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A S Meigooni

Publications and source records attributed to A S Meigooni.

At least 37 records · Page 2Linked to original sources
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

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

Instrumentation and dosimeter-size artifacts in quantitative thermoluminescence dosimetry of low-dose fields.

Thermoluminescence dosimetry is extensively used for quantitative dose measurements in various irradiation fields such as dosimetry of brachytherapy sources. In this application, small doses on the order of 0.5 cGy must be accurately measured, which requires careful control of instrumentation, energy-dependence, and nonlinearity of detector response. Several investigators have observed the presence of some undesirable signals when the thermoluminescent dosimeters (TLDs) were read without any nitrogen gas flow in the TLD reader. Others have indicated that the "prereadout" annealing technique is the same as the "preirradiation" technique for doses above 10 cGy, but they have not extended their study to lower doses. The goal of this study is to investigate dependence of sensitivity and linearity of the TLD response to the flow of nitrogen gas in the TLD reader at low dose level, annealing technique, and TLD size. The effect of nitrogen flow sensitivity and linearity of two different sizes of lithium fluoride TLD-100 chips has been studied. Our data indicate a large standard deviation of TLD sensitivity, up to a factor of 2, when TLDs were read without nitrogen gas flow in the TLD reader. In addition, a large deviation from linearity was observed for doses below 5 cGy. When the reading-chamber was purged with nitrogen gas, dispersion of the responses of the TLDs that were exposed to the same dose fell to within 5%. At precision levels of 2% and 5%, the low dose limits are 1 cGy and 0.5 cGy, respectively, for large chips and 15 cGy and 1 cGy for small chips, if TLDs are read with nitrogen gas flow in the TLD reader.(ABSTRACT TRUNCATED AT 250 WORDS)

Artifacts↗
PubMed1994

High dose-rate induced temperature artifacts: thermometry considerations for simultaneous interstitial thermoradiotherapy.

PURPOSE: The goal of the present study was to investigate the effect of high dose-rate radiation on a flouroptic thermometry system commonly used during microwave hyperthermia. METHODS AND MATERIALS: Measurements were performed by placing the flouroptic thermometry sensors at distances of < or = 1.5, 5, 10, and 15 mm from a remote afterloading high dose-rate 192Ir source in a water bath (at two different temperatures) and in a tissue equivalent radiation bolus medium. A simulated volumetric clinical setup using a radiation bolus medium was performed with thermometry sensors placed at 1.5, 7.5, 8.4, and 10.6 mm from a scanning high dose-rate source. RESULTS: It was found that high dose-rate radiation caused thermometry artifacts greater than 1.5 degrees C within 2 min for flouroptic thermometers placed 1.5 mm from a 5 Ci activity high dose-rate source. Simple calculations showed that artifacts of this magnitude could not be due to any heating caused by the energy deposited by the high dose-rate source. The artifact decayed, but was still evident 24 h after the exposure. The effect strongly depended on distance with a 0.7 degrees C artifactual increase in temperature seen for the probe 5 mm from the high dose-rate source. Moreover, experiments performed under conditions that represented a clinical setup with a 7 Ci high dose-rate source showed that for exposure times of 10 s, at distances of 1.5 mm, significant artifacts (> 0.5 degrees C) are produced. CONCLUSIONS: These findings indicate that high dose-rate-induced temperature artifacts should be taken into account in the quality assurance procedures for the treatment of patients with simultaneous interstitial thermoradiotherapy.

Artifacts↗
PubMed1994

Dosimetric characteristics, air-kerma strength calibration and verification of Monte Carlo simulation for a new Ytterbium-169 brachytherapy source.

PURPOSE: Ytterbium-169 (169Yb) is a promising new isotope for brachytherapy with a half life of 32 days and an average photon energy of 93 KeV. It has an Ir-192-equivalent dose distribution in water but a much smaller half-value layer in lead (0.2 mm), affording improved radiation protection and customized shielding of dose-limiting anatomic structures. The goals of this study are to: (a) experimentally validate Monte Carlo photon transport dose-rate calculations for this energy range, (b) to develop a secondary air-kerma strength standard for 169Yb, and (c) to present essential treatment planning data including the transverse-axis dose-rate distribution and dose correction factors for a number of local shielding materials. METHODS AND MATERIALS: Several interstitial 169Yb sources (type 6) and an experimental high dose-rate source were made available for this study. Monte-Carlo photon-transport (MCPT) simulations, based upon validated geometric models of source structure, were used to calculate dose rates in water. To verify MCPT predictions, the transverse-axis dose distribution in homogeneous water medium was measured using a silicon-diode detector. For use in designing shielded applicators, heterogeneity correction factors (HCF) arising from small cylindrical heterogeneities of lead, aluminum, titanium, steel and air were measured in a water medium. Finally, to provide a sound experimental basis for comparing experimental and theoretical dose-rate distributions, the air-kerma strength of the sources was measured using a calibrated ion chamber. To eliminate the influence of measurement artifacts on the comparison of theory and measurement, simulated detector readings were compared directly to measured diode readings. The final data are presented in the format endorsed by the Interstitial Collaborative Working Group. RESULTS: The in-air calibration revealed that the air-kerma strength per unit activity (mCi), as quoted by the vendor, varied from 1.30 to 1.57 cGy.cm2/mCi.h depending on seed design. The maximum difference between measured and MCPT-simulated absolute diode readings on the transverse axis was less than 2%, indicating that MCPT accurately predicts dose rate in medium for brachytherapy sources in this energy range. Comparison of measured and simulated HCFs for each of the 16 different cylindrical heterogeneities demonstrated 1-3% agreement. The HCFs vary by as much as 200% with respect to distance and by as much as 48% as a function of disk diameter, showing that HCF is strongly dependent on heterogeneity location and lateral dimensions as well as thickness. The dose-rate constant for water medium was found to be 1.225 cGy/h per kerma unit air-strength and 1.962 cGy/h per unit mCi as measured by the vendor. CONCLUSION: Monte Carlo simulation is an accurate and powerful tool for dosimetric characterization of brachytherapy sources in this energy range. Thin lead foils produce shielding factors comparable to standard shielded applicators for 137Cs. Meaningful theoretical absolute dose calculations in brachytherapy require accurately implemented air-kerma strength standards.

Brachytherapy↗
PubMed1994

A comparative study of dosimetric properties of Plastic Water and Solid Water in brachytherapy applications.

In this study the dosimetric properties of Plastic Water and Solid Water phantom materials are evaluated using Monte Carlo photon transport simulations. In particular, their water-equivalence with respect to absorption and attenuation of photons in the brachytherapy energy range are examined. For the given chemical compositions of the materials, the linear attenuation coefficients were calculated for photons of 1 keV-2 MeV. Moreover, absorbed doses to water in each phantom material were calculated at distances of 0.5-12.0 cm from point sources of 20 keV to 60Co gamma rays. These results show that at low photon energies (below 100 keV), there are substantial differences (up to a factor of 5) between the absorbed dose in Plastic Water and that in liquid water. The differences decrease as photon energy increases, and they become insignificant at 60Co gamma rays, as claimed by the manufacturer. In contrast, calculations show that the difference in absorbed dose in Solid Water from that in liquid water, over the entire range of photon energies employed in this study, is less than 25%. The results of this study demonstrate the necessity of careful dosimetric evaluation of a new phantom material, before its clinical application, particularly in energy ranges outside those referred to by the manufacturer.

Biophysical Phenomena↗
PubMed1993

Pre-installation empirical testing of room shielding for high dose rate remote afterloaders.

PURPOSE: Many facilities are acquiring high dose rate remote afterloading units. It is economical that these units be placed in existing shielded teletherapy rooms. Scatter-radiation barriers marginally protect uncontrolled areas from a high dose rate source especially in a room that houses a non-dynamic Cobalt-60 unit. In addition the exact thickness and material composition of the barriers are unknown and therefore, a calculation technique may give misleading results. Also, it would be impossible to evaluate an entire wall barrier by taking isolated core samples in order to assist in the calculations. A quick and inexpensive measurement of dose equivalent using a rented high activity 192Ir source evaluates the barriers and locates shielding deficiencies. METHODS AND MATERIALS: We performed transmission calculations for primary and scattered radiation based on National Council on Radiation Protection and Measurements Reports 49 and 51, respectively. We then rented a high activity 21.7 Ci (8.03 x 10(11) Bq) Ir-192 source to assess our existing teletherapy room shielding for adequacy and voids. This source was placed at the proposed location for clinical high dose rate treatment and measurements were performed. RESULTS: No deficiencies were found in controlled areas surrounding the room, but large differences were found between the calculated and measured values. Our survey located a region in the uncontrolled area above the room requiring augmented shielding which was not predicted by the calculations. A canopy shield was designed to potentially augment the shielding in the ceiling direction. CONCLUSION: Pre-installation testing by measurement is an invaluable method for locating shielding deficiencies and avoiding unnecessary enhancement of shielding particularly when there is lack of information of the inherent shielding.

Brachytherapy↗
PubMed1992

A comparison of radial dose functions for 103Pd, 125I, 145Sm, 241Am, 169Yb, 192Ir, and 137Cs brachytherapy sources.

Recently, encapsulated sources of 103Pd (21 keV average), 145Sm (41 keV average), 241Am (60 keV), and 169Yb (93 keV average) have been introduced as alternatives to conventional brachytherapy sources of 125I Model 6711 (27 keV average), 125I Model 6702 (28 keV average), 192Ir (369 keV average), and 137Cs (662 keV). To illustrate the dependence of the penetrating ability of photons from brachytherapy sources as a function of photon energy, a comparison of their radial dose functions is presented. Using the ITS Monte Carlo simulation code for photon-electron transport, the radial dose functions were calculated for monoenergetic photon sources with energies in the range of 30 keV to 1 MeV. Also, similar calculations were performed using the photon spectra emitted by the encapsulated brachytherapy sources. To verify the accuracy of Monte Carlo calculations, comparisons are made with our new measured data for 241Am and existing experimental and theoretical data from other investigators. A comparison of radial dose functions indicates that for 241Am, 169Yb, 192Ir and 137Cs sources radial dose functions are close to unity for distances up to 10 cm, for 145Sm the radial dose function drops to about 0.4 at 10 cm, and for 125I and 103Pd it drops precipitously to less than 0.20 at 7 cm. At 5 cm, the measured radial dose functions for 103Pd, 125I Model 6711, 125I Model 6702, 145Sm, 241Am, and 192Ir have values of 0.09, 0.34, 0.38, 0.86, 1.12, and 0.97, respectively. While all of these radioisotopes provide adequate penetrating power for interstitial brachytherapy, only the radioisotopes emitting photons with energies greater than about 40 keV can provide adequate depth dose (that is, small or no tissue attenuation) for intracavitary irradiation. Our criterion for choice of minimum photon energy suitable for intracavitary irradiation is that the radial dose function at 5 cm should not be less than 0.90. Also, note that photons with energies around 80 keV exhibit maximum penetrating ability in solid water for distances up to 5 cm.

Americium↗
PubMed1992

Some treatment planning considerations for 103Pd and 125I permanent interstitial implants.

Sealed sources of palladium-103 (103Pd), which decay with a half life of 17 days and emit on average 21 keV photons, are now in clinical use for permanent implants. For seed implantation of prostatic cancer, 103Pd implants are usually planned to deliver 115 Gy to full decay at an initial dose rate of 19.7 cGy/hr whereas 125I implants are usually planned to deliver 160 Gy at an initial dose rate of 7.72 cGy/hr. Because of the lower energy of photons emitted by 103Pd compared to the 125I sources (27 keV average energy), the tissue attenuation is more severe for 103Pd sources. The radial dose function drops more steeply with distance from the 103Pd sources compared to the 125I sources, raising a concern about the possibility of cold spots in the tumors implanted with 103Pd sources. To investigate this issue, a detailed analysis of the dependence of dose uniformity as a function of seed spacing for 125I and 103Pd sources in various cubic and spherical configurations was carried out. Using the measured single source dosimetry data as input, dose distributions for a variety of cubic and spherical implants were generated on a computerized treatment planning system. This study indicates that relative dose distributions for 125I and 103Pd implants with the same geometric configuration and number of seeds are very similar inside the implanted volume for implants. Dose uniformity within a target volume implanted with 103Pd seeds is also very similar to that for 125I. To expedite clinical implementation of 103Pd, an atlas of dose distributions for 103Pd implants has been produced for various seed configurations, seed spacings, and target volumes. Using 125I implants as a guideline, clinical procedures for planning of 103Pd implants have been developed. It was found that the total source strength implanted divided by the dimension of the implant can be expressed as an exponential function of implant size, resulting in a simple method for estimating the strength of seeds necessary in an implant. Also, the air kerma strength of 103Pd seeds is about 3.3 times that of 125I sources in an implant with the same geometric configuration and number of seeds, provided treatment doses of 115 Gy and 160 Gy are chosen for 103Pd and 125I implants, respectively.

Brachytherapy↗
PubMed1991

A simple technique for irradiation of recto-anal cancers with electrons using an internal anal shield.

A method of treating the primary site of recto-anal cancers using an en face electron beam in combination with an internal lead shield in the anus has been developed. Dose measurements using lithium fluoride thermoluminescent dosimetry indicate that the internal anal shield reduces the dose to the uninvolved anal wall by up to a factor of two while leaving the dose to the primary tumor site unaffected. Because the internal anal shield is placed in the anus during treatment, this system leads to a more precise daily positioning of the shield compared to the setup using an external shield alone. This technique has been used to treat two patients with anal cancer who tolerated the treatment well with no acute side effects. Both patients are now disease free, more than 30 months after their radiation treatment, without any treatment-related sequelae.

Aged↗
PubMed1988

On the choice of phantom material for the dosimetry of 192Ir sources.

Dosimetric characteristics of polystyrene, solid water, and polymethylmethacrylate were examined and compared to water to determine the suitability of these solid materials for the dosimetry of 192Ir. Ionization charge measured in each of the four media as a function of depth and depth-dose curves calculated by Monte Carlo simulation show that the three solids are equivalent to each other and to water under full scattering conditions. Photon energy spectra generated from the Monte Carlo simulation show little variation for the different media. Mass energy absorption coefficients and exposure-to-dose conversion factors were calculated as a function of depth for these spectra. Measured tissue attenuation factors are in excellent agreement with Meisberger's "selected" values. The radial dose function, which describes the change in dose with distance in phantom exclusive of the inverse square law, was calculated from the tissue attenuation factor and found to be in significant disagreement with Dale's Monte Carlo values. The reason for this discrepancy is discussed.

Brachytherapy↗
PubMed1988

Influence of the variation of energy spectra with depth in the dosimetry of 192Ir using LiF TLD.

Absolute dose measurements around low activity seeds used in brachytherapy are difficult to perform with ionisation chambers. The physical size of the chamber does not allow good resolution close to the seeds and the ionisation current generated is very low. Small thermoluminescent dosimeters (TLDs) overcome these problems but the energy dependence of their response should be considered. In this work, dose in polystyrene was measured at several distances from the high activity 192Ir source (370 GBq) of a remote afterloading device using an ionisation chamber and LiF TLD chips. These data show that over a range of 1-10 cm from the source the sensitivity of LiF varies by up to 8.5%. This is attributed to the higher response of LiF to the lower photon energies, and to the shift of the photon spectrum to lower energies with increasing depth. The sensitivity of LiF to 192Ir was also calculated by weighting the energy-dependent response of LiF by the Monte Carlo calculated photon spectra. The calculations give a similar change in sensitivity with distance from the source.

Brachytherapy↗
PubMed1984

Scattering cross-sections and partial kerma factors for neutron interactions with carbon at 20 less than En less than 65 MeV.

New measurements of differential elastic and inelastic neutron scattering from 12C at En = 20.8, 22, 24 and 26 MeV are presented and analysed in terms of both spherical and deformed optical model potentials. Results of these analyses are compared with other recent differential and total cross-section measurements in order to obtain a consistent representation of the energy dependence of neutron scattering over a broad energy range. Heavy ion recoil kerma factors are calculated and compared with earlier measurements and estimates of these quantities.

Carbon↗
PubMedDate not supplied

Dose perturbations at interfaces in photon beams.

A model based on an approximation called the partial fluence approximation is presented for the calculation of dose distributions in the vicinity of medium interfaces in photon beams. The predictions of the model are compared with dose distributions measured in layered phantoms consisting of aluminum and polystyrene, for photon beams ranging in energy from 60Co to 24 MV.

Humans↗