Volume 26, Issue 9, September 1999
Index of content:
- PH. D. THESES ABSTRACTS
26(1999); http://dx.doi.org/10.1118/1.598712View Description Hide Description
Exposure to lead is associated with a variety of detrimental health effects. After ingestion or inhalation, lead may be taken up from the bloodstream and retained by bone tissue.X-ray fluorescence was used to make in vivo measurements of bone lead concentration at the tibia and calcaneus for 367 active and 14 retired lead smelter workers. Blood lead levels following a labor disruption were used in conjunction with bone lead readings to examine the endogenous release of lead from bone. Relations between bone lead and a cumulative blood lead index differed depending on time of hiring. This suggests that the transfer of lead from blood to bone has changed over time, possibly as a result of varying exposure conditions. A common polymorphism in the δ-aminolevulinate dehydratase (ALAD) enzyme may influence the distribution of lead in humans. Blood lead levels were higher for smelter workers expressing the more rare allele. Bone lead concentrations, however, were not significantly different. This implies that a smaller proportion of lead in blood is distributed to tissue for individuals expressing the allele. The O’Flaherty physiological model of lead metabolism was modified slightly and tested with input from the personal exposure histories of smelter workers. The model results were consistent with observation in terms of endogenous exposure to lead and accumulation of lead in cortical bone. Modeling the calcaneus as a trabecular bone site did not reproduce observed trends. Variations in lead metabolism between different trabecular sites may therefore be significant. The model does not incorporate a genetic component, and its output did not reflect observed differences in this respect. This result provides further support for the influence of the ALAD polymorphism on lead metabolism. Experimental trials with a digital spectrometer revealed superior energy resolution and count throughput relative to the conventional x-ray fluorescence system. The associated reduction in the uncertainty of lead measurement has the potential to benefit future surveys and modeling efforts.
Computer-aided diagnosis: Automatic detection of microcalcifications on digital mammograms (in Spanish)26(1999); http://dx.doi.org/10.1118/1.598713View Description Hide Description
A computerized scheme to detect clustered microcalcifications in digital mammograms was developed. Detection of individual microcalcifications in regions of interest was also performed. Mammograms were previously classified into fatty or dense. The wavelet basis and reconstruction levels were selected. The symlets 8 were chosen for fatty tissue, and the Daubechies 4 for dense tissue. Two methods to detect individual microcalcifications were evaluated: two-dimensional and one-dimensional wavelet transform. The second technique yielded the best results, and was used to detect clustered microcalcifications in the complete mammogram, employing a scheme involving: (1) detection of the breast border, (2) application of one-dimensional wavelet transform, (3) application of gray level threshold, (4) clustering procedure, (5) reduction of false positives with discriminant analysis. When detecting individual microcalcifications, we have obtained a sensitivity of 66% and an average of 7.23 false positives per image with the two-dimensional method, and 71% of sensitivity at an average false positive rate of 6.12 with the one-dimensional technique. For the detection of clustered microcalcifications, the sensitivity was 80% with an average of 0.94 false positives per image for fatty mammograms. The area under the AFROC curve was For dense mammograms, 73% of sensitivity at a false positive detection rate of 2.21 per image was achieved Globally, a sensitivity of 76% and an average of 1.57 false detections per image were obtained. We conclude that a wavelet-based method to detect clustered microcalcifications would help radiologists as a “second opinion” in mammographic screening. The low false positive rate indicates that our technique would not confuse the radiologist by suggesting normal regions as suspicious.
Investigations on absolute portal dosimetry for the verification of static and dynamic dose delivery in radiotherapy (in English)26(1999); http://dx.doi.org/10.1118/1.598714View Description Hide Description
New in vivodosimetry procedures using electronic portal imaging devices(EPIDs) either compare measured to predicted transmission dose, or infer patient dose from measured transmission doseimages. These procedures demand a careful calibration of the portal imager. The EPID used in this research was a scanning liquid ionization chamber (SLIC). The first investigation was to see if a dosecalibration of the EPID, performed on the central axis, is sufficiently accurate to estimate the dose for off-axis points. The incident photon energy fluence distribution generated from the phase space of a linear accelerator was first convolved with Monte Carlo calculated dose spread kernels in water, which serves as the reference medium, and then in the detector. The kernels in water and the detector differ considerably at photon energies lower than 1 MeV or higher than 6 MeV. Transmission doses were then calculated in the SLIC-EPID and in water. The dose differences are due to changing beam quality. It is shown that depending on the type of calibration, the difference can be as large as 5%, which is unacceptable for accurate transmission dosemeasurements. To investigate the applicability of the EPID for dynamic dose delivery techniques, the dose response to a photon beam with variable intensity was measured and theoretically modeled. It was found that the accuracy using the detector for verification of the dose from intensity modulated treatment fields delivered by a dynamically modulated multileaf technique is limited. This is due to the detector’s nonlinear response. However, under certain acquisition conditions, the leaf movement during delivery can be precisely verified.
Monte Carlo studies for the optimization of hardware used in conformal radiation therapy (in English)26(1999); http://dx.doi.org/10.1118/1.598715View Description Hide Description
The Monte Carlo system GEANT was applied to characterize and develop devices used in conformal radiation therapy. First, the Siemens Primus accelerator head was modeled. Second, two alternative techniques for intensity modulation were investigated: (1) Two multileaf collimators were modeled with respect to their penumbra and leakage radiation. To reduce the penumbra, an optimized shape of the leaf face was determined. The effectiveness of a new collimator design for leakage reduction was confirmed, showing a decrease in the average leakage from 4.7% to 2.6%. (2) The feasibility of a scanned narrow photon beam was examined for accelerators with an electron energy of about 20 MeV. The simulated photon intensity distributions were verified by comparison with both data and the theoretical prediction. With a thin beryllium bremsstrahlung target and a purging magnet, a photon beamwidth of about 40 mm at 100 cm source-to-surface distance can be achieved. The cost of the narrow distribution is a lower yield, which increases the irradiation time. To narrow down the beamwidth, a design for a fixed collimator with parallel holes was developed that yields a lateral resolution of about 1.5 mm at 100 cm source-to-surface distance and a penumbra of less than 1 mm. The use of this collimator requires that the scanned incident electron beam is always perpendicular to the target. This approach has the potential to improve both the cost-effectiveness and the beam characteristics of intensity modulated radiation therapy.