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C G Soares

Publications and source records attributed to C G Soares.

At least 19 recordsLinked to original sources
PubMed2006

Experimental determination of the dose deposition profile of a 90Sr beta source.

Three different methods for characterising the dose deposition profile of a (90)Sr/(90)Y radioactive source are described: GAFChromic film dosimetry, Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). For the film measurements, GAFChromic film samples were stacked at different depths between polyethylene terephthalate (PET) foils. For TL, the thickness of a TLD-500 dosemeter was gradually reduced by polishing and the TL from chips of different thickness was used in conjunction with a mathematical model based on the exponential attenuation of dose inside the crystal to determine the decay constant for the dose-depth profile. Finally, an OSL reader with confocal stimulation / detection capabilities was used to map the two-dimensional dose distribution in TLD-500 dosemeters as a function of depth. The shapes of the dose deposition profiles obtained from all the investigated methods are in good agreement.

Beta Particles↗
PubMed2006

New developments in radiochromic film dosimetry.

NIST has been a pioneer in the use of radiochromic film for medical dosimetry applications. Beginning in 1988 with experiments with (90)Sr/Y ophthalmic applicators, this work has continued into the present. A review of the latest applications is presented, which include high activity low-energy photon source dosimetry and ultra-high resolution film densitometry for dose enhancement near stents and microbeam radiation therapy dosimetry. An exciting recent development is the availability of a new radiochromic emulsion which has been developed for IMRT dosimetry. This emulsion is an order of magnitude more sensitive than was previously available. Measurements of the sensitivity and uniformity of samples of this new film are reported, using a spectrophotometer and two scanning laser densitometers. A unique feature of the new emulsion is that the peak of the absorbance spectrum falls at the wavelength of the HeNe lasers used in the densitometer, maximising sensitivity. When read at a wavelength of 633 nm, sensitivities on the order of 900 mAU Gy(-1) were determined for this new film type, compared with about 40 mAU Gy(-1) for type HS film, 20 mAU Gy(-1) for type MD-55-2 film, and 3 mAU Gy(-1) for type HD-810. Film uniformities were found to be good, on the order of 6% peak to peak. However, there is a strong polarisation effect in the samples examined, requiring care in film orientation during readout.

Dose-Response Relationship, Radiation↗
PubMed2005

The three-dimensional scintillation dosimetry method: test for a 106Ru eye plaque applicator.

The need for fast, accurate and high resolution dosimetric quality assurance in radiation therapy has been outpacing the development of new and improved 2D and 3D dosimetry techniques. This paper summarizes the efforts to create a novel and potentially very fast, 3D dosimetry method based on the observation of scintillation light from an irradiated liquid scintillator volume serving simultaneously as a phantom material and as a dose detector medium. The method, named three-dimensional scintillation dosimetry (3DSD), uses visible light images of the liquid scintillator volume at multiple angles and applies a tomographic algorithm to a series of these images to reconstruct the scintillation light emission density in each voxel of the volume. It is based on the hypothesis that with careful design and data processing, one can achieve acceptable proportionality between the local light emission density and the locally absorbed dose. The method is applied to a Ru-106 eye plaque immersed in a 16.4 cm3 liquid scintillator volume and the reconstructed 3D dose map is compared along selected profiles and planes with radiochromic film and diode measurements. The comparison indicates that the 3DSD method agrees, within 25% for most points or within approximately 2 mm distance to agreement, with the relative radiochromic film and diode dose distributions in a small (approximately 4.5 mm high and approximately 12 mm diameter) volume in the unobstructed, high gradient dose region outside the edge of the plaque. For a comparison, the reproducibility of the radiochromic film results for our measurements ranges from 10 to 15% within this volume. At present, the 3DSD method is not accurate close to the edge of the plaque, and further than approximately 10 mm (<10% central axis depth dose) from the plaque surface. Improvement strategies, considered important to provide a more accurate quick check of the dose profiles in 3D for brachytherapy applicators, are discussed.

Brachytherapy↗
PubMed2004

Optically stimulated luminescence and thermoluminescence efficiencies for high-energy heavy charged particle irradiation in Al2O3:C.

The thermally and optically stimulated luminescence (TL and OSL) response to high energy heavy-charged particles (HCPs) was investigated for two types of Al2O3:C luminescence dosimeters. The OSL signal was measured in both continuous-wave (CW) and pulsed mode. The efficiencies of the HCPs at producing TL or OSL, relative to gamma radiation, were obtained using four different HCPs beams (150 MeV/u 4He, 400 MeV/u 12C, 490 MeV/u 28Si, and 500 MeV/u 56Fe). The efficiencies were determined as a function of the HCP linear energy transfer (LET). It was observed that the efficiency depends on the type of detector, measurement technique, and the choice of signal. Additionally, it is shown that the shape of the CW-OSL decay curve from Al2O3:C depends on the type of radiation, and, in principle, this can be used to extract information concerning the LET of an unknown radiation field. The response of the dosimeters to low-LET radiation was also investigated for doses in the range from about 1-1000 Gy. These data were used to explain the different efficiency values obtained for the different materials and techniques, as well as the LET dependence of the CW-OSL decay curve shape.

Aluminum Oxide↗
PubMed2002

National and international standards and calibration of thermoluminescence dosimetry systems.

Radiation protection for radiation workers, the public, and the environment is of international concern. The use of thermoluminescence dosemeters (TLD) is an acceptable method for dose recording in most countries. For reasons of consistency and data gathering (research) it is important that a Sievert (Sv) in one part of the world equals an Sv on the other side of the globe. To this end, much work has gone into the development of standards and calibration practices for TLD systems so that they compare not only with similar systems, but also with other forms of radiation measurement. While most national laboratories provide calibration services for these systems some, as in the United States, depend on services of secondary calibration laboratories that are traceable to the national laboratories through accreditation programmes. The purpose of this paper is to explain how TLD measurements are traceable to their respective national standards for both personnel and environmental dosimetry.

Calibration↗
PubMed2001

Dosimetry of beta-ray ophthalmic applicators: comparison of different measurement methods.

An international intercomparison of the dosimetry of three beta particle emitting ophthalmic applicators was performed, which involved measurements with radiochromic film, thermoluminescence dosimeters (TLDs), alanine pellets, plastic scintillators, extrapolation ionization chambers, a small fixed-volume ionization chambers, a diode detector and a diamond detector. The sources studied were planar applicators of 90Sr-90Y and 106Ru-106Rh, and a concave applicator of 106Ru-106Rh. Comparisons were made of absolute dosimetry determined at 1 mm from the source surface in water or water-equivalent plastic, and relative dosimetry along and perpendicular to the source axes. The results of the intercomparison indicate that the various methods yield consistent absolute dosimetry results at the level of 10%-14% (one standard deviation) depending on the source. For relative dosimetry along the source axis at depths of 5 mm or less, the agreement was 3%-9% (one standard deviation) depending on the source and the depth. Crucial to the proper interpretation of the measurement results is an accurate knowledge of the detector geometry, i.e., sensitive volume and amount of insensitive covering material. From the results of these measurements, functions which describe the relative dose rate along and perpendicular to the source axes are suggested.

Alanine↗
PubMed2001

Calculation of beta-ray dose distributions from ophthalmic applicators and comparison with measurements in a model eye.

Dose distributions throughout the eye, from three types of beta-ray ophthalmic applicators, were calculated using the EGS4, ACCEPT 3.0, and other Monte Carlo codes. The applicators were those for which doses were measured in a recent international intercomparison [Med. Phys. 28, 1373 (2001)], planar applicators of 106Ru-106Rh and 90Sr-90Y and a concave 106Ru-106Rh applicator. The main purpose was to compare the results of the various codes with average experimental values. For the planar applicators, calculated and measured doses on the source axis agreed within the experimental errors (<10%) to a depth of 7 mm for 106Ru-106Rh and 5 mm for 90Sr-90Y. At greater distances the measured values are larger than those calculated. For the concave 106Ru-106Rh applicator, there was poor agreement among available calculations and only those calculated by ACCEPT 3.0 agreed with measured values. In the past, attempts have been made to derive such dose distributions simply, by integrating the appropriate point-source dose function over the source. Here, we investigated the accuracy of this procedure for encapsulated sources, by comparing such results with values calculated by Monte Carlo. An attempt was made to allow for the effects of the silver source window but no corrections were made for scattering from the source backing. In these circumstances, at 6 mm depth, the difference in the results of the two calculations was 14%-18% for a planar 106Ru-l06Rh applicator and up to 30% for the concave applicator. It becomes worse at greater depths. These errors are probably caused mainly by differences between the spectrum of beta particles transmitted by the silver window and those transmitted by a thickness of water having the same attenuation properties.

Beta Particles↗
PubMed2000

Dosimetry characterization of 32P catheter-based vascular brachytherapy source wire.

Dosimetry measurements and Monte Carlo simulations for a catheter-based 32P endovascular brachytherapy source wire are described. The measured dose rates were obtained using both radiochromic dye film and an automated plastic scintillator. The investigated source has dimensions of 27 mm in length and 0.24 mm in diameter, and is encapsulated in NiTi. For the radiochromic film measurements, calibrated radiochromic dye film was irradiated at distances between 1 and 5 mm from the source axis in A-150 plastic, and read out with a high-resolution scanning densitometer. The depth-dose curve measured in A-150 is then converted to that in water using correction factors obtained from Monte Carlo calculations. For the scintillator system, direct measurements in water were acquired at distances between 1 and 6 mm from the center of the source, along the perpendicular bisector of the source axis. The scintillator was calibrated in terms of absorbed-dose rate in a reference beta-particle field at multiple depths. The measured dose rates obtained from the film and scintillator measurements were then normalized to the measured source activity, i.e., to convert the measured data to units of cGy/s/mCi. Theoretical dosimetry calculations of the catheter-based 32P wire geometry were also obtained from Monte Carlo simulations using the Electron Gamma Shower code (EGS4), the Monte Carlo N-particle transport code (MCNP4B), and CYLTRAN from the Integrated Tiger Series codes (ITS v.3) and found to be in good agreement. The results of both measurements and calculations are expressed as absorbed-dose rate in water per unit of contained activity (cGy/s/mCi). Comparisons indicate that the measured and calculated dosimetry are in good agreement (<10%) within the relevant treatment distances (1-5 mm). This work fully characterizes the radiation field around a novel 32P beta brachytherapy source in water. The depth-dose curve can be used to calculate the dose to the vessel wall from a 27 mm 32P source wire centered within the vessel lumen.

Algorithms↗
PubMed2000

A high-precision, high-resolution and fast dosimetry system for beta sources applied in cardiovascular brachytherapy.

A fast dosimetry system based on plastic scintillator detectors has been developed which allows three-dimensional measurement of the radiation field in water of beta-sources appropriate for application in cardiovascular brachytherapy. This system fulfills the AAPM Task Group 60 recommendations for dosimetry of cardiovascular brachytherapy sources. To demonstrate the use of the system, measurements have been performed with an 90Y-wire source. The dose distribution was determined with a spatial resolution of better than 0.2 mm, with only a few minutes needed per scan. The scintillator dosemeter was absolutely calibrated in terms of absorbed dose to water with a precision of +/-7.5%. The relative precision achievable is +/-2.5%. The response of the system is linear within +/-2% for dose rates from 0.5 mGy s(-1) to 500 mGy s(-1).

Beta Particles↗
PubMed1999

Determination of a calibration factor for the nondestructive assay of Guidant 32P brachytherapy sources.

A calibration factor ('dial setting') for the nondestructive assay of Guidant TiNi-encapsulated 32P intravascular brachytherapy wire sources has been determined for measurements with the Capintec CRC-12 (sic. 'dose calibrator') ionization chamber. The calibration factor was derived from ionization current measurements with the CRC-12 followed by very quantitative, destructive assays of the 32P content in two sources.

Brachytherapy↗
PubMed1998

Calibration and characterization of beta-particle sources for intravascular brachytherapy.

The calibration of a catheter-based system to be used for therapeutic radiation treatment to prevent restenosis following interventional coronary procedures is described. The primary dosimetry was performed ionometrically using an extrapolation chamber equipped with a 1-mm diameter collecting electrode to measure absorbed dose in tissue equivalent plastic at a depth of 2 mm. These results are compared with measurements with radiochromic dye film, which is also used to characterize sources for axial and trans-axial uniformity, and to determine dose distributions at various depths. A protocol for dose calculation based on that of AAPM TG43 is suggested for these sources, and examples of its use are given for the calculation of the enhancement effect on dose rate from a single seed source due to neighboring seeds. Monte Carlo calculations were also performed to validate the measured results.

Beta Particles↗
PubMed1995

Comparison of NIST and manufacturer calibrations of 90Sr+90Y ophthalmic applicators.

Since the resumption of the NIST calibration service for 90Sr+90Y beta-particle ophthalmic applicators, 65 sources have been calibrated using the revised technique [C. G. Soares, Med. Phys. 18, 787-793 (1991)]. For 59 of these sources, the manufacturer's calibration results were available for comparison to the NIST calibration results. The 59 sources represent eight different manufacturers, only one of which is still selling new sources. Manufacturer calibration dates range from the present back to 1954. The results of the comparisons are present, broken down by both manufacturer and calibration date; there are interesting and significant trends in both, with average differences of 20% not uncommon. The obsolete unit, "roentgen-equivalent-beta" (reb), in which some of the manufacturer calibrations are expressed, is discussed, and a factor (0.00982 Gy reb-1) for its conversion to absorbed dose is suggested.

Brachytherapy↗
PubMed1994

Extrapolation chamber measurements of 90Sr + 90Y beta-particle ophthalmic applicator dose rates.

Aspects of extrapolation chamber dose-rate measurements of 90Sr + 90Y beta-particle ophthalmic applicators are examined in this report, including the proper choice of collector electrode size, the gap width over which the measurement should be done, the effect of the entrance window materials, and the stopping-power ratio. Experiments, a simple analytic model for the effect of chamber geometry and nonzero gap width, and more detailed Monte Carlo simulations were used. The variation of the planar flux density as a function of angle for a thick 90Sr + 90Y source was measured and used as input for the model. From Monte Carlo simulation, the dose rate for tissue irradiation falls off by 8% between the surface and a depth of 7 mg/cm2. The derivative of chamber ionization as a function of gap width, needed for the dose-rate calibration, increases rapidly as the gap width decreases, typically by a factor of about 2 between gap widths of 1.5 and 0.15 mm. About half of this change is due to ionizing electrons leaving the collection volume at the larger gap widths as shown by the analytic model; the rest of the change is due to ionizing electrons which backscatter from the collector electrode and its backing as shown by Monte Carlo simulations. The backscattering effect increases the derived surface dose by a factor of 1.46. A satisfactory dose-rate extrapolation is obtained from gap widths of 0.1-0.25 mm, where the total ionization current is observed to be nearly linear in gap width.

Beta Particles↗
PubMed1994

The use of a radiochromic detector for the determination of stereotactic radiosurgery dose characteristics.

The measurement of absorbed dose as well as dose distributions (profiles and isodose curves) for small radiation fields (as encountered in stereotactic surgery) has been difficult due to the usual large detector size or densitometer aperture (> 1 mm) relative to the radiation field (as small as 4 mm). The radiochromic direct-imaging film, when read with a scanning laser microdensitometer (laser beam diameter 0.1 mm), overcomes this difficulty and has advantages over conventional film in providing improved precision, better tissue equivalence, greater dynamic range, higher spatial resolution, and room light handling. As a demonstration of suitability, the calibrated radiochromic film has been used to measure the dose characteristics for the 18-, 14-, 8-, and 4-mm fields from the gamma-ray stereotactic surgery units at Mayo Clinic and the University of Pittsburgh. Intercomparisons of radiochromic film with conventional methods of dosimetry and vendor-supplied computational dose planning system values indicate agreement to within +/- 2%. The dose, dose profiles, and isodose curves obtained with radiochromic film can provide high-spatial-resolution information of value for acceptance testing and quality control of dose measurement and/or calculation.

Film Dosimetry↗
PubMed1992

Effects of haloperidol, bromocriptine and amphetamine on the development of Ehrlich ascites carcinoma in mice.

We determined the effect of 13 days of treatment with 2.0 mg/kg haloperidol, 4.0 mg/kg bromocriptine or 2.0 mg/kg amphetamine on the number of tumor cells of mice bearing Ehrlich ascites carcinoma. The dopaminergic blocker significantly reduced the number of tumor cells of experimental mice, but the two dopamine-mimetic drugs used did not significantly affect tumor development. These results suggest that although neuroleptic drugs to inhibit Ehrlich ascites tumors, this effect does not seem to be related to changes in dopaminergic neuronal transmission.

Amphetamine↗
PubMed1991

Antitumor effects of dopaminergic blockers in mice bearing Ehrlich tumors.

We determined the effect of 13 days of treatment with 2.0 mg/kg haloperidol, 30.0 mg/kg metoclopramide or 4.0 mg/kg domperidone on the number of tumor cells of mice bearing Ehrlich ascites carcinoma. The three dopaminergic blockers significantly reduced the number of tumor cells of experimental mice. The mean +/- SEM number of tumor cells x 10(6)/ml saline lavage was 25.5 +/- 5.9 for the haloperidol group, 36.8 +/- 4.7 for the metoclopramide group, 25.3 +/- 3.5 for the domperidone group and 54.0 +/- 9.0 for the control mice (treated with 0.9% NaCl). In a second experiment, treatment with 0.5 and 2.0 mg/kg haloperidol showed that the antitumor effect of this drug was dose dependent. The possible mechanisms underlying these results (such as an increase in prolactin levels or a direct action of these drugs on lymphocytes) are discussed in light of the specific pharmacological properties of each dopaminergic blocker.

Analysis of Variance↗
PubMed1988

Difficulties encountered with some intermediate-atomic-number radiation-protection dosimeters irradiated on-phantom in low-energy photon beams.

During a measurement-assurance study done with a particular type of radiation-protection dosimeter, we obtained unexpectedly poor results when several of the dosimeters were irradiated simultaneously on-phantom in a low-energy bremsstrahlung beam. The dosimeters incorporated low- and intermediate-atomic-number (Z) thermoluminescence dosimeter elements with the intermediate-Z elements under Pb filters. The response of the low-Z elements was in good agreement with the predicted response regardless of dosimeter location on the phantom, while agreement for the intermediate-Z elements was poor for some locations not coinciding with the phantom center. We have since studied the behavior of this dosimeter type and of four other types in detail by irradiating them, one at a time, on- and off-phantom, with their geometric centers displaced up to 10 cm along the vertical and horizontal axes through the center of the beam cross section. Two low-energy bremsstrahlung beams and a 137Cs gamma-ray beam were used. Variation of response to low-energy photons with on-phantom location was observed to some degree in all intermediate-Z dosimeter elements covered with high-Z filters. The reason for the observed dependence of response upon on-phantom location seems to be insufficient side shielding against phantom and filter albedo, as proved in one case, in which added shielding removed this dependence for the element immediately adjacent to the added shield. Since the effective dose equivalent to the human body is lower for lateral incidence of low-energy photons than for the same photons incident from the front or back, dosimeters incorporating high-Z filters without side shielding (and thus expected to exhibit the phenomenon) do not lend themselves to the determination of the effective dose equivalent or any quantity proportional to it by currently employed methods. A detailed study of the angular dependence of dosimeter response performed on a suitable phantom should aid in eliminating such inadequate dosimeters from practical use.

Calibration↗