Biomedical subjects

J F Williamson

Publications and source records attributed to J F Williamson.

17 recordsLinked to original sources

PubMed1992

Source configuration and dose rates for the Selectron afterloading equipment for gynecologic applicators.

This paper describes source train configurations of the Selectron LDR-6 that can be used to duplicate commonly used manually afterloadable source configurations for gynecological intracavitary brachytherapy. We demonstrate that the recommended source configuration using 2.5 mm 137Cs spheres is dosimetrically equivalent to its manually afterloaded counterparts using 3M 137Cs sources (20 mm overall length, 14 mm active length). Isodose distributions are in agreement to within 5% for the Fletcher-Suit-Delclos family of tandems, colpostats, and vaginal cylinders. Our data also demonstrate that the isotropic point source model used by most computerized treatment planning systems is a good approximation to source trains of filtered, spherical sources interspersed with solid steel spacers. We conclude that the use of 2.5 mm 137Cs sources in the Selectron remote afterloading system can accurately reproduce the isodose distribution achieved with linear 137Cs manually afterloaded sources.

Brachytherapy↗

PubMed1992

Dosimetry and dose specification for a new gynecological brachytherapy applicator.

A new afterloadable gynecological intracavitary applicator has been designed and is now in use for the treatment of a wide range of vaginal, cervical, and endometrial cancers in a single application, with a dose distribution that more closely matches the prescribed treatment than previous methods. The plexiglass applicator consists of a bulbous section that is inserted up to the vaginal apex and loaded with right and left ovoid sources. The distal portion is cylindrical with a central channel for tandem sources. A straight tandem is used when the target volume extends to the vaginal apex, but an intrauterine tandem can also be used. Extensive dosimetric and radiographic evaluations were performed to guide design refinements and to validate the surface dose predictions of the brachytherapy treatment planning system. The final applicator design delivers 110-120 cGy/hr to the vaginal apex surface and 95-100 cGy/hr to the distal vaginal surfaces when loaded with a 144.6 U (20 mgRaEq) cesium tube in each ovoid channel and 72.3, 72.3, and 144.6 U (10, 10, and 20 mgRaEq) cesium tubes in the vaginal cylinder channel. A system for treatment dose specification has been established that includes dose tables for manually calculating surface doses to reference points. A dose distribution comparison with a sequential colpostat-vaginal cylinder treatment demonstrates that dose delivery is more precise and uniform with this new applicator.

Brachytherapy↗

PubMed1992

Rapid two-dimensional dose measurement in brachytherapy using plastic scintillator sheet: linearity, signal-to-noise ratio, and energy response characteristics.

Because of the large dose gradients encountered near brachytherapy sources, an efficient, accurate, low-atomic number areal detector, which can record dose at many points simultaneously, is highly desirable. We have developed a prototype of such a system using thin plates of plastic scintillator as detectors. A micro-channel plate (MCP) image intensifier was used to amplify the optical scintillation images produced by radioactive 125I and 137Cs sources in water placed 0.5-5.7 cm distance from the detector. A charge-coupled device (CCD) digital camera was used to acquire 2-D light-intensity distributions from the image intensifier output window. For both isotopes, a small area (2 x 3 mm2) PVT detector yields a CCD net count rate that is linear with respect to absorbed dose rate within +/- 3% out to 5.7 cm distance. Acquisition times range from 1.5-400 sec with a reproducibility of 0.5-5.5%. If a large-area (6 x 20 cm2) PVT detector is used, a four-fold increase in count rate and large deviations from linearity are observed, indicating that neighboring pixels contribute light to the signal through diffusion and scattering in PVT and water. A detailed noise analysis demonstrates that the image intensifier reduces acquisition time 10000-fold, reduces noise relative to signal 200-fold, and reduces amplifier gain noise as well.

Brachytherapy↗

PubMed1990

Dose calculations about shielded gynecological colpostats.

Although shielded gynecological colpostats have been shown experimentally to reduce doses to bladder and rectal tissue by as much as 50%, nearly all previously described dose computation algorithms ignore applicator heterogeneities. We describe the use of realistic Monte Carlo calculations to study the dosimetric effects of applicator structure. Use of sophisticated solid modeling techniques allows the complex internal structure of two commercially-available Fletcher-Suit colpostats, as well as that of 226Ra or 137Cs tubes, to be accurately simulated. Our results show significant differences among these source-applicator combinations. In addition, a novel dose computation algorithm for efficiently estimating absorbed dose near shielded applicators is described. Our approach is based upon empirical separation of primary- and scatter-dose components. The algorithm requires a small base of Monte Carlo-generated data, reproduces the Monte Carlo dose estimates within 3%, and is faster than Monte Carlo by a factor of 15,000. The scatter-separation method has the potential to make accurate dose estimates to bladder, rectum, tumor, and vagina available for clinical treatment planning and for extraction of more meaningful dose-response curves from clinical data.

Brachytherapy↗

PubMed1988

Monte Carlo and analytic calculation of absorbed dose near 137Cs intracavitary sources.

This paper presents dose-rate tables and treatment planning data needed to accurately implement the Sievert line-source integral, found on most commercial computer-aided treatment planning systems, for two recently introduced 137Cs intracavitary sources. One source uses a high-density active core designed to reproduce the non-elliptical isodose curves characteristic of the traditional radium tube. The other source consists of two or three discrete 137Cs seeds encapsulated in stainless steel. Using Monte Carlo dose-rate calculations as the standard of accuracy, we show that the Sievert model, using conventionally defined filtration corrections, overestimates kerma-rate in free space by as much as 20%. In addition, tissue attenuation and scatter build-up factors, derived from an isotropic point source, do not accurately characterize the distribution of scatter dose about heavily filtered sources. By varying the input parameters of the Sievert line-source integral so as to optimize its agreement with the more rigorous Monte Carlo data, accuracy of 3% can be achieved.

Brachytherapy↗

PubMed1987

Random selection of points distributed on curved surfaces.

Monte Carlo simulation is widely used in medical physics to obtain realistic solutions to radiation transport problems involving complex geometries. An important step is the random selection of the initial point in space from which the primary photon or charged particle originates. In many practical cases, the primary-particle sites are distributed over curved surfaces. We describe general sampling techniques that can be used to randomly choose the trajectory origin for sources distributed over any smooth surface that can be parametrically represented as a function of two variables. The source distribution need not be uniform. Implementation of these methods is particularly straight-forward for surfaces enclosing simple convex volumes such as spheres and ellipsoids.

Humans↗

PubMed1987

Ionizing radiation enhances malignant progression of mouse skin tumors.

Chemical carcinogenesis in mouse skin has been divided into the process of initiation, promotion and progression. Recently we have shown that ionizing radiation acts as an initiator in this model system. In this paper we describe a three-stage experiment using ionizing radiation in the third stage of mouse skin carcinogenesis. CD-1 mice were initiated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) followed by biweekly promotion with 12-O-tetradecanoylphorbol-13-acetate (TPA). After 20 weeks of promotion, the animals were treated with either acetone, TPA (twice a week for 2 weeks) or eight fractions of 1 MeV electrons (1 Gy/fraction over a period of 10 days). The conversion of papillomas to squamous cell carcinomas was 80% for animals treated with ionizing radiation compared with 25% for tumor-bearing animals treated with TPA. Ionizing radiation increased the number of cumulative carcinomas per group. The lack of an increase in the number of cumulative papillomas per group due to ionizing radiation suggests that the dose and fractionation protocol used in this study enhanced the progression of pre-existing papillomas.

Acetone↗

PubMed1987

Template-guided interstitial implants: Cs-137 reusable sources as a substitute for Ir-192.

Template-guided implantation of rigid steel or plastic guide needles for afterloading of radioactive sources is widely used in the treatment of gynecologic, rectal, and urologic malignant neoplasms. Iridium-192 is used almost universally, despite the high cost per implant, due to its short half-life and limited need for a flexible, trimmable source. A reusable afterloading system containing cesium-137 was developed. Each source has an effective active length of 6.8 cm and is encapsulated at the distal end of a 21-cm-long stainless steel tube. The sources can be afterloaded into the same plastic guide needles normally used for Ir-192 ribbons. Physical and dosimetric aspects of these sources are compared with those of Ir-192, and radiation protection and cost effectiveness are also discussed.

Brachytherapy↗

PubMed1986

The accuracy of the line and point source approximations in Ir-192 dosimetry.

The dosimetric approximations used in computer-aided treatment planning of Ir-192 seed implants generally ignore individual seed dimensions and internal structure. Most commonly, each seed is approximated by an isotropic point source. Alternately, each ribbon assembly consisting of uniformly spaced seeds is replaced by an unfiltered line source. Using filtration corrections applicable to platinum- and steel-encapsulated seeds calculated by the Monte Carlo method, the dosimetric errors introduced by these models into two-and-three-dimensional dose distributions arising from multiple plane implants are analyzed. Our results demonstrate that when anisotropy correction factors of 0.96 and 0.99 are used for platinum- and steel-filtered seeds, respectively, the point source model is accurate within 2%. The accuracy of the line source approximation depends significantly upon the details of its implementation. If the linear density of the line is set equal to individual seed strength for an inter-seed spacing of 1 cm, and filtration correction factors of 0.94 and 0.97 are used for platinum- and steel-clad seeds, respectively, the accuracy of the line source model is 1.5% near the implant center. The method of dose-volume histograms is used to compare the predictions of the different models.

Brachytherapy↗

PubMedDate not supplied

Radiation transport calculations in treatment planning.

This article reviews the role of radiation transport calculations in radiotherapy dose computation. The physical and mathematical principles underlying transport theory, as applied to electrons and photons, are discussed. Practical methods of solving the transport equation, with emphasis on Monte Carlo techniques, are reviewed. The contribution of analytic and Monte Carlo transport calculations to electron beam, photon beam, and brachytherapy treatment planning dosimetry is assessed. Currently, the most important roles of transport theory include using approximate solutions of the transport equation as theoretical foundations of dose computation algorithms, and using Monte Carlo simulation to calculate basic treatment planning data, characteristic of the treatment modality, which can serve as input to a dose computation algorithm.

Computer Simulation↗

PubMedDate not supplied

Monte Carlo evaluation of kerma at a point for photon transport problems.

Estimation of collision kerma at a geometric point arising from scattered photons is a potentially important application of Monte Carlo simulation, especially in the presence of steep flux gradients. We examine the usual method of extracting point-kerma estimates from randomly generated photon trajectories which consists of tallying the energy lost by photon collisions occurring in the vicinity of the point of interest. Several other methods derived from the equivalence of track length per unit volume and flux are evaluated as to accuracy and efficiency. Finally, a next-flight estimator is discussed in which the expected contribution of each simulated photon collision to kerma at the point of interest is calculated regardless of proximity of the collision to the point. All of these techniques are shown to involve a trade-off between statistical precision and spatial resolution: increasing the number of contributing collisions requires averaging kerma over a larger volume. Based upon both analytic models and realistic Monte Carlo simulations, use of next-flight and track-length estimators is shown to improve simulation efficiencies by factors of 2 to 20 compared to analog scoring. Practical guidelines as to choice of estimator and successful implementation are presented.

Models, Theoretical↗

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↗