Biomedical subjects

J F Williamson

Publications and source records attributed to J F Williamson.

98 records · Page 6Linked to original sources

PubMedDate not supplied

Theoretical evaluation of dose distributions in water about models 6711 and 6702 125I seeds.

The distribution of absorbed dose about models 6711 and 6702 125I seeds in water has been calculated from first principles using the Monte Carlo method. Dose is calculated as a function of angle with respect to the transverse seed bisector for distances from the seed center ranging from 0.1 to 7.5 cm. The computed results are compared to measured data. A truncated Fourier series is used to describe the Monte Carlo data in terms of a small number of coefficients, facilitating accurate and efficient dose calculations for clinical treatment planning.

Brachytherapy↗

PubMedDate not supplied

Monte Carlo evaluation of specific dose constants in water for 125I seeds.

To clinically utilize relative three-dimensional dose distribution data for 125I seeds in water, the specific dose constant must be known. This constant is the ratio of absolute dose rate in medium, 1 cm from the source on its transverse bisector, to source strength. We have used the Monte Carlo method to calculate specific dose constants in water for three types of 125I seeds (models 6711, 6701, and 6702). Our photon transport code realistically models the complex internal structure of these interstitial sources as well as the exposure standardization experiments conducted at the National Bureau of Standards which form the basis of 125I source strength specification. In addition, relative attenuation factors for the three seed types are presented. Our results indicate that published data overestimate the specific dose constant by 10% to 14%.

Air↗

PubMedDate not supplied

A convolution algorithm for brachytherapy dose computations in heterogeneous geometries.

Currently-available brachytherapy dose computation algorithms ignore heterogeneities such as tissue-air interfaces, shielded gynecological colpostats, and tissue-composition variations in 125I implants despite dose computation errors as large as 40%. To calculate dose in the presence of tissue and applicator heterogeneities, a computer code has been developed that describes scatter dose as a 3-D spatial integral which convolves primary photon fluence with a dose-spread array. The dose-spread array describes the distribution of dose due to multiple scattering about a single primary interaction site and is precomputed by the Monte Carlo method. To correct for heterogeneities traversed by the primary photons, the dose-spread array is renormalized to reflect the density and composition of the element, and the distance to the point of interest is scaled by the path-length of the intervening medium. Convolution calculations for 125I and 137Cs point sources in the presence of finite phantoms, air voids and high-density shields have been compared to the corresponding Monte Carlo calculations. The convolution code absolute and relative dose rate predictions are shown to agree with Monte Carlo calculations within 3%. Direct evaluation of the 3-D spatial convolution integral using 1-D adaptive integration reveals efficiency gains of 20-50 relative to Monte Carlo photon-transport calculations.

Algorithms↗

PubMedDate not supplied

Comparison of measured and calculated dose rates in water near I-125 and Ir-192 seeds.

Recent theoretical and experimental work indicates that currently accepted 125I dosimetry data may overestimate dose in water at 1 cm by 10%-24%. Among the most comprehensive measurements are those of the NCI-sponsored brachytherapy contract participants. Absolute dose rates in water calculated by the Monte Carlo method have been compared with the NCI dose measurements about 125I and 192Ir seeds embedded in solid-water phantoms. The photon transport code allows realistic geometric simulation of the complex internal seed structure, the National Institute of Standards and Technology air-kerma strength standardization geometry, and the dose measurement setup. When the appropriate measurement medium and geometry are assumed, agreement between theory and measurement is excellent, within 3% at 1 cm and averaging 3% at larger distances. However, the data do not support the water equivalence of solid water at 125I energies indicating that solid-water measurements underestimate 125I specific dose-rate constants in water by 4.3%. Because of its higher ratio of absorption to scatter, 125I dose distributions measured in solid water are less penetrating (by 35% at 10 cm) than those measured in liquid water. For model 6711, model 6702, and steel-clad 192Ir seeds, Monte Carlo calculations yielded specific dose-rate constants (assuming liquid water medium) of 0.877, 0.932, and 1.122 cGy cm2 h-1 per unit air-kerma strength, respectively. For 125I, currently accepted values are 18% and 11% larger for the two seed models.

Humans↗

PubMedDate not supplied

Clinical implementation of AAPM Task Group 32 recommendations on brachytherapy source strength specification.

Historically the strength of sealed brachytherapy sources has been described by many physical quantities, including true activity, apparent activity, and equivalent mass of radium. Recently, the AAPM Task Group 32 recommended that these quantities be replaced by a single quantity, air-kerma strength, with units of muGy m2h-1. A set of equations has been developed for unambiguously converting source strength estimates and renormalizing published dose-rate tables, which assume traditional quantities and units, into forms consistent with air-kerma strength. For commonly used brachytherapy sources, multiplicative conversion factors for each source-strength formalism and set of units are given. To convert equivalent mass of radium to air-kerma strength requires a single multiplicative factor, 7.23 muGy m2h-1/mgRaEq, applicable to all sources. Based upon a review of vendor source specification practices, the factors for converting source strength of 198Au, 103Pd, and 125I seeds from apparent mCi to air-kerma strength are 2.06, 1.29, and 1.27 muGy m2h-1/mCi(apparent), respectively. These factors are independent of source geometry but depend on the nominal exposure rate constant value selected by the vendor. Conversion factors applicable to mass of radium or true activity depend upon both source geometry and radionuclide identity. Because many of these conversion factors depend upon vendor choices of physical constants and exposure rate constants, readers are cautioned to carefully review vendor source strength specification practices before adopting these values clinically. Finally, the relationships between the various source strength quantities and absorbed dose rate in the medium surrounding the source are elucidated.

Brachytherapy↗

PubMedDate not supplied

Volume-based geometric modeling for radiation transport calculations.

Accurate theoretical characterization of radiation fields is a valuable tool in the design of complex systems, such as linac heads and intracavitary applicators, and for generation of basic dose calculation data that is inaccessible to experimental measurement. Both Monte Carlo and deterministic solutions to such problems require a system for accurately modeling complex 3-D geometries that supports ray tracing, point and segment classification, and 2-D graphical representation. Previous combinatorial approaches to solid modeling, which involve describing complex structures as set-theoretic combinations of simple objects, are limited in their ease of use and place unrealistic constraints on the geometric relations between objects such as excluding common boundaries. A new approach to volume-based solid modeling has been developed which is based upon topologically consistent definitions of boundary, interior, and exterior of a region. From these definitions, FORTRAN union, intersection, and difference routines have been developed that allow involuted and deeply nested structures to be described as set-theoretic combinations of ellipsoids, elliptic cylinders, prisms, cones, and planes that accommodate shared boundaries. Line segments between adjacent intersections on a trajectory are assigned to the appropriate region by a novel sorting algorithm that generalizes upon Siddon's approach. Two 2-D graphic display tools are developed to help the debugging of a given geometric model. In this paper, the mathematical basis of our system is described, it is contrasted to other approaches, and examples are discussed.

Algorithms↗

PubMedDate not supplied

Comparison of calculated and measured heterogeneity correction factors for 125I, 137Cs, and 192Ir brachytherapy sources near localized heterogeneities.

The influence of tissue and applicator heterogeneities on brachytherapy dose distributions is not well understood, despite widespread use of shielded applicators in intracavitary therapy. Heterogeneity correction factors (HCF) have been measured using a silicon diode detector arising from bounded heterogeneities consisting of lead, steel, titanium, silver, aluminum, and air cylinders near brachytherapy sources of 125I, 137Cs, and 192Ir. In addition, transverse-axis dose distributions for the three sources in homogeneous water were measured for distances of 0.2 to 16.0 cm. For each point of measurement, relative diode readings were simulated by a Monte Carlo photon transport code utilizing accurate models of the source internal structure, the experimental measure geometry and the source-strength calibration geometry. Comparison of measured and calculated HCF's reveals excellent agreement (1%-3% average) over a wide range of materials, diameters, and thicknesses. In addition, Monte Carlo simulation not only accurately reproduced the relative transverse-axis dose distributions in homogeneous medium, but was able to predict the variation of diode response with photon energy with an accuracy of 3% over the range of 30-662 keV. Our measurements demonstrate that HCF's vary by as much as 60%-100% with distance and heterogeneity diameter for a fixed thickness. Finally, silicon diode measurements of HCF (denied as reading with heterogeneity/reading in homogeneous medium) is shown to lead to errors of 5%-30% for 137Cs and 192Ir sources in the presence of high-atomic number shielding materials. This paper concludes, that Monte Carlo simulation is a powerful, convenient and accurate tool for investigating the long-neglected area of brachytherapy heterogeneity corrections.

Biometry↗

PubMedDate not supplied

One-dimensional scatter-subtraction method for brachytherapy dose calculation near bounded heterogeneities.

Neglect of tissue and applicator heterogeneities in brachytherapy treatment planning is due in part to lack of accurate, general, and fast three-dimensional (3D) dose-computation algorithms. A novel dose-calculation algorithm that accounts for the lateral dimensions and location of the heterogeneity as well as its thickness has been developed. For simple 2D, water-equivalent density heterogeneities, the algorithm is shown to be applicable to a wide range of photon energies and is 500-1000 times more efficient than Monte Carlo photon-transport calculations. The model is based upon reducing the bounded 2D heterogeneity problem to two 1D problems by dividing the scattering volume into two regions: A cone-shaped region that subtends the heterogeneity with its apex at the source and the complementary cone that contributes scatter dose by diffusion around the heterogeneity. The input data consist of precalculated scatter-to-primary ratios (SPRs) for collimated isotropic point sources. The central-axis "mini-beam" problem for a slab heterogeneity is solved by a simple 1D scatter integration model that accounts for both the thickness of the heterogeneity and its location relative to the point of interest. The scatter contribution arising outside the mini-beam is modeled as the difference in SPRs corrected for transmission through the heterogeneity. The algorithm agrees, on average, with sample Monte Carlo calculations within 1% to 7% for 125I, 192Ir, and 100 keV point sources along the axes of water-equivalent cylindrical heterogeneities (rho = 0-12.6 g/cm3, 3.6, and 24 mm diameters, and dose-perturbation factors of 0.44-1.33 relative to the homogeneous case). The potential of generalizing the scatter-subtraction approach to encompass 3D heterogeneities, those consisting of high-atomic number media, and those of irregular shape, is discussed.

Algorithms↗