PubMed Health⌕ Search

Biomedical subjects

A S Meigooni

Publications and source records attributed to A S Meigooni.

At least 19 recordsLinked to original sources
PubMed2004

Dosimetric characteristics of the new RadioCoil 103Pd wire line source for use in permanent brachytherapy implants.

Recently, a novel linear brachytherapy source in the form of a coiled wire has become available for use in interstitial implants of various treatment sites such as prostate gland. This source type employs a design completely different from that of most "seed" sources currently on the market, one which improves upon or eliminates several common problems with such sources. Dosimetric characteristics of these sources with active lengths 0.5 cm to 5.0 cm were determined for clinical application. For 0.5 cm and 1.0 cm active length sources, the dose rate constant, radial dose function, and two-dimensional (2D) anisotropy function were experimentally and theoretically determined following the updated AAPM Task Group 43 (TG-43U1) recommendations. Radial dose functions and/or "along-away" matrix functions were also obtained for sources with active lengths 2.0 cm to 5.0 cm. Measurements were performed with LiF thermoluminescent dosimeters in Solid Water phantoms. Measured data was compared to Monte Carlo simulated data in Solid Water utilizing the PTRAN code, version 7.43. After finding the data to be in agreement, Monte Carlo calculations were performed in liquid water to obtain clinically applicable dosimetric data as per TG-43U1 recommendations. The results indicated the dose rate constant of the 0.5 cm long RadioCoil 103Pd source in Solid Water to be 0.641 cGy h(-1) U(-1) when measured, and 0.636 cGy h(-1) U(-1) when simulated by Monte Carlo. The calculated dose rate constant in liquid water was found to be 0.650 cGy h(-1) U(-1). These values are comparable to other commercially available sources. Complete dosimetric data and simulation results are described in this paper. Per TG-43U1, clinical treatment planning systems should utilize the values reported for liquid water.

Body Burden↗
PubMed2002

Dosimetric characteristics with spatial fractionation using electron grid therapy.

Recently, promising clinical results have been shown in the delivery of palliative treatments using megavoltage photon grid therapy. However, the use of megavoltage photon grid therapy is limited in the treatment of bulky superficial lesions where critical radiosensitive anatomical structures are present beyond tumor volumes. As a result, spatially fractionated electron grid therapy was investigated in this project. Dose distributions of 1.4-cm-thick cerrobend grid blocks were experimentally determined for electron beams ranging from 6 to 20 MeV. These blocks were designed and fabricated at out institution to fit into a 20 x 20-cm(2) electron cone of a commercially available linear accelerator. Beam profiles and percentage depth dose (PDD) curves were measured in Solid Water phantom material using radiographic film, LiF TLD, and ionometric techniques. Open-field PDD curves were compared with those of single holes grid with diameters of 1.5, 2.0, 2.5, 3.0, and 3.5 cm to find the optimum diameter. A 2.5-cm hole diameter was found to be the optimal size for all electron energies between 6 and 20 MeV. The results indicate peak-to-valley ratios decrease with depth and the largest ratio is found at Dmax. Also, the TLD measurements show that the dose under the blocked regions of the grid ranged from 9.7% to 39% of the dose beneath the grid holes, depending on the measurement location and beam energy.

Dose Fractionation, Radiation↗
PubMed2002

Experimental determination of the TG-43 dosimetric characteristics of EchoSeed model 6733 I25I brachytherapy source.

Recently an improved design of a 125I brachytherapy source has been introduced for interstitial seed implants, particularly for prostate seed implants. This design improves the in situ ultrasound visualization of the source compared to the conventional seed. In this project, the TG-43 recommended dosimetric characteristics of the new brachytherapy source have been experimentally determined in Solid Water phantom material. The measured dosimetric characteristics of the new source have been compared with data reported in the literature for other source designs. The measured dose rate constant, A, in Solid Water was multiplied by 1.05 to extract the dose rate constant in water. The dose rate constant of the new source in water was found to be 0.99 +/- 8% cGy h(-1) U(-1). The radial dose function was measured at distances between 0.5 and 10 cm using LiF TLDs in Solid Water phantom. The anisotropy function, F(r, theta), was measured at distances of 2, 3, 5, and 7 cm.

Anisotropy↗
PubMed2001

Dosimetric characteristics of the bests double-wall 103Pd brachytherapy source.

103Pd and 125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, a double-wall 103Pd source has been introduced, which has a design different from that of sources presently on the market. Dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 recommendations and were related to the October 10, 2000 revision of the NIST 1999 SK Standard for 103Pd. Measurements were performed in a Solid Water phantom using LiF thermoluminescent dosimeters. For these measurements, slabs of Solid Water phantom material were machined to accommodate the source and LiF TLD chips of dimensions (3.1 x 3.1 x 0.8 mm3) and (1.0 x 1.0 x 1.0 mm3). The TLD chips were surrounded by at least 10 cm of Solid Water phantom material to provide full scattering conditions. The Monte Carlo simulations were performed in Solid Water and liquid water using the PTRAN code. The results of this investigation show an excellent agreement (within 5%) between the measured (0.67+/-8% cGy h(-1) U(-1)) and calculated (to be 0.65+/-3% cGy h(-1) U(-1)) dose rate constant in Solid Water. The Monte Carlo calculated dose rate constant of the Best 103Pd in water was found to be 0.67+/-0.02 cGy h(-1) U(-1). The radial dose function, g(r), of the new 103Pd source was measured at distances ranging from 0.5 and 7 cm using LiF TLD in Solid Water phantom material. Moreover, the radial dose function of the new source was calculated in liquid water and Solid Water at distances ranging from 0.1 to 7 cm using the PTRAN Monte Carlo Code. The anisotropy function, F(r, theta), of the new 103Pd source was also measured in Solid Water and calculated in both Solid Water and water phantom material. From the anisotropy functions, the anisotropy factors, and anisotropy constant were calculated for each medium. The results indicated that the measured anisotropy constant of the Best 103Pd source in Solid Water was 0.89+/-5%. Complete dosimetric data are described in this manuscript.

Anisotropy↗
PubMed2000

Experimental determination of dosimetric characteristics of Best 125I brachytherapy source.

125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, the Best 125I source became commercially available for interstitial brachytherapy treatment. Dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally determined, following the AAPM Task Group 43 recommendations, and were related to the NIST 1999 calibration assigned to this source. Measurements were performed in Solid Water phantom using LiF thermoluminescent dosimeters. The results indicated a dose rate constant, lambda, of 1.01 +/- 0.08 cGy h(-1) U(-1) for the new source. The radial dose function, g(r), of the new source was measured at distances ranging from 0.5 to 10.0 cm. The anisotropy function, F(r, theta), of the new source was measured at distances of 2, 5, and 7 cm from the source center. These data compare favorably with those from the Nycomed/Amersham Models 6711 and 6702 sources. The anisotropy constant, phi(an), of the Best 125I source was found to be 0.982. Complete dosimetric parameters of the new source are presented in this paper.

Anisotropy↗
PubMed2000

Dosimetric characteristics of the Pharma Seed model BT-125-I source.

125I brachytherapy sources are being used with increasing frequency for interstitial implants in tumor sites, especially the prostate. Recently, a new 125I source design has become commercially available for clinical applications. Dosimetric characteristics (i.e., dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 (TG-43) recommendations and were related to the 1999 NIST calibration assigned to this source [S(k), 99std]. Measurements were performed in a Solid Water phantom using LiF thermoluminescent dosimeters. The measured data were used to validate the Monte Carlo simulations that were performed in Solid Water using the PTRAN code. The Monte Carlo calculations were then performed in liquid water to obtain the dosimetric information for clinical applications in accordance with TG-43 recommendations. The results indicated that the dose rate constant, lambda, of the Pharma Seed model BT-125-I 125I source was 0.90 +/- 0.06 cGy h(-1) U(-1) using thermoluminescent dosimeter (TLD) measurements and 0.92 +/- 0.03 cGy h(-1) U(-1) using Monte Carlo simulations in Solid Water. The calculated value in liquid water was found to be 0.95 +/- 0.03 cGy h(-1) U(-1). The radial dose function, g(r), of the new 125I source was measured at distances ranging from 0.5 to 10 cm using LiF TLD in Solid Water phantom material. The Monte Carlo simulations were performed for distances ranging from 0.1 to 10 cm from the source center in Solid Water and liquid water. The anisotropy function, F(r, theta), was measured at distances of 2, 5, and 7 cm from the source center and calculated at distances of 0.5, 1, 2, 3, 5, and 7 cm from the source center. The anisotropy constant, phi(an), of the Pharma Seed source in water was found to be 0.975. Complete dosimetric data are described in this manuscript. Per TG-43, the values reported in water should be used for clinical treatment planning systems.

Anisotropy↗
PubMed2000

Dosimetric characteristics of a new 125I brachytherapy source.

125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, a new 125I source has been introduced, which has a design different from that of other sources presently on the market. Dosimetric characteristics of this source, including dose rate constant, radial dose function, and anisotropy function, were determined experimentally following the AAPM Task Group 43 recommendations. The characteristics were related to the 1999 NIST calibration assigned to this source [SK,99std]. Measurements were performed in a solid water phantom using LiF thermoluminescent dosimeters. For these measurements, slabs of solid water phantom material were machined to accommodate the source and LiF TLD chips of dimensions (3.1 x 3.1 x 0.8 mm3) and (1.0 x 1.0 x 1.0 mm3). The TLD chips were surrounded by at least 10 cm of solid water phantom material to provide full scattering conditions. The results indicated a dose rate constant, lambda, of 0.88 +/- 0.07cGyh(-1)U(-1) for the new 1251 source as compared to 0.98 and 1.04 cGy h(-1)U(-1) for the Nycomed/Amersham model 6711 and 6702 seeds, respectively. Per TG-43, the values reported here represent the dose absorbed by water at 1 cm from the source in a water medium. The radial dose function, g(r), of the new 125I source was measured at distances ranging from 0.5 to 10 cm. The anisotropy function, F(r,theta), of the new 125I source was measured at distances of 2 and 5 cm from the source center. Calculations of anisotropy and radial dose function were also made using a Monte Carlo code. These calculations were made for both solid water and liquid water, the former to validate the Monte Carlo code and the latter to provide results in liquid water for clinical use. All data compared favorably with those from the Nycomed/Amersham models 6711 and 6702 sources.

Anisotropy↗
PubMed2000

Dosimetric characteristics of the InterSource103 palladium brachytherapy source.

103Pd brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, the InterSource103 palladium source has been introduced, which has a design different from that of other sources presently on the market. Dosimetric characteristics (i.e., dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 (TG-43) recommendations and were related to the 1999 NIST calibration assigned to this source [Sk, 99std]. Measurements were performed in a solid water phantom using LiF thermoluminescent dosimeters. The measured data was compared with Monte Carlo simulations performed in solid water using the PTRAN code. The calculations were then performed in liquid water to obtain the dosimetric information for clinical applications as per TG-43 recommendation. The results indicated that the dose rate constant, lambda, of the InterSource103 palladium source was 0.664+/-5% cGy/h/U using TLD measurements and 0.660+/-3% cGy/h/U using Monte Carlo simulations in solid water. The calculated value in liquid water was found to be 0.696 +/- 3 % cGy/h/U. The radial dose function, g(r), of the new 103Pd source was measured at distances ranging from 0.5 to 10 cm using LiF TLD in solid water phantom material. The Monte Carlo simulations were performed at distances ranging from 0.1 to 10 cm from the source center in solid water and liquid water. The anisotropy function, F(r, theta), was measured at distances of 2, 3, 5, and 7 cm from the source center and calculated at distances of 0.5, 1, 2, 3, 5, and 7 cm from the source center. Complete dosimetric data are described in this paper. Per TG-43, the values reported in water should be used for clinical treatment planning systems.

Anisotropy↗
PubMed1998

Review of AAPM Task Group No. 43 recommendations on interstitial brachytherapy sources dosimetry. American Association of Physicists in Medicine.

In 1995, the American Association of Physicists in Medicine (AAPM) Task Group No. 43 (TG-43) published its recommendations on the dosimetry of interstitial brachytherapy sources. The report recommended the use of a new dose calculation formalism based on measured quantities. The formalism in modular form permits the computation of doses in two dimensions for 103Pd, 125I, and 192Ir sources. The TG-43 dose calculation formalism introduced new and updated quantities such as air kerma strength, dose rate constant, radial dose function, anisotropy function and anisotropy factor. The dose rate obtained using the TG-43 dose calculation formalism and updated source dosimetry data can be expected to be different from some of the currently used systems by as much as 17%. For the same treatment and implementing the TG-43 dosimetry with point source approximation, the widely prescribed dose of 160 Gy for 125I permanent implants using model 6711 sources changes to 144 Gy. In addition to the dose calculation formalism, TG-43 report also stated that the air kerma strength provided by NIST is estimated to be approximately 7-10% higher than it should be, due to low energy photon contamination for 125I. This difference has not been accounted for in the TG-43 report.

Brachytherapy↗
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

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↗
PubMed1997

Dosimetric characteristics of a new high-intensity 192Ir source for remote afterloading.

A new high-intensity 192Ir source has recently become commercially available for remote afterloading brachytherapy treatment. The dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally determined through the application of AAPM Task Group 43 recommendations. Complete dosimetric data are presented in this manuscript.

Brachytherapy↗
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

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↗
PubMed1996

Dosimetric characteristics of an improved radiochromic film.

Recently, a new model of radiochromic film has been developed for medical applications to provide a higher sensitivity and better uniformity of response than existing models (i.e., MD-55). Dosimetric characteristics including sensitivity, linearity, reproducibility, uniformity, and dependence on energy and time have been studied experimentally. The characteristics of the new films were compared with those of model MD-55. For these investigations, the two films were exposed to ionizing radiation in the dose range from 1-72 Gy, using gamma-rays from a 60Co teletherapy unit and 6- and 18-MV x rays from a linear accelerator. The response of the exposed film was measured with a helium-neon laser densitometer. The results indicated that the sensitivity of the improved film was about 40% greater than that of MD-55 film. Moreover, the response of the improved film was found to be uniform within 4% only in one direction of the film. The orthogonal direction indicated a nonuniformity of up to 15%, similar to that of model MD-55. Less than 5% energy dependence in the megavoltage photon range was observed for the new film. Complete dosimetric characteristics of the new film are presented.

Film Dosimetry↗
PubMed1995

Dosimetry and clinical implementation of dynamic wedge.

PURPOSE: Wedge-shaped isodoses are desired in a number of clinical situations. Physical wedge filters have provided nominal angled isodoses with dosimetric consequences of beam hardening, increased peripheral dosing, nonidealized gradients at deep depths, along with the practical consequences of filter handling and placement problems. Dynamic wedging uses a combination of a moving jaw and changing dose rate to achieve angled isodoses. The clinical implementation of dynamic wedge and an accompanying quality assurance program are discussed in detail. METHODS AND MATERIALS: The accelerator at our facility has two photon energies (6 MV and 18 MV), currently with dynamic wedge angles of 15 degrees, 30 degrees, 45 degrees, and 60 degrees. The segmented treatment tables (STT) that drive the jaw in concert with a changing dose rate are unique for field sizes ranging from 4.0 cm to 20.0 cm in 0.5 cm steps, resulting in 256 STTs. Transmission wedge factors were measured for each STT with an ion chamber. Isodose profiles were accumulated with film after dose conversion. For treatment-planning purposes, dmax orthogonal dose profiles were measured for open and dynamic fields. Physical filters were assigned empirically via the ratio of open and wedge profiles. RESULTS: A nonlinear relationship with wedge factor and field size was found. The factors were found to be independent of the stationary field setting or second order blocking. Dynamic wedging provided more consistent gradients across the field compared with physical filters. Percent depth doses were found to be closer to open field. The created physical filters provided planned isodoses that closely resembled measured isodoses. Comparative isodose plans show improvement with dynamic wedging. CONCLUSIONS: Dynamic wedging has practical and dosimetric advantages over physical filters. Table collisions with physical filters are alleviated. Treatment planning has been solved with an empirical solution. Dynamic wedge is a positive replacement for physical filters, and a first step for commercial introduction of dynamic conformal therapy.

Humans↗