摘要:

A biologically based model is developed to predict radiation‐induced normal tissue complication probability (NTCP) in inhomogeneously irradiated organs such as the lung or the kidney. The organ is assumed to be composed of independent functional subunits (FSUs) organized with a parallel architecture and it is assumed that the complication is produced only if a sufficiently large number of FSUs (the “functional reserve”) are destroyed. A general expression for NTCP is derived, as well as simple expressions for the mean and standard deviation of the radiation damage to the FSUs. It is demonstrated that these results for inhomogeneous irradiation reproduce those of the serial and tumor control models when the functional reserve consists of one or all of the FSUs, respectively. When the number of FSUs is large, the dose response for organs with identical characteristics is very steep. Since clinical dose‐response curves may arise from populations with varying functional reserves and radiosensitivities, we derive expressions for the NTCP for inhomogeneously irradiated organs that incorporate such variations.

ISSN:0269-3127    

DOI:10.1118/1.597056

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The equivalent tissue‐air‐ratio (ETAR) method employs three‐dimensional (3D) CT pixel information to approximate scatter dose contribution for inhomogeneity correction. In general, the calculation provides better agreement with measurements than the one‐dimensional (1D) methods typically used in commercial treatment planning systems. In its original implementation, the 3D formulation of the ETAR method is modified empirically as a 2D calculation in order to reduce computation time. The modification compromises the use of the method in several treatment geometries. An examination of the ETAR formulation shows that the calculation can be expressed as a convolution and thus can be performed in 3D using fast Fourier transform (FFT) techniques. The algorithm has been implemented as a 3D FFT convolution. Making use of the symmetric properties of the FFT, the new approach shows significant savings in computation time without excessive memory requirement. Despite its fundamental limitations when applied to regions of electronic disequilibrium, the ETAR method offers a practical solution to improving current dose calculation in 3D treatment planning, particularly when the more advanced scatter ray‐tracing dose calculation algorithms remain impractical for clinical use. Recent work to extend the method to approximate electron transport is also encouraging.

ISSN:0269-3127    

DOI:10.1118/1.597010

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Energy deposition around the trajectories of ionizing particles with linear energy transfer (LET) of 4, 40, and 400 keV/μm in water and subsequent diffusion of deposited heat is calculated using computational fluid dynamics. Immediately after the deposition of energy by the charged particle, the temperature and pressure in the vicinity of the particle track both increase dramatically, leading to the formation of a thermal spike and a pressure wave. Initially, the region of heat deposition is primarily localized to a region called the “thermal core,” which has dimensions of 0.3, 1, and 3 nm for particles with LETs of 4, 40, and 400 keV/μm, respectively. Instantaneous peak temperatures within the thermal core were 800 °C–2000 °C and peak pressures were about 25 000 atm. This sudden deposition of heat in a localized region leads to a very strong shock wave around the particle trajectory, which is shown to last for a duration of 10−9–10−8s. Even at distances beyond 10 nm away from the particle trajectory, pressures above 100 atm could exist for a duration of up to 10−11s. This local and transient environment, created by the passage of a charged particle in a medium, may lead to new mechanisms of radiation action leading to cell damage, as well as to the development of new radiation detectors.

ISSN:0269-3127    

DOI:10.1118/1.597011

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 with simulated annealing.

ISSN:0269-3127    

DOI:10.1118/1.597012

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597013

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597014

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The contrast and zero spatial frequency signal‐to‐noise ratio produced by a method for radiation therapy portal imaging known as low‐energy imaging with high‐energy bremsstrahlung beams have been mathematically analyzed. The analysis makes extensive use of Monte Carlo techniques and incorporates the detector, the spectrum, phantom, and geometry. The analysis is validated through comparison with measured data including subject contrast measurements and the attenuation of the beam with lead. Scatter reduction is found to be potentially the most effective method to improve contrast and SNR for a film based system. A large fraction of the scatter detected is of a much higher energy than that found in diagnostic radiology. Hence, traditional antiscatter grids, such as those used in diagnostic radiology, are ineffective. The analysis and theory from the literature are applied to design a new grid which is more appropriate for this application. The grid produces a modest improvement according to a contrast‐detail study.

ISSN:0269-3127    

DOI:10.1118/1.597015

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new method is presented for inspection of patient setup in radiation therapy by automatic comparison of the patient position relative to the beam position in portal and simulator images. Quantification of patient‐setup errors in terms of translation, rotation, and magnification is achieved by chamfer matching, a robust technique to match drawings and images, which is applied to both anatomy outlines and field edges. Applied to field edges, chamfer matching detects and visualizes deviations in field shape. Applied to anatomy outlines, the matching procedure quantifies and visualizes deviations in patient position relative to the radiation field. To test the method and to judge its feasibility, its behavior for four hundred different patient‐setup deviations, which were simulated in four clinical images, was examined. These images show a top view of the pelvic region. The performance was measured in terms of accuracy and success rate for numerous cost functions and distance codings associated with the chamfer matching procedure. An average accuracy of 1.8 mm was found, a success rate of 90%, and an average overall computation time of 3 s on a 486 microcomputer. The whole analysis procedure is fast enough to allow on‐line application.

ISSN:0269-3127    

DOI:10.1118/1.597016

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An uncertainty analysis of a field placement error measurement method using correlations between digital portal and simulation images has been performed. This analysis used the Monte Carlo method to simulate random uncertainties arising in image registration and portal field edge detection. The uncertainty analysis used data from a field placement error study to predict the dependences of the measurement method upon image matchpoint number and spatial distribution, image registration, and uncertainties arising in the detection and description of the field edge in a portal image. It was shown that the uncertainty in the field registration had the most significant effect upon the measurement uncertainty, whereas the number of matchpoints had an extremely limited effect (beyond the minimum required for the mathematical procedure used in registration). In contrast, it was shown that an accurate field placement error measurement required that the number of pixels describing a field edge used for comparisons should exceed 20, thus automatically setting a minimum field length necessary for field placement comparisons. It was also shown that the spatial uniformity of the matchpoints in an image pair had a limited effect upon the field placement error measurement, provided that the points are distributed over about 15% of the image area.

ISSN:0269-3127    

DOI:10.1118/1.597143

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An automated technique has been developed for the verification of treatment field size and shape in external beam radiation therapy. Portal images from film or digital on‐line imaging system are analyzed, and basic parameters are derived to describe the field size and shape from the contour points on the field boundary. The initial set of parameters included length of the perimeter, area, aspect ratio, and orientation angle. The parameters found for the actual field in the portal image are compared against those calculated for the prescribed field and any discrepancies indicated to the operator. The accuracy of the field parametrization scheme has been tested on a number of on‐line portal images with varying fields. The relative error did not exceed a few percent in perimeter and area or 2° in the angle, which should be sufficiently low for the detection of major errors in field shaping.

ISSN:0269-3127    

DOI:10.1118/1.597017

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY