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At least 145 records · Page 8Linked to original sources
PubMed1978

[Principles of optimization and scaling of certain processes in the vaccine production technology. 3. Method of determination of the optimal method of thermal sterilization of sugar solutions].

One of the most widespread methods of thermal sugar sterilization is the use of saturated water vapour under pressure in the capacity of heat source. Guaranteed sterility of solutions should be reached under conditions providing the least sugar splitting. However, in practice the integral effect of the thermal field on the solution is as a rule not taken into consideration. A new calculation method permitting to determine the amount of sugar split in the varying thermal field.

Carbohydrates↗
PubMed1983

[Stability of the optimal solution to the problem of determining optimal physicotechnical irradiation conditions].

The problem of determination of optimum physical and technical plans of irradiation often amounts to the problems of linear programming. To solve such a problem, it is necessary to calculate a matrix of conditions (dose rate in the system of distributed control points in the body during irradiation in different present directions). The purpose was to study the influence of errors of the calculation of dose rates upon the optimum irradiation plans. It has been established that though the task of determining optimum plans is a correct one, convergence of the solution of a disturbed problem to an initial one occurs extremely slowly. The optimum values of the functionals of a disturbed problem converge more rapidly to the functional of an initial problem. The comparison of optimum irradiation plans or optimum and traditional irradiation plans seems difficult. To compare different irradiation plans, it is more convenient to use the values of a specific function (optimum criteria).

Humans↗
PubMed1993

Optimization of conditioning therapy for leukemia prior to BMT. I. Optimal synergism between cyclophosphamide and total body irradiation for eradication of murine B cell leukemia (BCL1).

The doses of chemotherapy and radiotherapy and the sequence-dependent influence of CY administration and total body irradiation (TBI) on the eradication of murine B cell leukemia (BCL1) were investigated. Tumor-bearing BALB/c mice were treated with either CY followed by TBI (CY/TBI) or in the reverse (TBI/CY) at different time intervals after injection of 10(6) BCL1 cells. The presence of residual tumor cells after cytoreduction was assessed in secondary BALB/c recipients by monitoring leukemia-free survival. The anti-leukemic effect of CY followed by TBI was significantly better than TBI followed by CY. The survival of secondary adoptive recipients inoculated with 10(5) spleen cells obtained from mice treated with CY/TBI was 54% (57 of 105) compared with 25% (25 of 98) of recipients inoculated with 10(5) spleen cells obtained from mice treated with TBI/CY (p = 0.0001).

Animals↗
PubMedDate not supplied

[Optimization of radiation protection in the agrosphere: methods of optimization and computer systems for decision support with respect to management of protective measures].

Principles and methods of radiation protection optimisation for countermeasure implementation on areas subjected to radioactive contamination are discussed. Methodological aspects of the protective measures analysis in agrosphere are presented. Methods of estimation of intervention levels, examples of assessment and comparison of countermeasure effectiveness with the use of various criteria are considered. The brief review of up-to-date decision support systems is submitted along with the description of computer system PRANA DSS for estimation of the consequences and countermeasures management in agrosphere in the long-term period of liquidating the consequences of a nuclear accident.

Agriculture↗
PubMed2005

Simultaneous beam geometry and intensity map optimization in intensity-modulated radiation therapy.

PURPOSE: In current intensity-modulated radiation therapy (IMRT) plan optimization, the focus is on either finding optimal beam angles (or other beam delivery parameters such as field segments, couch angles, gantry angles) or optimal beam intensities. In this article we offer a mixed integer programming (MIP) approach for simultaneously determining an optimal intensity map and optimal beam angles for IMRT delivery. Using this approach, we pursue an experimental study designed to (a) gauge differences in plan quality metrics with respect to different tumor sites and different MIP treatment planning models, and (b) test the concept of critical-normal-tissue-ring--a tissue ring of 5 mm thickness drawn around the planning target volume (PTV)--and its use for designing conformal plans. METHODS AND MATERIALS: Our treatment planning models use two classes of decision variables to capture the beam configuration and intensities simultaneously. Binary (0/1) variables are used to capture "on" or "off" or "yes" or "no" decisions for each field, and nonnegative continuous variables are used to represent intensities of beamlets. Binary and continuous variables are also used for each voxel to capture dose level and dose deviation from target bounds. Treatment planning models were designed to explicitly incorporate the following planning constraints: (a) upper/lower/mean dose-based constraints, (b) dose-volume and equivalent-uniform-dose (EUD) constraints for critical structures, (c) homogeneity constraints (underdose/overdose) for PTV, (d) coverage constraints for PTV, and (e) maximum number of beams allowed. Within this constrained solution space, five optimization strategies involving clinical objectives were analyzed: optimize total intensity to PTV, optimize total intensity and then optimize conformity, optimize total intensity and then optimize homogeneity, minimize total dose to critical structures, minimize total dose to critical structures and optimize conformity simultaneously. We emphasize that the objectives that include optimizing conformity make use of the critical-normal-tissue-ring. Three tumor sites: head-and-neck, pediatric brain, and prostate are used for comparison. RESULTS: The critical-normal-tissue-ring acts as a good device for enforcing conformity. Trends in the characteristics and quality of plans resulting from each model were observed. Attempts to reduce dose to critical structures tend to worsen PTV conformity (1.542 to 3.092) and homogeneity (1.223 to 1.984), depending on the relative size and spatial distance of the critical structures to the PTV. When the critical structures are relatively small compared with the PTV (as in the case for the pediatric brain tumor, where each is less than 15% in volume), dose reduction to critical structures is accompanied by much worse scores in conformity (2.482) and homogeneity (1.984). When the critical structures are larger, as in the case of head-and-neck (approximately 50%), the conformity and homogeneity deterioration is less significant (1.542 and 1.239, respectively). There is a clear tradeoff between homogeneity, conformity, and minimum dose to organs at risk (OARs). For head-and-neck and pediatric brain tumor, the model that minimizes total dose to critical structures and optimizes conformity simultaneously offers a compromise among these factors, resulting in reduced critical structure dose with conformal and homogeneous plans. In the prostate case, the tumor is smaller than the two large nearby critical structures, and all models provide very homogeneous PTV dose distribution. However, minimizing dose to critical structures worsens conformity, as it spreads the radiation to the area surrounding the PTV. The maximum dose to the critical structures also increases slightly. Compared with plans used in the clinic which generally have uniformly spaced beam angles, the optimal clinically acceptable plans obtained via the methods herein do not have equispaced beams. The optimal beam angles returned appear to be nonintuitive, and depend on PTV size and geometry and the spatial relationship between the tumor and critical structures. CONCLUSIONS: The MIP model described allows simultaneous optimization over the space of beamlet fluence weights and beam and couch angles. Based on experiments with tumor data, this approach can return good plans that are clinically acceptable and practical. This work distinguishes itself from recent IMRT research in several ways. First, in previous methods beam angles are selected before intensity map optimization. Herein, we employ 0/1 variables to model the set of candidate beams, and thereby allow the optimization process itself to select optimal beams. Second, instead of incorporating dose-volume criteria within the objective function as in previous work, herein, a combination of discrete and continuous variables associated with each voxel provides a mechanism to strictly enforce dose-volume criteria within the constraints. Third, using the construct of critical-normal-tissue-ring within the objective function can enhance the achievement of conformal plans. Based on the three tumor sites considered, it appears that volume and spatial geometry with respect to the PTV are important factors to consider when selecting objectives to optimize, and in estimating how well suited a particular model is for achieving a specified goal.

Algorithms↗
PubMed1983

Reduced optimality in pre- and perinatal conditions in a Swedish newborn population.

The optimality concept of Prechtl was adopted for the investigation of a series of 229 newborn infants, selected by paired sampling as controls to 116 dyskinetic cerebral palsy cases. A list of 34 conditions, subgrouped into 14 prepartum, 10 partum and 10 postpartum items, was designed and criterias for optimality were defined. When a condition fell short of criteria for optimality, the infant concerned was given a non-optimal point. The sum of non-optimal points for each infant was defined as the reduced optimality score and the proportion of non-optimal points as the reduced optimality rate. The self-weighting capacity of the reduced optimality score system was confirmed. Lack of optimality compared to optimality in a particular item was generally related to a significant increase of the reduced optimality rates of remaining items. Reduced optimality compared to optimality was in the prepartum period significantly more often followed by reduced optimality than optimality in the partum and postpartum periods. In clinical categories--preterm birth, low birth weight, small for gestational age, and combinations of these--a significant increase of the reduced optimality rates were found. Outcome in items comparable to vital statistics were all close to the expected. The series was considered representative for Swedish children born during the period from 1959-70.

Birth Weight↗
PubMed2002

Beyond bixels: generalizing the optimization parameters for intensity modulated radiation therapy.

Intensity modulated radiation therapy (IMRT) treatment planning systems optimize fluence distributions by subdividing the fluence distribution into rectangular bixels. The algorithms typically optimize the fluence intensity directly, often leading to fluence distributions with sharp discontinuities. These discontinuities may yield difficulties in delivery of the fluence distribution, leading to inaccurate dose delivery. We have developed a method for decoupling the bixel intensities from the optimization parameters; either by introducing optimization control points from which the bixel intensities are interpolated or by parametrizing the fluence distribution using basis functions. In either case, the number of optimization search parameters is reduced from the direct bixel optimization method. To illustrate the concept, the technique is applied to two-dimensional idealized head and neck treatment plans. The interpolation algorithms investigated were nearest-neighbor, linear and cubic spline, and radial basis functions serve as the basis function test. The interpolation and basis function optimization techniques were compared against the direct bixel calculation. The number of optimization parameters were significantly reduced relative to the bixel optimization, and this was evident in the reduction of computation time of as much as 58% from the full bixel optimization. The dose distributions obtained using the reduced optimization parameter sets were very similar to the full bixel optimization when examined by dose distributions, statistics, and dose-volume histograms. To evaluate the sensitivity of the fluence calculations to spatial misalignment caused either by delivery errors or patient motion, the doses were recomputed with a 1 mm shift in each beam and compared to the unshifted distributions. Except for the nearest-neighbor algorithm, the reduced optimization parameter dose distributions were generally less sensitive to spatial shifts than the bixel optimization. These results indicate that significant reductions in optimization parameter sets can be accomplished with a negligible reduction in dose distribution quality. The decreased parameters can result in a reduced optimization time, or can be used to allow an improved and consequently more computation-intensive dose calculation for more accurate dose calculations during the optimization process. The basis functions may be generalized to model the accelerator motion for direct computation of the accelerator motion sequence, removing the need for developing an independent leaf sequence step.

Algorithms↗
PubMed1997

Effect of geometrical optimization on the treatment volumes and the dose homogeneity of biplane interstitial brachytherapy implants.

BACKGROUND AND PURPOSE: The isodose distributions of HDR stepping source brachytherapy implants can be modified by changing dwell times and this procedure is called optimization. The purpose of this study is to evaluate the effect of geometrical optimization on the brachytherapy volumes and the dose homogeneity inside the implant and to compare them with non-optimized counterparts. MATERIAL AND METHODS: A set of biplane breast implants consisting of 84 different configurations have been digitized by the planning computer and volumetric analysis was performed for both non-optimized and geometrically optimized implants. Treated length (TL), treated volume (V100), irradiated volume (V50), overdose volume (V200) and quality index (QI) have been calculated for every non-optimized implant and compared to its corresponding geometrically optimized implant having a similar configuration and covering the same target length. RESULTS: The mean TL was 74.48% of the active length (AL) for non-optimized implants and was 91.87% for optimized implants (P < 0.001). The mean QI was 1.83 for non-optimized implants and 2.17 for optimized implants (P < 0.001). The mean V50/V100 value was 2.71 for non-optimized implants and 2.65 for optimized implants (P < 0.001) and the mean V200/V100 value was 0.09 for non-optimized implants and 0.10 for optimized implants (P < 0.001). CONCLUSIONS: By performing geometrical optimization it is possible to implant shorter needles for a given tumour to adequately cover the target volume with the reference isodose and thus surgical damage is reduced. The amount of healthy tissues outside the target receiving considerable radiation is significantly reduced due to the decrease in irradiated volume. Dose homogeneity inside the implant is significantly improved. Although there is a slight increase of overdose volume inside the implant, this increase is considered to be negligible in clinical applications.

Brachytherapy↗
PubMed2004

Influence of dose point and inverse optimization on interstitial cervical and oropharyngeal carcinoma brachytherapy.

BACKGROUND AND PURPOSE: Evaluation of the use of optimization methods in interstitial cervical and oropharyngeal brachytherapy; evaluation of the conformal index (COIN) and the natural dose ratio (NDR) to quantify the implant quality. MATERIAL AND METHODS: CT-based dose distributions were obtained for seven implants according to the Paris system. CT-based implants were used to assess the dose point and inverse optimization methods. To compare the results of these planning methods, the coverage index (CI), normal tissue irradiation (NTI), and the protection of organs at risk (OARs) were evaluated using cumulative dose volume histograms (CDVH). RESULTS: In regular cervical implants, a CI of 94 and 96%; a NTI of 35 and 28% resulted for non-optimized and optimized implants, respectively. In irregular cervical implants, a CI of 88, 96, and 90%; a NTI of 44, 37, and 44% resulted for non-optimized, dose point optimized, and inverse optimized implants, respectively. Compared to the non-optimized implants; both optimization methods resulted in better protection for the bladder wall. As for the protection of the rectal wall, only the inverse optimization gave a better result. In oropharyngeal implants, a better CI resulted after dose point optimization. Irradiation of the contralateral parotid were improved after both optimization methods. The maximum change in COIN that could have been achieved by optimization was 3%, as CI and NTI increased similarly. For the same value of COIN, an underdosage of PTV was avoided by the optimization methods as NDR increased from 0.86 to 1.01. CONCLUSION: CT-based optimized implant allows conformation of the dose distribution to the PTV while sparing normal tissue and organs at risk. COIN and NDR should be used together to evaluate both doses to normal tissue and organs at risk, and an under- or overdose inside the PTV.

Brachytherapy↗
PubMed2006

Combined genetic algorithm and multiple linear regression (GA-MLR) optimizer: Application to multi-exponential fluorescence decay surface.

The optimization approach based on the genetic algorithm (GA) combined with multiple linear regression (MLR) method, is discussed. The GA-MLR optimizer is designed for the nonlinear least-squares problems in which the model functions are linear combinations of nonlinear functions. GA optimizes the nonlinear parameters, and the linear parameters are calculated from MLR. GA-MLR is an intuitive optimization approach and it exploits all advantages of the genetic algorithm technique. This optimization method results from an appropriate combination of two well-known optimization methods. The MLR method is embedded in the GA optimizer and linear and nonlinear model parameters are optimized in parallel. The MLR method is the only one strictly mathematical "tool" involved in GA-MLR. The GA-MLR approach simplifies and accelerates considerably the optimization process because the linear parameters are not the fitted ones. Its properties are exemplified by the analysis of the kinetic biexponential fluorescence decay surface corresponding to a two-excited-state interconversion process. A short discussion of the variable projection (VP) algorithm, designed for the same class of the optimization problems, is presented. VP is a very advanced mathematical formalism that involves the methods of nonlinear functionals, algebra of linear projectors, and the formalism of Fréchet derivatives and pseudo-inverses. Additional explanatory comments are added on the application of recently introduced the GA-NR optimizer to simultaneous recovery of linear and weakly nonlinear parameters occurring in the same optimization problem together with nonlinear parameters. The GA-NR optimizer combines the GA method with the NR method, in which the minimum-value condition for the quadratic approximation to chi(2), obtained from the Taylor series expansion of chi(2), is recovered by means of the Newton-Raphson algorithm. The application of the GA-NR optimizer to model functions which are multi-linear combinations of nonlinear functions, is indicated. The VP algorithm does not distinguish the weakly nonlinear parameters from the nonlinear ones and it does not apply to the model functions which are multi-linear combinations of nonlinear functions.

Algorithms↗
PubMed1994

Optimization of interstitial volume implants.

For interstitial applications of high dose rate (HDR) afterloading brachytherapy, generally a single stepping iridium-192 source is used, enabling optimization of the dose distribution by optimization of the relative time (dwell time) that the source remains at a certain position (dwell position). We analysed the effects of geometric optimization in a regular volume implant, with strictly parallel catheters, and in an irregular volume implant, such as an implant for tumours of the base of the tongue characterized by a non-parallel geometry and varying catheter separations. In both examples the reference dose is specified at 85% of the mean central dose (as is done in the Paris system for dose specification) in the non-optimized as well as the optimized plan. The irradiated volume, the dose uniformity, and the choice of the reference dose of optimized and non-optimized dose distributions were compared. This was done by isodose plots for representative planes, volume dose histograms (distributed, contiguous, and natural), and dose non-uniformity ratios (DNRs). For the regular implant, optimization results in a 28% increase in the treated volume with a similar increase in the overdosed volumes. In order to keep the treated volume comparable with the non-optimized dose distribution, 90-95% of the mean central dose should be chosen as a reference dose or the range of active dwell positions should be shortened in case of optimization. In the case of the irregular volume implant at the base of the tongue, the method for dose specification should be kept unchanged after geometric optimization as the volume enclosed by the reference isodose does not increase. It is clear from the volume-dose histograms that there is a reduction of the overdosed volume due to optimization. This is accompanied by an increase in the uniformity index and a decrease of the DNR. In conclusion, geometric optimization appears to be an effective tool to improve the dose distribution of interstitial volume implants. Contiguous and natural volume dose histograms appear, apart from planar dose plots, valuable methods for evaluating the dose distribution of an implant.

Brachytherapy↗
PubMed2003

Prospective evaluation of a D-optimal designed population pharmacokinetic study.

Recently, methods for computing D-optimal designs for population pharmacokinetic studies have become available. However there are few publications that have prospectively evaluated the benefits of D-optimality in population or single-subject settings. This study compared a population optimal design with an empirical design for estimating the base pharmacokinetic model for enoxaparin in a stratified randomized setting. The population pharmacokinetic D-optimal design for enoxaparin was estimated using the PFIM function (MATLAB version 6.0.0.88). The optimal design was based on a one-compartment model with lognormal between subject variability and proportional residual variability and consisted of a single design with three sampling windows (0-30 min, 1.5-5 hr and 11-12 hr post-dose) for all patients. The empirical design consisted of three sample time windows per patient from a total of nine windows that collectively represented the entire dose interval. Each patient was assigned to have one blood sample taken from three different windows. Windows for blood sampling times were also provided for the optimal design. Ninety six patients were recruited into the study who were currently receiving enoxaparin therapy. Patients were randomly assigned to either the optimal or empirical sampling design, stratified for body mass index. The exact times of blood samples and doses were recorded. Analysis was undertaken using NONMEM (version 5). The empirical design supported a one compartment linear model with additive residual error, while the optimal design supported a two compartment linear model with additive residual error as did the model derived from the full data set. A posterior predictive check was performed where the models arising from the empirical and optimal designs were used to predict into the full data set. This revealed the "optimal" design derived model was superior to the empirical design model in terms of precision and was similar to the model developed from the full dataset. This study suggests optimal design techniques may be useful, even when the optimized design was based on a model that was misspecified in terms of the structural and statistical models and when the implementation of the optimal designed study deviated from the nominal design.

Adult↗