The biological basis and clinical application of three-dimensional conformal external beam radiation therapy in carcinoma of the prostate.
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Carcinoma↗
Biomedical subjects
G S Mageras
Publications and source records attributed to G S Mageras.
Advances in precision treatment: some aspects of 3D conformal radiation therapy.
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Humans↗
Compensators for three-dimensional treatment planning.
Presented here is a method of designing compensators for a single beam or one or more pairs of beams, not necessarily parallel opposed. The objective is to produce a flat distribution in a plane that may be perpendicular to the central ray or may be an arbitrarily oriented plane, for example, a plane that bisects the hinge angle between two beams. The method takes into account not only surface irregularities but also tissue inhomogeneities, hinge angles between beams, distance from the source, and even "horns" in the beam. The design process employs convolution of Monte Carlo generated pencil beams with photon fluence distributions, appropriately modified for the presence of beam modifiers (blocks and compensators), to compute dose in a flat homogeneous phantom. Corrections for inhomogeneities and surface curvature are applied by using computerized tomography information to determine the effective path length through tissue. Multiple interactions are used to arrive at a compensator that properly incorporates changes in radiation transport, and therefore dose distribution, resulting from the presence of beam-shaping devices. In each iteration it is assumed that the required reduction in dose at a point can be achieved by reducing the fluence along the ray joining the source to computation point proportionately. The compensator design is represented as a finely spaced matrix of thickness values which is entered into a prorammable milling maching for fabrication. Dose measurements in phantom exposed to 6-MV x rays with and without compensation are presented.
Equipment Design↗
Clinically relevant optimization of 3-D conformal treatments.
In this paper a method of computer-aided optimization of 3-D conformal treatment plans is presented which incorporates models to predict the clinical consequences of resulting dose distributions. Even though these models are simplistic, it is submitted that their intelligent use leads to treatment plans which indicate lower normal tissue complications and higher tumor control. Dose distribution data, biological models, and observed normal tissue and tumor response data are used to compute tumor control and normal tissue complication probabilities for each of the critical normal structures encountered in a treatment plan. These quantities are combined into a single score using an objective function which incorporates the importance of each end point as assessed by the physician. Using the "simulated annealing" method of optimization, the beam weights are adjusted to maximize the score. Additional constraints are applied to ensure consistency of the results of optimization with the judgment of the physician. These optimization methods have been applied to conformal treatment plans consisting of multiple fixed fields with conformal field shaping. The results indicate that the methods presented have considerable potential.
Aged↗
A model for computer-controlled delivery of 3-D conformal treatments.
Three-dimensional conformal radiation treatments are highly complex and may comprise a large number of coplanar and noncoplanar beams, virtually all of which are shaped and may also employ arbitrary intensity modulation. The delivery of such treatments with conventional means is highly labor intensive and limited to a relatively small number of fields. A generalized model for a system that can deliver 3-D conformal treatments rapidly and safely using a computer-controlled treatment machine equipped with a multileaf collimator is presented. The model emphasizes the separation of tasks between an external computer programmed by the user institution and a control computer proprietary to the treatment machine manufacturer. The treatment scheme that is employed consists of a sequence of fixed fields, called segments, which are delivered in succession without human intervention. Settings for all segments, derived from a 3-D conformal treatment planning system, are downloaded by the external computer into the control computer of the treatment machine. During patient setup, the control computer enables the operator to step through computer-controlled setup of the segments without radiation to ensure the absence of collisions, and to allow the adjustment of setup parameters if necessary. The external computer verifies the treatment setup and permits the control computer to carry out the treatment segment by segment. Safety aspects of the model and anomalous situations which may arise are discussed.
Computer Simulation↗
Automatic generation of beam apertures.
In order to specify arbitrarily shaped beam apertures for three-dimensional radiation treatment planning, aperture contours (or outlines) are often manually drawn using a beam's eye view display of the target volume and nearby normal structures. This can be a very time consuming process, and can be impractical for multileaf collimation and computer-aided optimization of a large number of fields. A method has been developed that allows automatic generation of aperture shapes that outline the target volume and may spare neighboring structures whenever desired. Margins of user-specified sizes (positive or negative) around the target and normal structures are also incorporated. For a chosen beam orientation, a 3D surface of each anatomic structure of interest is formed and projected onto a plane at the beam's isocenter. The outlines of each projected object are detected by an edge following algorithm, and margins are added. The outlines of normal structures are combined with that of the target volume to obtain the final aperture shape. This is done by overlaying filled versions of the outlines in such a way that regions of the target overlapped by normal structures are cut away, leaving only the target volume region to be irradiated. The remaining target volume outline is again detected to produce an aperture contour. Normal structures may split the aperture into several pieces, so this method detects any number of disjoint aperture contours. The results of the algorithm are illustrated with apertures generated for nasopharynx and prostate tumors, including sparing of normal tissues.
Algorithms↗
Application of fast simulated annealing to optimization of conformal radiation treatments.
Applications of simulated annealing to the optimization of radiation treatment plans, in which a set of beam weights are iteratively adjusted so as to minimize a cost function, have been motivated by its potential for finding the global or near-global minimum among multiple minima. However, the method has been found to be slow, requiring several tens of thousands of iterations to optimize 50 to 100 variables. A technique to improve the efficiency for finding a solution is reported, which is generally applicable to the optimization of continuous variables. In previous applications of simulated annealing to treatment planning optimization, only one or two weights are varied each iteration. This approach is to change all weights simultaneously, using random changes that are initially large to coarsely sample the cost function, then are reduced with iteration to probe finer structure. The performance of different methods are compared in optimizing a plan for treatment of the prostate, in which the search space consists of 54 noncoplanar beams and the cost function is based on tumor control and normal tissue complication probabilities. The proposed method yields solutions with similar values of the cost function in only a fraction of the iterations compared either to a fixed single weight adjustment technique, or to a method which combines the Nelder and Mead downhill simplex simulated annealing.
Humans↗