S. Giannouli

Global convergence analysis of fast multiobjective gradient-based dose optimization algorithms for high-dose-rate brachytherapy

We consider the problem of the global convergence of gradient-based optimization algorithms for interstitial high-dose-rate (HDR) brachytherapy dose optimization using variance-based objectives. Possible local minima could lead to only sub-optimal solutions. We perform a configuration space analysis using a representative set of the entire non-dominated solution space. A set of three prostate implants is used in this study. We compare the results obtained by conjugate gradient algorithms, two variable metric algorithms and fast-simulated annealing. For the variable metric algorithm BFGS from numerical recipes, large fluctuations are observed. The limited memory L-BFGS algorithm and the conjugate gradient algorithm FRPR are globally convergent. Local minima or degenerate states are not observed. We study the possibility of obtaining a representative set of non-dominated solutions using optimal solution rearrangement and a warm start mechanism. For the surface and volume dose variance and their derivatives, a method is proposed which significantly reduces the number of required operations. The optimization time, ignoring a preprocessing step, is independent of the number of sampling points in the planning target volume. Multiobjective dose optimization in HDR brachytherapy using L-BFGS and a new modified computation method for the objectives and derivatives has been accelerated, depending on the number of sampling points, by a factor in the range 10-100.

CT imaging based digitally reconstructed radiographs and their application in brachytherapy

The aim of our study was to develop an algorithm to simulate the digitally reconstructed radiograph (DRR) calculation process for different beam qualities (photon energies) in the range 50 keV to 12 MeV. This was achieved using volumetric anatomical data for the patient obtained from three-dimensional diagnostic CT images. These DRR images can be used in three-dimensional treatment planning for external beam radiotherapy as well as for brachytherapy in the same way as conventional radiographic films. The advantages of using such DRRs in modern 3D brachytherapy treatment planning are shown. A number of tools are described, illustrating that the application of DRRs in brachytherapy is very useful.

Generation of uniformly distributed dose points for anatomy-based three-dimensional dose optimization methods in brachytherapy.

We have studied the accuracy of statistical parameters of dose distributions in brachytherapy using actual clinical implants. These include the mean, minimum and maximum dose values and the variance of the dose distribution inside the PTV (planning target volume), and on the surface of the PTV. These properties have been studied as a function of the number of uniformly distributed sampling points. These parameters, or the variants of these parameters, are used directly or indirectly in optimization procedures or for a description of the dose distribution. The accurate determination of these parameters depends on the sampling point distribution from which they have been obtained. Some optimization methods ignore catheters and critical structures surrounded by the PTV or alternatively consider as surface dose points only those on the contour lines of the PTV. D(min) and D(max) are extreme dose values which are either on the PTV surface or within the PTV. They must be avoided for specification and optimization purposes in brachytherapy. Using D(mean) and the variance of D which we have shown to be stable parameters, achieves a more reliable description of the dose distribution on the PTV surface and within the PTV volume than does D(min) and D(max). Generation of dose points on the real surface of the PTV is obligatory and the consideration of catheter volumes results in a realistic description of anatomical dose distributions.

Catheter autoreconstruction in computed tomography based brachytherapy treatment planning

The aim of this study is to develop an automatic reconstruction of brachytherapy catheters using CT (computed tomography) data. Previously no such automatic facility has existed in any treatment planning software. To achieve this facility we have developed tools for the automatic reconstruction (which we term autoreconstruction) of plastic and metallic catheters. These algorithms overcome a number of difficulties which arise when a large number of catheters are present. These include situations with intersecting catheters and with loop techniques. The time required for the catheter reconstruction process using our autoreconstruction method is significantly reduced. The accuracy of our autoreconstruction is at least as high as the classical manual slice-by-slice method.

A dosimetric comparison of versus for HDR prostate brachytherapy

For the purpose of evaluating the use of Yb169 for prostate High Dose Rate brachytherapy (HDR), a hypothetical Yb169 source is assumed with the exact same design of the new microSelectron source replacing the Ir192 active core by pure Yb169 metal. Monte Carlo simulation is employed for the full dosimetric characterization of both sources and results are compared following the AAPM TG-43 dosimetric formalism. Monte Carlo calculated dosimetry results are incorporated in a commercially available treatment planning system (SWIFTTM), which features an inverse treatment planning option based on a multiobjective dose optimization engine. The quality of prostate HDR brachytherapy using the real Ir192 and hypothetical Yb169 source is compared in a comprehensive analysis of different prostate implants in terms of the multiobjective dose optimization solutions as well as treatment quality indices such as Dose Volume Histograms (DVH) and the Conformal Index (COIN). Given that scattering overcompensates for absorption in intermediate photon energies and distances in the range of interest to prostate HDR brachytherapy, Yb169 proves at least equivalent to Ir192 irrespective of prostate volume. This has to be evaluated in view of the shielding requirements for the Yb169 energies that are minimal relative to that for Ir192.

Application of the Monte Carlo integration (MCI) method for calculation of the anisotropy of brachytherapy sources

Source anisotropy is a very important factor in the brachytherapy quality assurance of high-dose rate (HDR) 192Ir afterloading stepping sources. If anisotropy is not taken into account then doses received by a brachytherapy patient in certain directions can be in error by a clinically significant amount. Experimental measurements of anisotropy are very labour intensive. We have shown that within acceptable limits of accuracy, Monte Carlo integration (MCI) of a modified Sievert integral (3D generalization) can provide the necessary data within a much shorter time scale than can experiments. Hence MCI can be used for routine quality assurance schedules whenever a new design of HDR or PDR 192Ir is used for brachytherapy afterloading. Our MCI calculation results are compared with published experimental data and Monte Carlo simulation data for microSelectron and VariSource 192Ir sources. We have shown not only that MCI offers advantages over alternative numerical integration methods, but also that treating filtration coefficients as radial distance-dependent functions improves Sievert integral accuracy at low energies. This paper also provides anisotropy data for three new 192Ir sources, one for the microSelectron-HDR and two for the microSelectron-PDR, for which data are currently not available. The information we have obtained in this study can be incorporated into clinical practice.

Autoactivation of source dwell positions for HDR brachytherapy treatment planning.

The most accurate classical dose optimization algorithms in HDR brachytherapy strongly depend on an appropriate selection of source dwell positions which fulfill user-defined geometrical boundary conditions which are relative to patient anatomy. Most anatomical situations, such as for prostate and head and neck tumors, are complex and can require geometries with 5-15 catheters with 48 possible dwell positions per catheter depending on the tumor volume. The manual selection of dwell positions using visual checks by trial and error is very time consuming. This can only be improved by the use of a technique which automatically recognizes and selects the optimum dwell positions for each catheter. We have developed an algorithm, termed an autoactivation algorithm, which improves implant planning by providing a facility for the necessary automatic recognition of HDR source dwell positions.

A dosimetric comparison of 169Yb versus 192Ir for HDR prostate brachytherapy.

For the purpose of evaluating the use of Yb169 for prostate High Dose Rate brachytherapy (HDR), a hypothetical Yb169 source is assumed with the exact same design of the new microSelectron source replacing the Ir192 active core by pure Yb169 metal. Monte Carlo simulation is employed for the full dosimetric characterization of both sources and results are compared following the AAPM TG-43 dosimetric formalism. Monte Carlo calculated dosimetry results are incorporated in a commercially available treatment planning system (SWIFTTM), which features an inverse treatment planning option based on a multiobjective dose optimization engine. The quality of prostate HDR brachytherapy using the real Ir192 and hypothetical Yb169 source is compared in a comprehensive analysis of different prostate implants in terms of the multiobjective dose optimization solutions as well as treatment quality indices such as Dose Volume Histograms (DVH) and the Conformal Index (COIN). Given that scattering overcompensates for absorption in intermediate photon energies and distances in the range of interest to prostate HDR brachytherapy, Yb169 proves at least equivalent to Ir192 irrespective of prostate volume. This has to be evaluated in view of the shielding requirements for the Yb169 energies that are minimal relative to that for Ir192.

Brachytherapy dose–volume histogram computations using optimized stratified sampling methods

A stratified sampling method for the efficient repeated computation of dose-volume histograms (DVHs) in brachytherapy is presented as used for anatomy based brachytherapy optimization methods. The aim of the method is to reduce the number of sampling points required for the calculation of DVHs for the body and the PTV. From the DVHs are derived the quantities such as Conformity Index COIN and COIN integrals. This is achieved by using partial uniform distributed sampling points with a density in each region obtained from a survey of the gradients or the variance of the dose distribution in these regions. The shape of the sampling regions is adapted to the patient anatomy and the shape and size of the implant. For the application of this method a single preprocessing step is necessary which requires only a few seconds. Ten clinical implants were used to study the appropriate number of sampling points, given a required accuracy for quantities such as cumulative DVHs, COIN indices and COIN integrals. We found that DVHs of very large tissue volumes surrounding the PTV, and also COIN distributions, can be obtained using a factor of 5-10 times smaller the number of sampling points in comparison with uniform distributed points.

A dosimetric comparison of Yb169 versus Ir192 for HDR prostate brachytherapy

For the purpose of evaluating the use of Yb169 for prostate High Dose Rate brachytherapy (HDR), a hypothetical Yb169 source is assumed with the exact same design of the new microSelectron source replacing the Ir192 active core by pure Yb169 metal. Monte Carlo simulation is employed for the full dosimetric characterization of both sources and results are compared following the AAPM TG-43 dosimetric formalism. Monte Carlo calculated dosimetry results are incorporated in a commercially available treatment planning system (SWIFTTM), which features an inverse treatment planning option based on a multiobjective dose optimization engine. The quality of prostate HDR brachytherapy using the real Ir192 and hypothetical Yb169 source is compared in a comprehensive analysis of different prostate implants in terms of the multiobjective dose optimization solutions as well as treatment quality indices such as Dose Volume Histograms (DVH) and the Conformal Index (COIN). Given that scattering overcompensates for absorption in intermediate photon energies and distances in the range of interest to prostate HDR brachytherapy, Yb169 proves at least equivalent to Ir192 irrespective of prostate volume. This has to be evaluated in view of the shielding requirements for the Yb169 energies that are minimal relative to that for Ir192.