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J F Williamson

Publications and source records attributed to J F Williamson.

At least 55 records · Page 3Linked to original sources
PubMed1998

Monte Carlo-aided dosimetry of a new high dose-rate brachytherapy source.

In this article we introduce a new high-intensity 192Ir source design for use in a recently reengineered microSelectron-HDR remote afterloading device for high dose-rate (HDR) brachytherapy. The maximum rigid length and outer diameter of the new source are reduced to 4.95 and 0.90 mm, respectively, compared to 5.50 and 1.10 mm for the previous source design introduced in 1991. In addition, a smaller diameter and more flexible steel cable are used, allowing the source cable to negotiate smaller diameter catheters or more tortuously curved catheters. Using Monte Carlo photon transport simulation, the complete two-dimensional (2-D) dose-rate distribution is calculated over the 0.1-7 cm distance range and are presented both as conventional 2-D Cartesian lookup tables and in the formalism recommended by the American Association of Physicists in Medicine Task Group 43 (TG-43) Report. The dose distribution of this source is very similar to that of its predecessor, except near the source tip and in the shadow of the cable assembly, where differences of 5%-8% are apparent. The accuracy of various methods for extrapolating beyond the tubulated anisotropy functions to short distances is evaluated. It is demonstrated that linear extrapolation from the anisotropy functions defined by TG-43 accurately (+/- 2%) estimates dose rate at short and long distances lying outside the radial distance range of the original measured data from which the anisotropy and radial dose functions were derived. In contrast, the algorithm used on the vendor's planning system results in large calculation errors at distances less than 5 mm.

Anisotropy↗
PubMed1997

Validation of Monte Carlo dose calculations near 125I sources in the presence of bounded heterogeneities.

PURPOSE: Dose distributions around low energy (< 60 keV) brachytherapy sources, such as 125I, are known to be very sensitive to changes in tissue composition. Available 125I dosimetry data describe the effects of replacing the entire water medium by heterogeneous material. This work extends our knowledge of tissue heterogeneity effects to the domain of bounded tissue heterogeneities, simulating clinical situations. Our goals are three-fold: (a) to experimentally characterize the variation of dose rate as a function of location and dimensions of the heterogeneity, (b) to confirm the accuracy of Monte Carlo dose calculation methods in the presence of bounded tissue heterogeneities, and (c) to use the Monte Carlo method to characterize the dependence of heterogeneity correction factors (HCF) on the irradiation geometry. METHODS AND MATERIALS: Thermoluminescent dosimeters (TLD) were used to measure the deviations from the homogeneous dose distribution of an 125I seed due to cylindrical tissue heterogeneities. A solid water phantom was machined accurately to accommodate the long axis of the heterogeneous cylinder in the transverse plane of a 125I source. Profiles were obtained perpendicular to and along the cylinder axis, in the region downstream of the heterogeneity. Measurements were repeated at the corresponding points in homogeneous solid water. The measured heterogeneity correction factor (HCF) was defined as the ratio of the detector reading in the heterogeneous medium to that in the homogeneous medium at that point. The same ratio was simulated by a Monte Carlo photon transport (MCPT) code, using accurate modeling of the source, phantom, and detector geometry. In addition, Monte Carlo-based parametric studies were performed to identify the dependence of HCF on heterogeneity dimensions and distance from the source. RESULTS: Measured and calculated HCFs reveal excellent agreement (< or = 5% average) over a wide range of materials and geometries. HCFs downstream of 20 mm diameter by 10 mm thick hard bone cylinders vary from 0.12 to 0.30 with respect to distance, while for an inner bone cylinder of the same dimension, it varies from 0.72 to 0.83. For 6 mm diameter by 10 mm thick hard bone and inner bone cylinders, HCF varies 0.27-0.58 and 0.77-0.88, respectively. For lucite, fat, and air, the dependence of HCF on the 3D irradiation geometry was much less pronounced. CONCLUSION: Monte Carlo simulation is a powerful, convenient, and accurate tool for investigating the long neglected area of tissue composition heterogeneity corrections. Simple one dimensional dose calculation models that depend only on the heterogeneity thickness cannot accurately characterize 125I dose distributions in the presence of bone-like heterogeneities.

Iodine Radioisotopes↗
PubMed1997

Two-dimensional scatter integration method for brachytherapy dose calculations in 3D geometry.

In brachytherapy clinical practice, applicator shielding and tissue heterogeneities are usually not explicitly taken into account. None of the existing dose computational methods are able to reconcile accurate dose calculation in complex three-dimensional (3D) geometries with high efficiency and simplicity. We propose a new model that performs two-dimensional integration of the scattered dose component. The model calculates the effective primary dose at the point of interest and estimates the scatter dose as a superposition of the scatter contributions from pyramid-shaped minibeams. The approach generalizes a previous scatter subtraction model designed to calculate the dose for axial points in simple cylindrically symmetric geometry by dividing the scattering volume into spatial regions coaxial with the source-to-measurement point direction. To allow for azimuthal variation of the primary dose, these minibeams were divided into equally spaced azimuthally distributed pyramidal volumes. The model uses precalculated scatter-to-primary ratios (SPRs) for collimated isotropic sources. Effective primary dose, which includes the radiation scattered in the source capsule, is used to achieve independence from the source structure. For realistic models of the 192Ir HDR and PDR sources, the algorithm agrees with Monte Carlo within 2.5% and for the 125I type 6702 seed within 6%. The 2D scatter integration (2DSI) model has the potential to estimate the dose behind high-density heterogeneities both accurately and efficiently. The algorithm is much faster than Monte Carlo methods and predicts the dose around sources with different gamma-ray energies and differently shaped capsules with high accuracy.

Biophysical Phenomena↗
PubMed1997

Quantitative evaluation of radiochromic film response for two-dimensional dosimetry.

Radiochromic film (RCF) is attractive as a thin, high resolution, 2D planar dosimeter. We have studied the uniformity, linearity, and reproducibility of a commercially supplied RCF system (model MD-55). Forty 12 cm long strips of RCF were exposed to uniform doses of 6 MV x rays. Optical density (OD) distributions were measured by a helium-neon scanning laser (633 nm) 2D densitometer and also with a manual densitometer. All film strips showed 8%-15% variations in OD values independent of densitometry technique which are evidently due to nonuniform dispersal of the sensor medium. A double exposure technique was developed to solve this problem. The film is first exposed to a uniform beam, which defines a pixel-by-pixel nonuniformity correction matrix. The film is then exposed to the unknown dose distribution, rescanned, and the net OD at each pixel corrected for nonuniformity. The double exposure technique reduces OD/unit dose variation to a 2%-5% random fluctuation. RCF response was found to deviate significantly from linearity at low doses (40% change in net OD/Gy from 1 to 30 Gy); a finding not previously reported. To study the tradeoff between statistical noise and spatial resolution, OD was averaged over blocks of adjacent 50 microns pixels (ranging from 1 x 1 to 10 x 10 pixels). Reproducibility, defined as the standard deviation of repeated single-pixel measurements on separate film pieces, was 2% at 30 Gy for a resolution of 0.25 mm. With careful correction for nonlinearity and nonuniformity, RCF is a promising quantitative 2D dosimeter for radiation oncology applications.

Absorptiometry, Photon↗
PubMed1996

The Sievert integral revisited: evaluation and extension to 125I, 169Yb, and 192Ir brachytherapy sources.

PURPOSE: The goal of this study is to assess the accuracy of the Sievert integral dose calculation model for medium and low-energy brachytherapy sources (photon energy: 25-500 keV). A simple modification of the basic model, the isotropic scattering correction, is proposed that significantly improves its accuracy. METHODS AND MATERIALS: Both the classical model and revised Sievert algorithms were tested against 2D dose distributions derived from Monte Carlo photon transport (MCPT) calculations for the following sources: a 169Yb interstitial source, pulsed and high dose rate 192Ir sources and the model 6702 125I source. The Sievert model was implemented as a 3D numerical integral over the radioactivity distribution and included photon attenuation and scattering by the surrounding medium. The Sievert filtration coefficients were approximated by linear energy absorption coefficients, parameters of best fit, and curve fits to simulated open-air transmission measurements. The revised model consists of using the Sievert integral only to calculate the primary dose distribution using contact absorber filtration coefficients. The dose component due to photon scattering in the medium is assumed to be isotropically distributed and is modeled by point-source scatter-to-primary dose ratios. RESULTS: The classical Sievert integral produces maximum and RMS average dose-calculation errors ranging from -53 to -20% and 3 to 19%, respectively. In contrast, the revised model reproduces the MCPT dose distribution with maximum and RMS mean errors ranging from 5 to 13% and 1 to 6%, respectively. CONCLUSIONS: The classical Sievert model fails to accurately describe brachytherapy dose distributions around heavily filtered sources emitting photons with average energies of 28 to 400 keV. The revised Sievert model accurately models single-source dose distributions for a wide range of sources, using well-defined filtration coefficients and scatter ratios that can be measured or calculated without knowledge of the final dose distribution. The model is potentially useful as a single-source dose-array generator for clinical treatment planning in the low energy domain.

Algorithms↗
PubMed1996

Design and dosimetric characteristics of a high dose rate remotely afterloaded endocavitary applicator system.

PURPOSE: An applicator is described for endocavitary treatment of rectal cancers using a high dose rate (HDR) remote afterloading system with a single high-intensity 192Ir source as an alternative to the 50 kVp x-ray therapy contact unit most frequently used in this application. METHODS AND MATERIALS: The applicator consists of a tungsten-alloy collimator with a 45 degree beveled end, placed in a protoscope with an elliptical cross-section. The resultant 3 cm diameter circular treatment aperture, located in the beveled face of the proctoscope, is irradiated by circular array of dwell positions located about 6.5 mm from the applicator surface. This beveled end allows patients with posterior wall tumors to be treated in the dorsal lithotomy position. The dose-rate distributions about the applicator were determined using a combination of thermoluminescent dosimetry (TLD-100 detectors) and radiochromic film dose measurement techniques along with Monte Carlo dosimetry calculations. TLD-100 (3 x 3 x 0.9 mm3 chips) measurements were used to measure the distribution of dose over the proctoscope surface as well as the central axis dose-rate distribution. Relative radiochromic film measurements were used to measure off-axis ratios (flatness and penumbra width) within the treatment aperture. These data were combined with Monte Carlo simulation results to obtain the final dose distribution. RESULTS: The tungsten collimator successfully limits the dose to the tissue in contact with the proctoscope walls to less than 12% of the prescribed dose. These results indicate that the HDR applicator system has slightly more penetrating depth-dose characteristics than the most widely used contact therapy x-ray machine. Flatness characteristics of the two treatment delivery systems are comparable, although the HDR endocavitary applicator has a significantly wider penumbra. Finally, the HDR applicator has a lower surface dose rate (1.5-4 Gy/min of dwell time) compared to 9-10 Gy/min for the x-ray unit. CONCLUSIONS: An applicator system has been developed for endocavitary treatment of early stage rectal carcinoma that uses a single-stepping source HDR remote afterloading system as a radiation source. The advantages of the HDR-based system over x-ray therapy contact units currently used in this clinical application are (a) enhanced flexibility in applicator design and (b) widespread availability of single-stepping source HDR remote afterloading systems.

Brachytherapy↗
PubMed1996

Accuracy of Monte Carlo photon transport simulation in characterizing brachytherapy dosimeter energy-response artefacts.

Practical dosimeters in brachytherapy, such as thermoluminescent dosimeters (TLD) and diodes, are usually calibrated against low-energy megavoltage beams. To measure absolute dose rate near a brachytherapy source, it is necessary to establish the energy response of the detector relative to that of the calibration energy. The purpose of this paper is to assess the accuracy of Monte Carlo photon transport (MCPT) simulation in modelling the absolute detector response as a function of detector geometry and photon energy. We have exposed two different sizes of TLD-100 (LiF chips) and p-type silicon diode detectors to calibrated 60Co, HDR source (192Ir) and superficial x-ray beams. For the Scanditronix electron-field diode, the relative detector response, defined as the measured detector readings per measured unit of air kerma, varied from 38.46 V cGy-1 (40 kVp beam) to 6.22 V cGy-1 (60Co beam). Similarly for the large and small chips the same quantity varied from 2.08-3.02 nC cGy-1 and 0.171-0.244 nC cGy-1, respectively. Monte Carlo simulation was used to calculate the absorbed dose to the active volume of the detector per unit air kerma. If the Monte Carlo simulation is accurate, then the absolute detector response, which is defined as the measured detector reading per unit dose absorbed by the active detector volume, and is calculated by Monte Carlo simulation, should be a constant. For the diode, the absolute response is 5.86 +/- 0.15 (V cGy-1). For TLDs of size 3 x 3 x 1 mm3 the absolute response is 2.47 +/- 0.07 (nC cGy-1) and for TLDs of 1 x 1 x 1 mm3 it is 0.201 +/- 0.008 (nC cGy-1). From the above results we can conclude that the absolute response function of detectors (TLDs and diodes) is directly proportional to absorbed dose by the active volume of the detector and is independent of beam quality.

Biophysical Phenomena↗
PubMed1996

Measurement and calculation of heterogeneity correction factors for an Ir-192 high dose-rate brachytherapy source behind tungsten alloy and steel shields.

Shields made of high atomic number material are commonly used in vaginal applicators with high dose-rate (HDR) 192Ir remotely afterloaded brachytherapy sources. However little data is available for the dose distribution around such shields. Heterogeneity correction factors (HCFs) are defined as the ratio of the dose to a point with the heterogeneity (shield) in place, divided by the dose to the same point with no heterogeneity. Using thermoluminescent dosimeters (TLDs) in solid water phantom we have measured the HCFs behind 6 and 20 mm diam tungsten alloy disks, 4 and 2 mm thick and a 4 mm thick steel disk, positioned 15 mm from the source. For each measurement point, the heterogeneity correction factors were also inferred from Monte Carlo simulations, which accurately modeled the experimental geometry. The agreement between measured and calculated HCFs on the average was within 6%. Tungsten alloy disks resulted in about two times greater dose reduction in water (HCF approximately 0.4, for 20 x 4 mm disk) than for a steel disk with the same dimensions (HCF approximately 0.85). Reducing the disk diameter to 6 mm increased the dose transmission up to about 25%. Increasing the source-to-detector distance from 4 to 7 cm caused a change in HCF from 2% to more than 20%, depending on disk material and diameter. The detector artifact effects arising from the finite size and different composition of the TLD chips were determined.

Alloys↗
PubMed1995

Clinical evaluation of an interstitial remote afterloading device for multichannel intracavitary irradiation.

PURPOSE: The purpose of this report is to describe the clinical implementation and evaluation of an interstitial remote afterloading device for multichannel intracavitary brachytherapy. METHODS AND MATERIALS: Two 15-channel low dose rate devices were adapted for use with Fletcher-Suit tandem and ovoids and Simon-Heyman capsules. The technical records for 103 intracavitary brachytherapy procedures performed from February 1989 through February 1991 were reviewed. RESULTS: Isodose distributions from fixed source trains for Microselectron low dose rate gynecologic applicators closely approximate standard manual afterloading sources and applicators. Device malfunctions occurred in 51% (53 out of 103) of the procedures. Malfunctions by applicator type were 70% (51 out of 73) for tandem, ovoids, and capsules, 12% (2 out of 17) for tandem and ovoids, and none (0 out of 13) for ovoids only. The most common malfunction occurred during source transfer. Total implant time was prolonged 0 to 4% by malfunction and 10% by patient care interruptions, depending on applicator type. CONCLUSION: The adaptation of the Microselectron device for multichannel gynecologic intracavitary brachytherapy results in similar dose distributions as standard manual after loading sources and a decreased radiation exposure to nursing personnel. The system has a high rate of malfunctions but a low overall prolongation of implant time due to malfunction.

Brachytherapy↗
PubMed1995

TLD, diode and Monte Carlo dosimetry of an 192Ir source for high dose-rate brachytherapy.

Very few dosimetry data are available for the current generation of high-dose-rate (HDR) 192Ir sources, which have broad application in remotely afterloaded brachytherapy. We have measured the two-dimensional dose rate distribution around a microSelectron-HDR source and used the results to validate Monte Carlo simulations. Thermoluminescent dosimeters (TLDs) in solid-water phantoms were used to measure the transverse-axis dose rates in the distance range 0.5-10 cm and the polar dose-rate profiles at 1.5, 3 and 5 cm distance from the source. At close distances, 2-40 mm from the HDR source, we performed transverse axis dose-rate measurements with a Si diode in water. We performed diode measurements at the same distances also for a pulsed dose-rate (PDR) source to compare the results for 192Ir sources with different encapsulation. Both the HDR and the PDR sources were decayed, separated from their cables and calibrated prior to the measurements. The measured dose rates were compared with Monte Carlo photon transport calculations, which realistically modelled the experimental and source geometry at each measurement point. Agreement between Monte Carlo photon transport absolute dose-rate calculations and measurements was, on average, within 5%. From the transverse-axis experimental data, we deduced a value for the dose-rate constant lambda 0 of 192Ir HDR sources of 1.14 cGy h-1 U-1 +/- 5%. This value agrees within the experimental error with the Monte Carlo estimate of 1.115 cGy h-1 U-1 +/- 0.5%. Excellent agreement with previously measured anisotropy functions was observed. Higher anisotropy is observed for the point at 0 degree along the source cable for which no previous data have been reported.

Biophysical Phenomena↗
PubMed1995

A secondary air kerma strength standard for Yb-169 interstitial brachytherapy sources.

Ytterbium-169 (169Yb) is a promising new intermediate low-energy isotope for interstitial implantation. To date, no air kerma strength (SK) standard for this source exists that can serve as a sound foundation for comparing various dose measurements and theoretical calculations. We have solved this problem by adapting the free air measurement technique of Goetsch et al, originally developed for 192Ir. Using a 100 cm3 spherical ion chamber with NIST traceable external beam calibrations in a free air geometry, we have measured the air kerma strength of six different source batches (two type 6 batches, three type 8 batches, and one experimental high-intensity source). Room scatter corrections, derived from an empirical fit to the data (following Goetsch et al) and/or directly by Monte Carlo simulation, yielded identical results with a reproducibility of 1%. The ratio [SK/Avendor] of measured SK to the vendor's contained activity assay averaged 1.554 cGy cm2 mCi-1 h-1 (0.0420 microGy m2 MBq-1 h-1), in conflict with the expected value of 1.34 (0.0362), derived from Monte Carlo calculations. The measured [SK/Avendor] for the type 8 seeds varies by as much as 10% whereas the SK/dose calibrator reading ratio varies by no more than 0.3%, suggesting that the reproducibility of Avendor is relatively poor. These discrepancies may help explain the variation (as large as 28%) in published dose rate constants for 169Yb.

Air↗
PubMed1995

Monte Carlo aided dosimetry of the microselectron pulsed and high dose-rate 192Ir sources.

Despite the large number of single-stepping source pulsed and high dose-rate (HDR) remote after-loading devices in clinical use, the published literature contain little data characterizing dose-rate distributions around the high-intensity (4 x 10(3)-4 x 10(4) microGy m2h-1) 192Ir sources currently used in these devices. We have used the Monte Carlo method to calculate complete two-dimensional dose-rate distributions about the most widely used high dose-rate source design, as well as the Nucletron pulsed dose-rate (PDR) 192Ir source. A Monte Carlo photon transport code, incorporating the detailed internal geometry of the source, was used to calculate the dose rate per unit air-kerma strength in water medium on the transverse bisecting axis over the 0.15-12 cm distance range. In addition, polar dose profiles were calculated at distances ranging from 0.25 to 5 cm. The PDR and HDR dose-rate distributions are tabulated using the formalism endorsed by the Interstitial Collaborative Working Group and the AAPM Task Group 43, and includes dose-rate constant, radial dose function, anisotropy function, geometry function, and anisotropy factors. The dose-rate constants, lambda, of the MicroSelectron/HDR and PDR sources were found to be 1.115 and 1.128 cGy h-1 per unit air-kerma strength, respectively, in good agreement with previously published data for low dose-rate interstitial 192Ir sources. Oblique filtration by the high-density iridium metal core resulted in deviations from anisotropy as large as 35%-55% near the longitudinal axis of the source. Dose-rate distributions are also presented in Cartesian ("away" and "along") coordinates.

Brachytherapy↗
PubMed1995

Experimental validation of Monte Carlo dose calculations about a high-intensity Ir-192 source for pulsed dose-rate brachytherapy.

Despite widespread use of high-intensity Ir-192 remotely afterloaded sources, no published measured or calculated dose-rate tables for currently used source designs are available. For a pulsed dose-rate Ir-192 source, both transverse axis (0.5-10 cm) and two-dimensional polar dose-rate profiles (1.5, 3, and 5 cm) were measured with thermoluminescent dosimetry in a solid water phantom. Dose rates were normalized to measured air-kerma strength, and the source geometry was verified by pinhole autoradiography and transmission radiography. At each measurement point, dose rates were calculated by a Monte Carlo photon transport (MCPT) code, which realistically modeled the experimental phantom, source, and detector geometry. Agreement between MCPT absolute dose-rate calculations and measurements averaged 3% and was less than 5%, demonstrating that Monte Carlo simulation is an accurate and powerful tool for two-dimensional dosimetric characterization of high activity Ir-192 sources.

Brachytherapy↗
PubMed1995

Quantitative verification of 192Ir PDR and HDR source structure by pin-hole autoradiography.

For precise localization of the center and determination of the dimensions of the radioactive material within the capsule of brachytherapy sources, we have developed a method based on simultaneous pin-hole autoradiography of two sources. We constructed a variable magnification pin-hole camera consisting of two telescopically fitted Plexiglas cylinders which can accommodate two radioactive sources on the plate covering the top cylinder. The 192Ir pulsed and high dose-rate sources were studied and an 192Ir seed was used as a reference source. The magnification factor was determined from the dimensions of the 192Ir seed image, which was geometrically well defined by a separate transmission radiography experiment. The observed position for the center of radioactivity in the PDR and the HDR source capsules are in agreement with the vendor specifications. The distance from the tip of the PDR capsule to its center of radioactivity was found in this way to be 0.79 +/- 0.21 mm, which agrees with the position (0.85 mm) of the pellet situated closer to the tip as specified by the vendor. Quantitative verification of the internal source structure using this method enhances the accuracy with which the dose distribution near brachytherapy sources can be predicted by three-dimensional Monte Carlo calculations.

Autoradiography↗
PubMed1995

The evaluation of optimized implants for idealized implant geometries.

The purpose of this paper is to investigate the utility of implant quality measures on single stepping-source brachytherapy treatment plans. Four dwell weight optimization algorithms were applied to four regular geometric implants: single plane, double plane, cuboid, and cylindrical. The dwell weight optimization schemes included equal weighing, two commercial optimization schemes (dose-point and geometric) and a variation of the Paterson-Parker distribution rules. The implant quality measures were investigated as a function of dose-per-integrated reference air kerma (IRAK) to eliminate bias resulting from a prescription choice. A particular dose per IRAK refers to a dose surface that is a function only of the relative dwell weight distribution and is therefore well suited to investigate dwell weight optimization schemes. The implant quality measures included the dose-nonuniformity ratio (DNR) developed by Saw and a coverage index to assess the isodose coverage relative to the implanted volume. These were termed direct quantities due to their clear clinical significance. Additional measures include the ratio of the implant dose-volume histogram (DVH) to that of a point source exhibiting the same IRAK (Rp) and the ratio of the optimized DVH to the equally weighted DVH (EWR). The widths of the Rp curves and depths of the EWR curves were used to characterize these indirect implant quality measures. To evaluate the effectiveness of both the direct and indirect measures, they were correlated with the DNR for an isodose surface that covered the implant (D0). The efficiency of the dwell weight distribution was examined by noting the dose-per-IRAK surface D0.(ABSTRACT TRUNCATED AT 400 WORDS)

Algorithms↗