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W E Bolch

Publications and source records attributed to W E Bolch.

At least 55 records · Page 3Linked to original sources
PubMed1999

MIRD pamphlet No. 17: the dosimetry of nonuniform activity distributions--radionuclide S values at the voxel level. Medical Internal Radiation Dose Committee.

The availability of quantitative three-dimensional in vivo data on radionuclide distributions within the body makes it possible to calculate the corresponding nonuniform distribution of radiation absorbed dose in body organs and tissues. This pamphlet emphasizes the utility of the MIRD schema for such calculations through the use of radionuclide S values defined at the voxel level. The use of both dose point-kernels and Monte Carlo simulation methods is also discussed. PET and SPECT imaging can provide quantitative activity data in voxels of several millimeters on edge. For smaller voxel sizes, accurate data cannot be obtained using present imaging technology. For submillimeter dimensions, autoradiographic methods may be used when tissues are obtained through biopsy or autopsy. Sample S value tabulations for five radionuclides within cubical voxels of 3 mm and 6 mm on edge are given in the appendices to this pamphlet. These S values may be used to construct three-dimensional dose profiles for nonuniform distributions of radioactivity encountered in therapeutic and diagnostic nuclear medicine. Data are also tabulated for 131I in 0.1-mm voxels for use in autoradiography. Two examples illustrating the use of voxel S values are given, followed by a discussion of the use of three-dimensional dose distributions in understanding and predicting biologic response.

Animals↗
PubMed1998

Product yields from irradiated glycylglycine in oxygen-free solutions: Monte Carlo simulations and comparison with experiments.

The radiation chemistry of photon-irradiated aqueous solutions of biological molecules may be considered under four distinct time regimes: physical transport (< or = 10(-15) s); prechemical conversion of H2O+, H2O*, and subexcitation electrons into free radicals and molecular products (10(-15) s to 10(-12) s); chemical reactions within individual electron tracks (10(-12) s to 10(-6) s); and chemical reactions within overlapping tracks (>10(-6) s). We have previously reported of the use of the Monte Carlo radiation transport/chemistry codes OREC and RADLYS to model the radiolysis of glycylglycine in oxygen-free solution to a time of 1 micros. These simulations successfully predicted the yields of free ammonia, an end product created solely in the reaction of the hydrated electron with the solute within individual tracks. Other measurable products are only partially created during intratrack reactions, and thus one must additionally consider the late, intertrack chemistry of this system. In this paper, we extend our simulations of glycylglycine radiolysis to model for the first time the events which occur during this late chemistry stage. The model considers the product rates of the reactants in bulk solution by using previously available microsecond intratrack yields given by single-track OREC/RADLYS simulations and an x-ray dose rate of 2.80 Gy min(-1) as used in a companion experimental program. These rates are then applied in a series of coupled, differential rate equations that describe the solution chemistry of glycylglycine radiolysis. Product yields are reported as a function of time over a total irradiation period of 10(4) s. Excellent overall agreement is seen between the theoretical predictions and measurements of five radiolysis end products: free ammonia, acetylglycine, diaminosuccinic acid, aspartic acid, and succinic acid. The model also gives the explicit contributions of intratrack and intertrack reactions to the various end products. For example, the model predicts that approximately 56% and 93% of succinic acid and aspartic acid, respectively, are produced during intertrack reactions at a solute concentration of 0.05 M; these contributions drop to 0.07% and 11%, respectively, at 1.2 M.

Computer Simulation↗
PubMed1998

NMR microscopy of trabecular bone and its role in skeletal dosimetry.

One of the more intractable problems in internal dosimetry is the assessment of energy deposition by alpha and beta particles within trabecular, or cancellous, bone. In the past few years, new technologies have emerged that allow for the direct and nondestructive 3D imaging of trabecular bone with sufficient spatial resolution to characterize trabecular bone structure in a manner needed for radiation dosimetry models. High-field proton nuclear magnetic resonance (NMR) imaging is one such technology. NMR is an ideal modality for imaging trabecular bone due to the sharp contrast in proton density between the bone matrix and bone marrow regions. In this study, images of the trabecular regions within the bodies of a human thoracic vertebra have been obtained at a field strength of 14.1 T. These images were digitally processed to measure chord length distribution data through both the bone trabeculae and marrow cavities. These distributions, which were found to be qualitatively consistent with those measured by F. W. Spiers and colleagues at the University of Leeds using physical sectioning and automated light microscopy, yielded a mean trabecular thickness of 201 microm and a mean marrow cavity thickness of 998 microm. The NMR techniques developed here for vertebral imaging may be extended to other skeletal sites, allowing for improved site-specific skeletal dosimetry.

Bone Marrow↗
PubMed1996

Calculation of absorbed energy in the gastrointestinal tract.

One goal of this research was to reproduce the photon specific absorbed fraction calculations of Cristy and Eckerman using their gastrointestinal (GI) tract model. A second goal was to calculate photon specific absorbed fraction values for their GI tract model using electron tracking techniques. A final goal was to calculate electron absorbed fraction values for their GI tract model. This paper summarizes the work performed using the currently accepted model of the GI tract provided by Cristy and Eckerman. Their model was coded into the Electron Gamma Shower 4 (EGS4) computational package for calculation of photon specific absorbed fraction values. To benchmark the initial code, the EGS4 program was run so that all secondary particles deposited their energy at the site of the primary photon interaction (i.e., without electron tracking). The results obtained from these preliminary calculations were compared to those provided by Cristy and Eckerman to verify and benchmark the program. Next, specific absorbed fraction values were calculated for twelve discrete photon energies using the electron tracking capabilities of EGS4. These photon specific absorbed fraction values were compared to those calculated without electron tracking. Finally, absorbed fraction values were calculated for twelve discrete electron energies. The electron absorbed fraction values were compared to those calculated without electron tracking. Finally, absorbed fraction values were calculated for twelve discrete electron energies. The electron absorbed fraction values were compared to the ICRP "one-half assumption" for electron energy deposition in the wall of the GI tract.

Digestive System↗
PubMed1996

A revised model for the calculation of absorbed energy in the gastrointestinal tract.

The goal of this research was to develop a more complete gastrointestinal (GI) tract model for use in internal dose assessment. This paper summarizes the development of a revised mathematical model of the GI tract. The current GI tract model assumes the wall can be represented as a single soft tissue layer without regard to the radiosensitivity of the cells. The goal of the GI tract revision was to develop geometric regions that separate the radiosensitive cells from the less radiosensitive cells. Once the model was revised, it was coded into the Electron Gamma Shower 4 (EGS4) computational package for calculation of photon and electron absorbed fraction values. Photon absorbed fraction values were calculated for twelve discrete energies. For the photon absorbed fraction calculations, the EGS4 program was run so that secondary particles created in photon interactions were followed using the electron tracking capabilities of EGS4. The results of the photon absorbed fraction calculations provided better estimates of the energy deposited in the radiosensitive cells when the target organ was the source. In cases where the target organ was not the source, the photon absorbed fraction values did not provide better estimates than those obtained using the current GI tract model. An increase in the number of photon histories should provide better estimates of the photon absorbed fraction for these cases. Electron absorbed fraction values also were calculated for twelve discrete electron energies. The results of these calculations provided the expected pattern of energy deposition and better estimates than those currently available. The annual limit on intake was recalculated for a single radionuclide to demonstrate the affect of these improved absorbed fraction values on internal dose assessment. The radionuclide was selected for two reasons: 1) it was a beta emitting radionuclide; and 2) the annual limit on intake for ingestion was based on the non-stochastic committed dose equivalent limit to the lower large intestine. The calculated annual limit on intake was found to be three times greater than the annual limit on intake provided in ICRP Publication 30. There are many radionuclides that have a section of the GI tract as the limiting organ for ingestion. It is expected that the annual limit on intake value for these radionuclides would increase when the revised GI tract model is employed for internal dose assessment.

Digestive System↗
PubMed1996

A revised dosimetric model of the adult head and brain.

UNLABELLED: During the last decade, several new radiopharmaceuticals have been introduced for brain imaging. The marked differences of these tracers in tissue specificity within the brain and their increasing use for diagnostic studies support the need for a more anthropomorphic model of the human brain and head. Brain and head models developed in the past have comprised only simplistic representations of this anatomic region. METHODS: A new brain model has been developed which includes eight subregions: the caudate nucleus, the cerebellum, the cerebral cortex, the lateral ventricles, the lentiform nucleus, the thalamus, the third ventricle and the white matter. This brain model has been included within a slightly modified version of the head model developed by Poston et al. in 1984. The head model, which includes both the thyroid and eyes, was modified in this work to include the cerebrospinal fluid within the cranial and spinal regions. RESULTS: Absorbed fractions of energy for photon and electron sources located in thirteen source regions within the new head model were calculated using the EGS4 Monte Carlo radiation transport code for radiations in the energy range 10 keV to 4 MeV. CONCLUSION: S-values were calculated for five radionuclides used in brain imaging (11C, 15O, 18F, 99(m)Tc and 123I) and for three radionuclides showing selective uptake in the thyroid (99(m)Tc, 123I, and 131I). S-values were calculated using 100 discrete energy points in the beta-emission spectrum of the different radionuclides.

Adult↗
PubMed1994

Monte Carlo track-structure calculations for aqueous solutions containing biomolecules.

Detailed Monte Carlo calculations provide a powerful tool for understanding mechanisms of radiation damage to biological molecules irradiated in aqueous solution. This paper describes the computer codes, OREC and RADLYS, which have been developed for this purpose over a number of years. Some results are given for calculations of the irradiation of pure water. Comparisons are presented between computations for liquid water and water vapor. Detailed calculations of the chemical yields of several products from X-irradiated, oxygen-free glycylglycine solutions have been performed as a function of solute concentration. Excellent agreement is obtained between calculated and measured yields. The Monte Carlo analysis provides a complete mechanistic picture of pathways to observed radiolytic products. This approach, successful with glycylglycine, will be extended to study the irradiation of oligonucleotides in aqueous solution.

Amino Acids, Diamino↗
PubMed1992

Measurement of products from X-irradiated glycylglycine in oxygen-free aqueous solutions.

The product yields in X-irradiated aqueous solutions of glycylglycine (0.05 M and 1.0 M) were measured under deoxygenated conditions. Comparison was made between the results obtained from X- and 60Co gamma-irradiated glycylglycine solutions reported by Garrison, Sokol, and Bennett-Corniea (Radiat. Res. 53, 376-384, 1973). The mechanisms proposed by Garrison et al. were tested by evaluating the stoichiometric relationships. The two intermediate radicals, deamination and H-abstraction radicals, were produced in the initial interactions of glycylglycine with reactive species (e-aq, OH, H) formed in H2O. Although the difference was fairly large at 0.05 M, the production of deamination radicals agreed well with the consumption of the radicals at 1.0 M. The production and the consumption of H-abstraction radicals were within the estimated experimental error in dilute solutions. Among all the products only the G value of aspartic acid decreased with increasing concentration of glycylglycine. This could be attributed to the fact that more acetylglycine is formed at the expense of aspartic acid at 1.0 M than at 0.05 M glycylglycine solutions. Competitive reactions involved with deamination radicals under conditions of homogeneously distributed reactants are discussed to elucidate the radiation chemistry of glycylglycine.

Cobalt Radioisotopes↗
PubMed1991

Radiation damage to a biomolecule: new physical model successfully traces molecular events.

For the first time, a complete computer simulation of physical and chemical reactions at the molecular level has been used to calculate the yield of a chemical species resulting from irradiation of a biological molecule in aqueous solution. Specifically, when a solution of glycylglycine is irradiated anaerobically, an ammonia molecule is released by the action of a hydrated electron, which is produced by irradiation of water. In the computations, Monte Carlo techniques are used to simulate the statistical progression of molecular events as they are assumed to occur. These include the initial physical ionization and excitation of water molecules along a particle track in the liquid; the subsequent formation of free radicals and other species: and the random diffusion and chemical reactions of the species with each other, the solvent, and solute molecules. We have calculated and measured the yield of ammonia from irradiation of glycylglycine with 250 kVp x-rays as a function of glycylglycine concentration between 0.01 and 1.2 M. Excellent agreement is obtained between predicted and measured results. The literal simulation of events, combined with specific experimental measurements, offers a powerful new tool for studying mechanisms of radiation action and damage at the molecular level.

Ammonia↗
PubMed1991

Estimates of beta absorbed fractions in small tissue volumes for selected radionuclides.

Energy deposition patterns are dependent upon the size and geometry of the source region, distribution of radioactive material, types of radiations and energies emitted by the radionuclide, as well as interfaces between different materials which may exist within the region. Commonly, in absorbed dose calculations for internally deposited beta-emitting radionuclides, it is assumed that the absorbed fraction of energy for the mean beta energy is a sufficient representation of the beta spectrum. The accuracy of this assumption was tested by comparing absorbed fractions calculated using actual beta spectral energies with those obtained using the mean beta energy for several radionuclides commonly used in nuclear medicine. A sphere composed of tissue was chosen as the preferred geometry. Spheres of 0.1, 0.5, 1.0, and 2.0 cm radius were used, and absorbed fractions were calculated as a function of surface-to-volume ratios. This allows the assessment of absorbed fraction in spheres where there is a uniform distribution of a radionuclide.

Beta Particles↗
PubMed1990

Monte Carlo calculations of free ammonia production in deoxygenated solutions of glycylglycine irradiated by X rays and 60Co gamma rays.

Detailed-history Monte Carlo computer codes were used to simulate the formation, diffusion, and chemical reaction of free-radical species within deoxygenated aqueous solutions of glycylglycine irradiated by 250-kVp X rays and by 60Co gamma rays. In one reaction, hydrated electrons react with the glycylglycine solute to produce unbound, or free, ammonia. This reaction is complete by 10(-6) s within individual electron tracks for glycylglycine concentrations greater than or equal to 0.025 M. For solute concentrations from 0.025 to 1.2 M, calculated G values of free ammonia are in excellent agreement with measured values. In addition, the computer model predicts a statistically significant difference between the G value of free ammonia produced under X irradiation and that produced under 60Co gamma irradiation.

Ammonia↗
PubMed1990

Measurement of free ammonia produced by X irradiation of glycylglycine in aqueous solution.

This research was initiated to test the validity of predictions based on Monte Carlo calculations of the effect of ionizing radiation on a simple dipeptide. The mechanism for the formation of ammonia, proposed by Garrison, Sokol, and Bennett-Corniea (Radiat. Res. 53, 376-384, 1973), was reevaluated by measuring the yields under deoxygenated and oxygenated conditions. Although free ammonia was formed under both conditions, the yields were different, depending on the concentrations of solute and molecular oxygen. The reaction probabilities of the specific interactions of free radicals formed in pure water with solute and oxygen are discussed to account for the observed difference. Our results obtained after low-dose-rate X irradiation are compared with those obtained by Garrison et al. after high-dose-rate 60Co gamma irradiation.

Ammonia↗
PubMed1987

A method of obtaining neutron dose and dose equivalent from digital measurements and analysis of recoil-particle tracks.

The feasibility of a digital approach to neutron dosimetry has been investigated. Such an approach uses an ionization detector capable of measuring the numbers of electrons produced within various subvolumes of a chamber gas along a charged-particle track. In addition, a computer algorithm is used to infer absorbed dose, LET, and dose equivalent given this digital track-structure information for each event. This paper describes one detector design capable of providing digital track-structure information and discusses examples of proton and C-ion tracks calculated from a Monte Carlo charged-particle transport code. The associated computer algorithm is presented next with its verification accomplished by running a variety of recoil particles through a simulated detector volume and comparing the resulting average energy deposition and dose equivalent to those unfolded by the algorithm.

Neutrons↗
PubMed1983

Indoor radon progeny exposure in the Florida phosphate mining region: a review.

This paper reviews the data generated in studies of land radioactivity and indoor airborne radon progeny associated with mined and reclaimed phosphate lands in Florida. Highest indoor radon progeny levels are associated with the slab-on-grade type of construction. Concentrations exceeding 0.03 WL are associated with overburden soils, deposits and fill, while concentrations up to about 0.03 WL are associated with tailings. The lower limit for distinguishing increases above non-enhanced natural concentrations is on the order of 0.01-0.02 WL. Results of this study show that about 25% of the land produced by present methods of mining and reclamation practices would require restrictions on the type of construction or would require special construction methods. It is projected that with modification of mining and tailings disposal practices, virtually all of the land produced by mining and reclamation would be satisfactory for unrestricted construction use.

Background Radiation↗