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Biomedical subjects

L A DeWerd

Publications and source records attributed to L A DeWerd.

At least 37 records · Page 2Linked to original sources
PubMed1996

TLD measurements of in vivo mammographic exposures and the calculated mean glandular dose across the United States.

An analysis is presented of the exposures received by TLDs placed on the breasts of 4400 women obtaining mammograms at 170 institutions across the United States. Mean glandular dose and exposure were examined as a function of compressed breast thickness. The exposure and mean glandular dose were found to increase linearly with breast thickness. The mean glandular dose typically delivered by the institutions was well below the limit of 3.0 mGy for the 4.5-cm breast. However, some institutions tend to uniformly give higher doses. In such institutions approximately 25% of 4.5-cm-thick breasts received a mean glandular dose exceeding 3.0 mGy.

Biophysical Phenomena↗
PubMed1995

Accuracy of the point source approximation to high dose-rate Ir-192 sources.

The accuracy of the point source approximation used in dose calculations for an implant comprised of multiple high dose rate (HDR) Ir-192 source dwell positions is investigated. First, a single dwell position implant is modeled. The exposure rate about the source is calculated using both the point source approximation and the more rigorous line source formalism. A comparison of these calculated exposure rates is made. It is found that for each HDR Ir-192 source dwell position, the point source approximation results in a dose overestimation of 1% at a distance of 1 cm on the source transverse axis, while dose underestimations of more than 2% can be found at a distance of 1 cm on the source longitudinal axis. Even larger errors occur closer to the source. The results of this academic study are then extended to two clinical cases--an endobronchial treatment and a tandem and ovoids setup, both involving multiple source dwell positions. Since clinical HDR Ir-192 implants are comprised of many individual source dwell positions, there will be inaccuracy in the calculated overall dose distribution leading to dose delivery errors. For example, the dose delivered to a prescription point located 0.5 cm from an endobronchial applicator will be 3% lower than prescribed. Similar errors are produced in gynecologic implants. To decrease below 0.5% the dose delivery error resulting from the point source approximation, prescription points should be at a distance of at least 1 cm from any applicator. Since the dosimetry error is a direct result of the choice of model used to describe the source, the use of anisotropy factors accounting for the variation of photon fluence around the HDR Ir-192 source will not completely correct the calculation.

Brachytherapy↗
PubMed1995

Quality assurance tool for high dose rate brachytherapy.

An insert for a well ionization chamber is described for quality assurance tests for high dose rate remote afterloaders. The use of this insert over time has shown its applicability for efficient QA tests for the high dose rate afterloader. The measurement results are six times greater than the uncertainties involved for the measurements. Checks on position can be made better than 1 mm and dwell times better than 1 s. The overall measurements can be done to better than 1.5%. QA checks can be accomplished in a shorter time period with as great or better accuracy than prior film techniques.

Brachytherapy↗
PubMed1994

A comparison of methods for calibrating parallel-plate chambers/.

All dosimetry protocols for calibrating the output of electron beams recommend the use of parallel-plate ionization chambers, but the method of determining their value of Ngas is a matter of concern. The AAPM Protocol (TG 21) recommends a direct comparison with a calibrated cylindrical chamber in phantom at dmax with the highest available electron energy beam. This must be done by the user. Since all calibration laboratories traditionally use 60Co for megavoltage chamber calibrations, two alternate procedures based on exposures in-air, or in-phantom, have been proposed. All methods use correction factors in the data reduction. To verify the consistency of the three methods, we have measured Ngas using each of these techniques for six of the most commonly used and commercially-available parallel-plate ionization chambers. The paired cylindrical and parallel-plate ionization chambers, and phantom materials/buildup caps were matched to the wall composition of the plane chambers, as recommended in TG 39. A 22 MeV electron beam was used for the electron irradiations. The ionization chambers were then taken to an Accredited Dosimetry Calibration Laboratory (ADCL), where 60Co calibrations were performed. The results demonstrate that, by using the appropriate correction factors for the chambers described in this work, all three methods yield values for Ngas that are within 1% of each other.

Biophysical Phenomena↗
PubMed1992

A new re-entrant ionization chamber for the calibration of iridium-192 high dose rate sources.

A re-entrant (well-type) ionization chamber has been designed and fabricated at the University of Wisconsin for use with iridium-192 high dose-rate (HDR) remote after-loading brachytherapy devices. The chamber was designed to provide an ionization current of about 10(-8) ampere with a nominal 10 curie iridium-192 source. A narrow opening is provided into the sensitive volume of the chamber to insert a Nucletron MicroSelectron catheter, or catheters with similar diameters from other HDR manufacturers. The chamber exhibits a flat response (+/- 0.1%) for any source position within +/-4 mm of the chamber center. A 300 volt chamber bias yields a 99.96% ion collection efficiency. The chamber is capable of being calibrated directly with an iridium-192 source which has in turn been calibrated with thimble-type ion chambers. Reproducibility for readings in the current mode for 10 consecutive insertions of the MicroSelectron iridium-192 HDR source is within 0.02% or less. Two thimble chambers calibrated by the U.S. National Institute of Standards and Technology provide calibration traceability of iridium-192 HDR sources and re-entrant chambers to a primary national standards laboratory. Results of activity measurements of 6 commercial iridium-192 HDR sources are reported.

Calibration↗
PubMed1990

Evaluation of a commercial diode monitor for mailed quality control of therapy units.

A commercial diode dosimeter was evaluated for use in a mailed radiotherapy quality control package. The quality control package contained the commercial diode dosimeter and four thermoluminescent dosimeters to measure the given dose, and it also contained a photographic film to measure the light/radiation field coincidence. The film also served as a backup dosimeter. Twenty small Midwestern hospitals participated in field measurements; approximately half of the hospitals had a consulting physicist and the others had one full-time physicist. Each hospital was asked to deliver 1.5 Gy (150 rads) to the device, which was returned to our laboratory for evaluation. Reliable results were obtained from three measurements at each hospital at about 3-month intervals. The diode dosimeter was more precise than thermoluminescent dosimeters or film.

Calibration↗
PubMed1978

Mailed thermoluminescent dosimeter determination of entrance skin exposure and half-value layer in mammography.

A method of providing in vivo measurement of entrance skin exposure for mammography using mailed thermoluminescent dosimeters has been developed. The dosimeters are calibrated to 137Cs (662 keV), and a correction is applied to account for energy response. A mailed dosimeter packet which can be used to measure the half-value layer at mammography energies is also described. The results for both systems agree well (to within 10%) with careful ion chamber determinations. These dosimeter measurements provide a simple. Inexpensive means to reliably determine exposure and to monitor machine parameters on a periodic basis.

Cesium Radioisotopes↗
PubMedDate not supplied

Direct-response ultraviolet thermoluminescent dosimeter.

An integrating solid-state dosimeter has been developed to detect ultraviolet radiation using the process of "direct" ultraviolet stimulation. This process yields a thermoluminescent signal after an exposure to ultraviolet radiation, without prior exposure to ionizing radiation. Magnesium oxide crystals were found to show a directly stimulated thermoluminescent glow peak at 145 degrees C which can be utilized as a linear, energy-independent ultraviolet dosimeter.

Magnesium Oxide↗
PubMedDate not supplied

Solid-state electrophotography with Al2O3.

Solid-state thermocurrent and radioconductivity (electrical conductivity during irradiation) experiments are described with attention to the feasibility of using ionic solids for radiographic imaging. The radioconductivity varies with temperature giving rise to temperature windows of potential usefulness. The electrophotographic image is formed at atmospheric pressure using Al2O3, collecting the charge on mylar film. Development is by the power cloud technique. A transparency, which can be viewed as a conventional radiographic image, is easily produced. A successful transfer image, free of electric discharge artifact, was produced.

Aluminum↗
PubMedDate not supplied

Mailed thermoluminescent dosimetry reviews in radiation therapy.

The six Centers for Radiological Physics performed periodic thermoluminescent dosimetry (TLD) reviews for photon beams at 276 facilities across the country. The purpose of the reviews and the techniques in use are briefly described. Results from 2413 mailed reviews of megavoltage units indicate that, in 92% of the reviews, a specified dose was delivered within +/- 5% of the TLD-measured dose. A comparison is made to other mailed TLD review systems.

Postal Service↗
PubMedDate not supplied

The application of CaSO4:Dy (TLD-900) to diagnostic x-ray exposures.

The properties of a new commercial thermoluminescent dosimeter, CaSO4:Dy without a LiF binder (TLD-900) was studied for low-exposure measurements in diagnostic radiology. The former TLD-900 had a LiF binder (herein referred to as "TLD-900/LiF"). The principle features of this dosimeter are its high sensitivity to low-energy radiation and its relatively low fading which permits measurements down to less than 2.6 X 10(-8) C kg (0.1 mR) with an accuracy better than 20%. The characteristics, annealing procedures, light sensitivity, energy response, reproducibility, and fading, of TLD-900 are discussed and compared with TLD-900/LiF. When the precautions presented in this paper are used, the dosimeters can be used for the measurement of x-ray exposures.

Calcium Sulfate↗
PubMedDate not supplied

Calibration and use of the Wisconsin kVp test cassette.

The Ardran-Crooks kVp test cassette is widely used in diagnostic radiology to provide a rapid, simple, noninvasive measurement of x-ray tube potential. A modified version of this cassette called the Wisconsin kVp Test Cassette was introduced commercially in the U. S. in 1972. Since then, the method of calibration of these cassettes has changed significantly. Wisconsin kVp Test Cassettes calibrated by the manufacturer prior to August 1982 may yield underestimates of kVp measurements, particularly when using the 90-110 and 110-130 kVp regions with single-phase units. In August 1982 significant improvements in the calibration methods were implemented. The resultant change in calibration is demonstrated by data from the Centers for Radiological Physics. Present calibration methods are believed to be accurate within the greater of +/- 2 kVp or 2% of actual peak tube potential. Proper use of the cassette is necessary to achieve this level of accuracy.

Biophysical Phenomena↗
PubMedDate not supplied

Radiation therapy and clinical physics practices in the midwest (1982-1986).

As a part of an on-site physics review procedure, the Midwest Center for Radiological Physics (MWCRP) interviewed midwest clinical radiotherapy physicists and radiation therapists in institutions that participated in federally funded Cancer Control Programs. Information from 77 institutions was compiled regarding types of megavoltage units, daily patient load, radiotherapy personnel, treatment planning responsibilities, dosimetric practices, and treatment planning computers. Even though some practices, such as frequency of patient charge checks, were consistent throughout the midwest, other practices varied considerably such as patient load with the number of megavoltage units. This information may be useful either for planning a new facility, considering the needs for staff, or for comparing existing practices and responsibilities.

Cancer Care Facilities↗