from a dose-volume model

High dose-rate brachytherapy is a method for cancer treatment where the radiation source is placed within the body, inside or close to a tumour. For dose planning, mathematical optimization techniques are being used in practice and the most common approach is to use a linear model which penalizes deviations from specified dose limits for the tumour and for nearby organs. This linear penalty model is easy to solve, but its weakness lies in the poor correlation of its objective value and the dose-volume objectives that are used clinically to evaluate dose distributions. Furthermore, the model contains parameters that have no clear clinical interpretation. Another approach for dose planning is to solve mixed-integer optimization models with explicit dose-volume constraints which include parameters that directly correspond to dose-volume objectives, and which are therefore tangible. The two mentioned models take the overall goals for dose planning into account in fundamentally different ways. We show that there is, however, a mathematical relationship between them by deriving a linear penalty model from a dose-volume model. This relationship has not been established before and improves the understanding of the linear penalty model. In particular, the parameters of the linear penalty model can be interpreted as dual variables in the dose-volume model.Funding Agencies|Swedish Research Council [VR-NT 2015-04543]; Swedish Cancer Foundation [CAN 2015/618]</p

Evaluation of a lithium formate EPR dosimetry system for dose measurements around Ir-192 brachytherapy sources

A dosimetry system using lithium formate monohydrate (HCO2Li center dot H2O) as detector material and electron paramagnetic resonance (EPR) spectroscopy for readout has been used to measure absorbed dose distributions around clinical Ir-192 sources. Cylindrical tablets with diameter of 4.5 mm, height of 4.8 mm, and density of 1.26 g/cm(3) were manufactured. Homogeneity test and calibration of the dosimeters were performed in a 6 MV photon beam. Ir-192 irradiations were performed in a PMMA phantom using two different source models, the GammaMed Plus HDR and the microSelectron PDR-v1 model. Measured absorbed doses to water in the PMMA phantom were converted to the corresponding absorbed doses to water in water phantoms of dimensions used by the treatment planning systems (TPSs) using correction factors explicitly derived for this experiment. Experimentally determined absorbed doses agreed with the absorbed doses to water calculated by the TPS to within +/- 2.9%. Relative standard uncertainties in the experimentally determined absorbed doses were estimated to be within the range of 1.7%-1.3% depending on the radial distance from the source, the type of source (HDR or PDR), and the particular absorbed doses used. This work shows that a lithium formate dosimetry system is well suited for measurements of absorbed dose to water around clinical HDR and PDR Ir-192 sources. Being less energy dependent than the commonly used thermoluminescent lithium fluoride (LiF) dosimeters, lithium formate monohydrate dosimeters are well suited to measure absorbed doses in situations where the energy dependence cannot easily be accounted for such as in multiple-source irradiations to verify treatment plans. Their wide dynamic range and linear dose response over the dose interval of 0.2-1000 Gy make them suitable for measurements on sources of the strengths used in clinical applications. The dosimeter size needs, however, to be reduced for application to single-source dosimetry.Original Publication: Laura Antonovic, Håkan Gustafsson, Gudrun Alm Carlsson and Åsa Carlsson Tedgren, Evaluation of a lithium formate EPR dosimetry system for dose measurements around Ir-192 brachytherapy sources, 2009, MEDICAL PHYSICS, (36), 6, 2236-2247. http://dx.doi.org/10.1118/1.3110068 Copyright: American Institute of Physics http://www.aip.org/</p

Optimization in treatment planning of high dose‐rate brachytherapy : Review and analysis of mathematical models

Treatment planning in high dose‐rate brachytherapy has traditionally been conducted with manual forward planning, but inverse planning is today increasingly used in clinical practice. There is a large variety of proposed optimization models and algorithms to model and solve the treatment planning problem. Two major parts of inverse treatment planning for which mathematical optimization can be used are the decisions about catheter placement and dwell time distributions. Both these problems as well as integrated approaches are included in this review. The proposed models include linear penalty models, dose–volume models, mean‐tail dose models, quadratic penalty models, radiobiological models, and multiobjective models. The aim of this survey is twofold: (i) to give a broad overview over mathematical optimization models used for treatment planning of brachytherapy and (ii) to provide mathematical analyses and comparisons between models. New technologies for brachytherapy treatments and methods for treatment planning are also discussed. Of particular interest for future research is a thorough comparison between optimization models and algorithms on the same dataset, and clinical validation of proposed optimization approaches with respect to patient outcome.Funding: Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR-NT 2015-04543]; Swedish Cancer SocietySwedish Cancer Society [CAN 2017/1029, CAN 2018/622]</p