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electron pencil-beam redefinition algorithm dose calculations in the presence of heterogeneities

MEDICAL PHYSICS 2001年第10期28卷 2096-2104页

作者： Boyd, RA Hogstrom, KR Starkschall, G Univ Texas MD Anderson Canc Ctr Dept Radiat Phys Houston TX 77030 USA

The electron pencil-beam redefinition algorithm (PBRA) is currently being refined and evaluated for clinical use. The purpose of this work was to evaluate the accuracy of PBRA-calculated dose in the presence of heterogeneities and to benchmark PBRA dose accuracy for future improvements to the algorithm. The PBRA was evaluated using a measured electron beam dose algorithm verification data set developed at The University of Texas M. D. Anderson Cancer Center. The data set consists of measurements made using 9 and 20 MeV beams in a water phantom with air gaps, internal air and bone heterogeneities, and irregular surfaces. Refinements to the PBRA have enhanced the speed of the dose calculations by a factor of approximately 7 compared to speeds previously reported in published data;a 20 MeV, 15X15 cm(2) field electron-beam dose distribution took approximately 10 minutes to calculate. The PBRA showed better than 4% accuracy in most experiments. However, experiments involving the low-energy (9 MeV) electron beam and irregular surfaces showed dose differences as great as 22%, in albeit a small fractional region. The geometries used in this study, particularly those in the irregular surface experiments, were extreme in the sense that they are not seen clinically. A more appropriate clinical evaluation in the future will involve comparisons to Monte Carlo generated patient dose distributions using actual computed tomography scan data. The present data also serve as a benchmark against which future enhancements to the PBRA can be evaluated. (C) 2001 American Association of Physicists in Medicine.

关键词： electron pencil beam electron beam dose algorithm dose algorithm

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A measured data set for evaluating electron-beam dose algorithms

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MEDICAL PHYSICS 2001年第6期28卷 950-958页

作者： Boyd, RA Hogstrom, KR Antolak, JA Shiu, AS Univ Texas MD Anderson Canc Ctr Dept Radiat Phys Houston TX 77030 USA

The purpose of this work was to develop an electron-beam dose algorithm verification data set of high precision and accuracy. Phantom geometries and treatment-beam configuration!, used in this study were similar to those in a subset of the verification data set produced by the electron Collaborative Working Group (ECWG). Measurement techniques and quality-control measures were utilized in developing the data set to minimize systematic errors inherent in the ECWG data set. All measurements were made in water with p-type diode detectors and using a Wellhofer dosimetry system. The 9 and 20 MeV, 15 x 15 cm(2) beams from a single linear accelerator composed the treatment beams. Measurements were made in water at 100 and 110 cm source-to-surface distances. Irregular surface measurements included a "stepped surface" and a "nose-shaped surface." Internal heterogeneity measurements were made for bone and air cavities in differing orientations. Confidence in the accuracy of the measured data set was reinforced by a comparison with Monte Carlo (MC)-calculated dose distributions. The MC-calculated dose distributions were generated using the OMEGA/beam code to explicitly model the accelerator and phantom geometrics of the measured data set. The precision of the measured data, estimated from multiple measurements, was better than 0.5% in regions of low-dose gradients. In general, the agreement between the measured data and the MC-calculated data was within 2%. The quality of the data set was superior to that of the ECWG data set, and should allow for a more accurate evaluation of an electron beam dose algorithm. The data set will be made publicly available from the Department of Radiation Physics at The University of Texas M. D. Anderson Cancer Center. (C) 2001 American Association of Physicists in Medicine.

关键词： electron beam dose algorithm verification data Monte Carlo

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