Volume 31, Issue 4, April 2004
Index of content:
- PH. D. THESES ABSTRACTS
Inverse planning for low-dose-rate prostate brachytherapy by simulated annealing under fuzzy expert control31(2004); http://dx.doi.org/10.1118/1.1650526View Description Hide Description
Simulated annealing (SA) is a multivariate combinatorial optimization process that searches the configuration space of possible solutions by a random walk, guided only by the goal of minimization of the objective function. The decision-making capabilities of a fuzzy inference system are applied to guide the SA search, to look for solutions which, in addition to optimizing a plan in dosimetric terms, also present some clinically desirable spatial features. No a priori constraints are placed on the number or position of needles or on the seed loading sequence of individual needles. These additional degrees of freedom are balanced by giving preference to consider plans with seed distributions that are balanced in the right/left and anterior/posterior halves in each axial slice, and with local seed density that is about uniform. Piecewise linear membership functions are constructed to represent these requirements. Before a step in the random search is subject to the SA test, the expert functions representing the spatial seed-distribution requirements are evaluated. Thus, the expert planner’s knowledge enters into the decision as to the “goodness” of a seed configuration regarding the spatial seed-distribution goals. When a step in the random walk yields a seed configuration that is found wanting, a specific number of additional steps in the local neighborhood is attempted until either improvement in the spatial requirements is achieved, or the allowed number of attempts is exhausted. In the latter case, the expert system desists and the unfavorable step is taken, moving on to the simulated annealing test. The number of attempts is determined by the fuzzy logic inference engine and depends on just how badly the expert requirement is not met. The program is interfaced with a commercial treatment planning system (TPS) to import optimized seed plans for isodose display and analysis. Execution in a 1.5 GHz computer is less than a minute, adequate for real-time planning. Results for phantom and real patients are dosimetrically comparable to generic methods such as modified peripheral loading, but in some cases, fewer needles and less radioactivity are required. Sensitivity to simulated random seed placement error is similar to manual plans.
31(2004); http://dx.doi.org/10.1118/1.1650527View Description Hide Description
A 120 leaf collimator of high resolution has been constructed to shape a fast neutron therapy (FNT) beam produced from a superconducting cyclotron using the reaction. The computer controlledmultileaf collimator(MLC) replaces an aging, manually operated multirod collimator (MRC). The MLC was built to address two problems: The need to increase the efficiency of FNT at the Gershenson RadiationOncology Center of the Karmanos Cancer Institute at Detroit, MI, and the desire to implement intensity modulated neutron radiotherapy for which a suitable computer controlled beam shaping device of high resolution and rapid shape changing not currently exist. The specific aims were to build a neutronMLC that would solve these problems and then verify its radiological performance as being clinically acceptable. The MLC leaves project 5 mm in the iso-centric plane perpendicular to the beam axis. A taper has been included on the leaves matching the beam divergence along one axis. A 5 mm leaf projection width was chosen to give high resolution conformality across the entire field. The maximum field size provided is To reduce the interleaf transmission a 0.254 mm blocking step has been included. End-leaf steps totaling 0.762 mm were also included allowing adjacent leaf pairs to close off within the primary radiation beam. The neutronMLC also includes individual and automated universal tungsten wedges. All electro-mechanical functions of the MLC are governed by the multileaf collimatorcontrol system (MLCCS). The MLCCS control functions are leaf and wedge motion control, leaf and wedge position verification, switching on and off the radiation beam, and inputting certain cyclotron interlocks statuses as well as outputting MLC interlocks. The MLCCS is responsible for setting required shapes on the MLC, and verifying those shapes to be accurate to within for each leaf’s projection in the plane perpendicular to the beam axis at the position of the iso-center. The position verification function uses a machine vision system that images optical targets on the leaves to verify set field shape accuracy within at the level of iso-center. The MLC transmission was measured to be 3.9±0.5% with an 11±2% gamma component which is slightly lower than the MRC measured transmission of 4.6±0.5% with a 17±2% gamma component. The difference in the measured gamma component ratio for the closed collimators agrees within 40% to a theoretical approximation of for tungsten to steel when considering equal attenuation lengths. The actual MLC penumbral measurements in water for a field at 5 cm depth were and along the focused and unfocused axes, respectively. The MRC penumbra measured under the same conditions along both of its focused axes suggesting penumbral equivalence between the MLC and MRC along the focused axis with slight degradation in the unfocused direction of the MLC. The many benefits of the fully automatic MLC over the semi-manual MRC are considered to justify this compromise.
31(2004); http://dx.doi.org/10.1118/1.1655708View Description Hide Description
Intensity modulated radiation therapy(IMRT) uses nonuniform intensity distributions to conform high dose to a tumor and low dose to surrounding sensitive structures. Because of the large number of beams (5–11) and the wide range of intensities, treatment planning is typically an inverse problem in which the intensity distributions are optimized. Three areas addressed in this thesis are plan complexity, beam directions, and dose–volume constraints. Inverse treatment planning is flexible and can deliver complex dose distributions that are sometimes not warranted. The first goal of this thesis is to demonstrate simple alternatives to inverse planning that use just enough degrees of freedom for the problem so that the solution is not overly sensitive to a slight change in dose constraints and patient geometry. With the addition of simple beam direction optimization, a suitable IMRT plan can be created while maintaining clinical practicality. The second goal of the thesis is to introduce and analyze a new algorithm which systematically analyzes and selects beam directions in the fewest number of beams possible. In IMRT, the optimization of beam directions is complicated due to the interdependence with beam intensities. Our beam direction algorithm has the capability of achieving plans that are better than standard IMRT techniques, often with a fewer number of beams. The third goal of this thesis is to propose a new formulation of the inverse treatment planningoptimization problems that include dose–volume constraints which are known to destroy convexity. This is compared to a formulation that has been addressed in the literature. We solve both formulations with a new technique based on direct search optimization with a systematic search region reduction. This is compared to a standard fast simulated annealing technique. The results of using the new formulation show a direct correspondence between the minimum objective function values and the resulting dose distributions and dose–volume histograms. The research of this thesis is performed using examples of lung, prostate, and brain stem radiotherapy. We provide evidence that dose–volume based formulations of inverse treatment planningoptimization for IMRT have the ability to achieve optimal plans that are clinically relevant.
Metrological and treatment planning improvements on external beam radiotherapy. Detector size effect and dose calculation in low-density media (in Spanish)31(2004); http://dx.doi.org/10.1118/1.1667332View Description Hide Description
The objective of this thesis is the improvement of the measurement and calculation accuracy for radiation therapy fields. Basically, it deals with two questions: the detector size effect and the heterogeneity dose calculation. The author analyzes both the metrological and computational effects and its clinical implications by simulation of the radiotherapytreatments in a treatment planning system. The detector size effect leads up to smoothing of the radiation profile increasing the penumbra (20%–80%) and beam fringe (50%–90%) values with the consequent clinical effect of over-irradiation of the organs at risk close to the planning target volume (PTV). In this thesis this problem is analyzed finding mathematical solutions based on profile deconvolution or the use of radiation detectors of adequate size. On the other side, the author analyzes the dose computation on heterogeneous media by the superposition algorithms versus classical algorithms. The derived conclusion from this thesis is that in locations like lung and breast, the classical algorithms lead to a significant underdosage of the PTV with an important decrease of tumor control probability (TCP). On this basis, the author does not recommend the clinical use of these algorithms in the mentioned tumor locations.