A revised model for electron dosimetry in the human small intestine.

In this study, the absorbed dose was calculated to the small intestine (SI) wall of an adult human from electrons in its lumen contents. The effects on dose due to variations in the lumen radius and wall-thickness also were studied. The SI model was based on values gleaned from anatomic and histologic reviews of the adult human SI. Histologic and radiological analyses of the SI suggested the microscopic intricacy of this walled organ could be avoided for dosimetric purposes and a set of concentric cylinders could be used to model the SI. The model was input into the Monte Carlo N-Particle (MCNP) version 4A computational package, which was used to simulate energy deposition in the SI by electrons of fifty discrete energies ranging 10-500 keV. The source electrons as well as all resulting particles, such as knock-on electrons, bremsstrahlung, and electrons created from bremsstrahlung interactions, were transported until the particle energies fell below the 1 keV low-energy cutoff. Detailed physics treatments for secondary photons were made. With a reasonable number of histories, appropriate variance reduction techniques were used to improve the precision of the Monte Carlo calculations. The model used very small tally regions, which ranged in thickness from 0.5 microm to 200 microm depending on the electron energy studied and tally location in the wall. Relative errors associated with these calculations were maintained at less than 5%. The large number of tally results across the wall for each of the energies studied enabled the construction of the energy-specific depth dose curves in the wall. Each of these curves was consistent with the anticipated energy deposition pattern. These curves showed that only a small fraction of the energy absorbed at the contents-mucus interface reaches the stem cell layers because the cells are located deep in the mucosa. This fraction was found to vary from 1.66 x 10(-6) to 1.21 x 10(-1) over the energy range 10-500 keV. These results demonstrated the interface dose, which has been routinely reported as the "wall" dose, is a significant overestimate of the actual dose to the stem cells. The dose uncertainties associated with variations of the critical cell depth were shown to be very high for electrons whose CSDA ranges in the soft tissue exceeded the depth of the critical cells. This study showed that the uncertainty in the wall-thickness had no effect on depth doses while variation in the lumen radius significantly changes depth doses. The results suggest that these changes could be approximated by the inverse square of the lumen radius.

Radiological assessment of petroleum pipe scale from pipe-rattling operations.

Petroleum pipe scale, consisting of concentrated inorganic solids such as barium sulfate, can deposit on the inside of down-hole pipes during the normal course of oil field pumping operations. A portion of this scale has been shown to contain naturally occurring radioactive materials (NORM), predominantly compounds of radium. When these pipes are removed from the well, there is a potential for radiation doses to the oil field workers handling the pipes, especially as the pipes are cleaned for reuse. A thorough sampling and measurement protocol was applied under a variety of weather conditions in an outdoor laboratory to obtain an accurate indication of the radiological and aerodynamic characteristics of scale release and dust dispersion during petroleum pipe scale removal from out-of-service pipes with a restored, historically relevant outdoor pipe-cleaning machine. Exposure rate data were also obtained for both the pre-cleaned pipes, and the general area inhabited by workers during the descaling operation. Four radiation exposure pathways were investigated: inhalation of pipe scale dust generated during pipe rattling, incidental ingestion of the pipe scale dust, external exposure from uncleaned pipes, and external exposure from pipe scale dispersed on the ground. Pipes from three oil fields were rattled to collect as much industry-representative data as possible. The Ra specific activity of the pipe scale ranged from 33.6 +/- 0.4 to 65.5 +/- 0.7 Bq g, depending on the formation. A median atmospheric dust loading of 0.13 mg m was measured in the operator breathing zone. The respirable fraction was observed to be about 42% to 46%. Based on cleaning 20 pipes per day,250 d per year on average, annual committed effective doses for the operator and helper ranged from 0.11 mSv (11 mrem) to 0.45 mSv(45 mrem) for inhalation and from 19 microSv (1.9 mrem) to 97 microSv (9.7 mrem) for incidental ingestion. Worker annual external dose from the pipe racks ranged from 0 to 0.28 mSv (28 mrem). In the deposition experiment, more than 99% by weight of the deposited scale fell within 2 m of the machine centerline, the vast majority of which was in the downwind direction. The dose from this deposited material dominated the worker dose estimates. The annual external dose from dispersed material was estimated to be 2.8 mSv (280 mrem) for the operator and 4.1 mSv (410 mrem) for the helper.

A dosimetric model for inhaled radioactive gases.

Mathematical simulation models have been used to study transport of insoluble and nonreactive gases for more than twenty years. However, gas and vapor transport and uptake still are not well understood, and a mathematical model for slightly soluble and nonreactive gas transport and uptake still has not been developed. This paper describes the development of a mathematical model of diffusion, convection, lateral transport into the airway wall, and alveolar absorption for inhaled radioactive gases in human conductive and respiratory airways. The model is based on a single-path trumpet-bell model. Sensitivity studies were conducted to ascertain the influence on the final model of the functional residual capacity, the tidal volume and diffusivity and solubility. Results obtained with this model are presented for HT gas exposure and are compared with other findings. In general, the results obtained in this research are in good agreement with other mean experimental results.

Absorbed dose from traversing spherically symmetric, Gaussian radioactive clouds.

If a large radioactive cloud is produced, sampling may require that an airplane traverse the cloud. A method to predict the absorbed dose to the aircrew from penetrating the radioactive cloud is needed. Dose rates throughout spherically symmetric Gaussian clouds of various sizes, and the absorbed doses from traversing the clouds, were calculated. Cloud size is a dominant parameter causing dose to vary by orders of magnitude for a given dose rate measured at some distance. A method to determine cloud size, based on dose rate readings at two or more distances from the cloud center, was developed. This method, however, failed to resolve the smallest cloud sizes from measurements made at 1,000 m to 2,000 m from the cloud center.

MIRD Pamphlet No. 15: Radionuclide S values in a revised dosimetric model of the adult head and brain. Medical Internal Radiation Dose.

UNLABELLED: Current dosimetric models of the brain and head lack the anatomic detail needed to provide the physical data necessary for suborgan brain dosimetry. 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 for use in estimating regional absorbed dose within the brain and its adjacent structures. METHODS: A new brain model has been developed that includes eight subregions: the caudate nuclei, the cerebellum, the cerebral cortex, the lateral ventricles, the lentiform nuclei, the thalami, the third ventricle and the white matter. This brain model is incorporated within a total revision of the head model presented in MIRD Pamphlet No. 5 Revised. Modifications include the addition of the eyes, the teeth, the mandible, an upper facial region, a neck region and the cerebrospinal fluid within both the cranial and spinal regions. RESULTS: Absorbed fractions of energy for photon and electron sources located in 14 source regions within the new model were calculated using the EGS4 Monte Carlo radiation transport code for particles in the energy range 10 keV-4 MeV. These absorbed fractions were then used along with radionuclide decay data to generate S values for 24 radionuclides that are used in clinical or investigational studies of the brain, 12 radionuclides that localize within the cranium and spinal skeleton and 12 radionuclides that selectively localize in the thyroid gland. CONCLUSION: A substantial revision to the dosimetric model of the adult head and brain originally published in MIRD Pamphlet No. 5 Revised is presented. This revision supports suborgan brain dosimetry for a variety of radiopharmaceuticals used in neuroimaging. Dose calculations for the neuroimaging agent 1231-tropane provide an example of the new model and yield mean brain doses that are consistent with published values. However, the absorbed dose to subregions within the brain such as the caudate and lentiform nuclei may exceed the average brain dose by a factor of up to 5.

Calculation of effective doses for broad parallel photon beams.

Values of effective dose (E) were calculated for the entire range of incident directions of broad parallel photon beams for selected photon energies using the Monte Carlo N-Particle (MCNP) transport code with a hermaphroditic phantom. The calculated results are presented in terms of conversion coefficients transforming air kerma to effective dose. This study also compared the numerical values of E and H(E) over the entire range of incident beam directions. E was always less than H(E) considering all beam directions and photon energies, but the differences were not significant except when a photon beam approaches some specific directions (overhead and underfoot). This result suggests that the current H(E) values can be directly interpreted as E or, at least, as a conservative value of E without knowing the details of irradiation geometries. Finally, based on the distributions of H(E) and E over the beam directions, this study proposes ideal angular response factors for personal dosimeters that can be used to improve the angular response properties of personal dosimeters for off-normal incident photons.

Effective dose equivalent and effective dose for photon exposures from point and disk sources on the floor.

A complete set of H(E) and E values were calculated for photon exposures from point and disk sources on the floor using the MCNP code and a "hermaphroditic" phantom. It was found that a male can receive a higher H(E) or health risks than a female by a factor of two from an identical point source on the floor when source distance is less than 50 cm. Conversely, if the source distance becomes larger than 100 cm, the female receives H(E) higher than the male by up to 40%. For identical sources, both the male and female experience significantly higher H(E) from front-located sources than from back- or side-located sources. For a 100-cm source distance, male H(E) from a front-located source is greater than that from a side-located source by factors of 4, 3, and 2 for 0.08, 03, and 1.0 MeV photons, respectively. In the female cases, the differences are somewhat smaller but still differ by factors of 3, 2, and 1.7. It was also found that both the highest male and female H(E) values occur when a source is within 40-60 cm in front of the phantom. The maximum male H(E) is 1.8 x 10(-18), 6.6 x 10(-18), and 2.1 x 10(-17) Sv per photon emission for 0.08, 03, and 1.0 MeV photons, respectively. For females, these maximum values are slightly smaller, 1.4 x 10(-18), 5.3 x 10(-18), and 1.9 x 10(-17) Sv/photon, respectively. Tissue kerma free-in-air at 100 cm above a disk source (Ktissue) was found to greatly overestimate H(E) if the source radius is less than 200 cm. For radii larger than 200 cm, the Ktissue gives a relatively better estimate of H(E), overestimating by not more than 100%. The point source H(E) values were directly integrated to estimate H(E) for simple non-self-shielding sources such as disk, circle, and line sources. This simple approach was found to overestimate H(E) by less than 10% for these irradiation geometries. Finally, the comparison of H(E) and E showed that for most cases these values are almost identical. For point sources, when source distance is larger than 50 cm, the difference between H(E) and E was always less than 23% over photon energies between 0.08 and 1.0 MeV. For disk sources of radius larger than 50 cm, the difference was even smaller (<12%).

Calculation of the dose distribution in water from 71Ge K-shell x-rays.

The dose distribution in water from 71Ge K-shell x-rays (Eave = 9.44 keV) was calculated for various source configurations using both analytic and EGS4 Monte Carlo calculations. The point source kernel and the buildup factor are presented. The buildup factor for a point source in water has been found to increase up to about 1.1 as radial distance approaches 1 cm. Comparison between 71Ge and 90Sr/Y shows a similarity between their relative dose distribution in water. The dose distribution from a disc source was calculated using the EGS4 code and compared with the results from analytic calculation. Excellent agreement was observed, confirming the validity of analytic calculations. The dose rate at 0.01 cm from a 71Ge disc source was calculated to be about 1.3 x 10(-5) Gy MBq-1 s-1. Based on the results from this study, 71Ge activity of the order of 3.7 x 10(10) Bq (approximately 1 Ci) might be necessary to obtain dose rates typical of 90Sr/Y ophthalmic applicators. The possibility of using 71Ge as a source of radioactive stents was also investigated. A 71Ge stent was modelled as a cylindrical shell source and the dose rates were determined by Monte Carlo calculations. Some calculated results are compared with published values for a 32P-coated stent. The dose rate at 0.01 cm from a 71Ge stent has been calculated to be about 6.5 x 10(-3) Gy MBq-1 h-1, which is much lower than the reported dose rate at the same distance from a 32P-coated stent. However, an initial source activity of the order of 3.7 x 10(7) Bq (approximately 1 mCi) would easily result in a typical target dose (approximately 24 Gy) needed for intravascular stent applications. In conclusion, 71Ge sources could be used as alternatives to beta sources and, unlike high-energy (approximately MeV) beta sources, may provide easily predictable dose distributions in heterogeneous media and low dose rates, which might be beneficial for some clinical applications.

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.

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.

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.

Dose equivalent near the bone-soft tissue interface from nuclear fragments produced by high-energy protons.

During manned space missions, high-energy nucleons of cosmic and solar origin collide with atomic nuclei of the human body and produce a broad linear energy transfer spectrum of secondary particles, called target fragments. These nuclear fragments are often more biologically harmful than the direct ionization of the incident nucleon. That these secondary particles increase tissue absorbed dose in regions adjacent to the bone-soft tissue interface was demonstrated in a previous publication. To assess radiological risks to tissue near the bone-soft tissue interface, a computer transport model for nuclear fragments produced by high energy nucleons was used in this study to calculate integral linear energy transfer spectra and dose equivalents resulting from nuclear collisions of 1-GeV protons transversing bone and red bone marrow. In terms of dose equivalent averaged over trabecular bone marrow, target fragments emitted from interactions in both tissues are predicted to be at least as important as the direct ionization of the primary protons-twice as important, if recently recommended radiation weighting factors and "worst-case" geometry are used. The use of conventional dosimetry (absorbed dose weighted by aa linear energy transfer-dependent quality factor) as an appropriate framework for predicting risk from low fluences of high-linear energy transfer target fragments is discussed.

Experience in the development of a postmarketing surveillance network: the pharmacy medication monitoring program.

OBJECTIVE: To describe the pilot and early implementation phase of a system for assembling and recruiting cohorts of patients taking selected prescription medications and prospectively monitoring them for new health events. DESIGN: Prospective observational study, based on telephone interviews, of 1475 patients filling prescriptions for a nonsteroidal antiinflammatory drug (NSAID). Patients were interviewed by telephone using trained interviewers at a central site. Hospitalizations and deaths were followed up and reviewed by an independently physician. SETTING: Community setting in a region of Hamilton, Ontario, Canada. PARTICIPANTS: All consenting patients filling new or repeat prescriptions for NSAIDs at participating pharmacies. MAIN OUTCOME MEASURES: The authors report on the development and assessment of systems for: (1) ongoing recruitment of patients through community pharmacies; (2) data transfer from pharmacies to the coordinating center; (3) surveying patients; (4) classifying, coding, and evaluating new health events; and (5) following up on new serious adverse events. RESULTS: Fifty-one percent of patients approached were recruited, and 83% of these provided completed interviews. For patients picking up their own medications, pharmacy workload varied from 4 to 10 minutes per patient approached. Nineteen percent of patients reported having a new health problem or unusual symptom at the initial telephone interview. Reported health-related events were similar to those described in other studies of NSAIDs. CONCLUSIONS: Most aspects of the monitoring system performed well. One limitation was the low recruitment rate for patients who did not directly drop off or pick up their own prescriptions. Even so, this method of patient accrual may complement alternative monitoring programs.

Development of a method for calibrating in vivo measurement systems using magnetic resonance imaging and Monte Carlo computations.

Research efforts towards developing a new method for calibrating in vivo measurement systems using magnetic resonance imaging (MRI) and Monte Carlo computations are discussed. The method employs the enhanced three-point Dixon technique for producing pure fat and pure water MR images of the human body. The MR images are used to define the geometry and composition of the scattering media for transport calculations using the general-purpose Monte Carlo code MCNP, Version 4. A sample case for developing the new method utilizing an adipose/muscle matrix is compared with laboratory measurements. Verification of the integrated MRI-MCNP method has been done for a specially designed phantom composed of fat, water, air, and a bone-substitute material. Implementation of the MRI-MCNP method is demonstrated for a low-energy, lung counting in vivo measurement system. Limitations and solutions regarding the presented method are discussed.

A study of the angular dependence problem in effective dose equivalent assessment.

The newly revised American National Standard N13.11 (1993) includes measurements of angular response as part of personnel dosimeter performance testing. However, data on effective dose equivalent (HE), the principle limiting quantity defined in International Commission on Radiological Protection (ICRP) Publication 26 and later adopted by U.S. Nuclear Regulatory Commission (NRC), for radiation incident on the body from off-normal angles are little seen in the literature. The absence of scientific data has led to unnecessarily conservative approaches in radiation protection practices. This paper presents a new set of fluence-to-HE conversion factors as a function of radiation angles and sex for monoenergetic photon beams of 0.08, 0.3, and 1.0 MeV. A Monte Carlo transport code (MCNP) and sex-specific anthropomorphic phantoms were used in this study. Results indicate that Anterior-posterior (AP) exposure produces the highest HE per unit photon fluence in all cases. Posterior-anterior (PA) exposure produces the highest HE among beams incident from the rear half-plane of the body. HE decreases dramatically as one departs from the AP and PA orientations. The results also indicate that overestimations caused by using isotropic dosimeters in assessing effective dose equivalent from near-overhead and near-underfoot exposures are 550%, 390%, and 254% for 0.08, 0.3, and 1.0 MeV, respectively. Comparisons of the angular dependence of HE with those based on the secondary quantities defined in International Commission on Radiation Units and Measurements (ICRU) Reports 39, 43, and 47 show significant differences. This paper discusses why more accurate assessments of HE are necessary and possible. An empirical equation is proposed which can be used as the optimum dosimeter angular response function for radiation angles ranging from 0 degrees to 90 degrees for dosimeter calibration, performance testing, and design.

Adverse drug effects and the need for drug information.

The information needs of a group of patients taking antihypertensive medication were assessed with special emphasis on the influence of perceived symptoms of high blood pressure and adverse drug effects. All patients of a hypertension clinic currently on antihypertensive medication were included in the study. The response rate to the questionnaire was 85%. Of the 623 patients included, only 31% expressed satisfaction with the amount of information received on adverse effects of their antihypertensive drugs. Patients younger than 50 years explicitly expressed a need for information more often than those older than 64. There were no differences in the expressed information needs between men and women. The reported experience of symptoms related to high blood pressure and adverse drug effects was more common among younger patients than among the elderly. Of those who experienced both adverse drug effects and symptoms, 57% expressed a need for more information, whereas only 30% of those who had no such experiences expressed a need for more information on adverse drug effects. It was concluded that there is a substantial need for more information on adverse drug effects, especially among those who have experienced adverse drug effects or some symptoms of hypertension.

Absorbed dose calculations to blood and blood vessels for internally deposited radionuclides.

At present, absorbed dose calculations for radionuclides in the human circulatory system used relatively simple models and are restricted in their applications. To determine absorbed doses to the blood and to the surface of the blood vessel wall, EGS4 Monte Carlo calculations were performed. Absorbed doses were calculated for the blood and the blood vessel wall (lumen) for different blood vessels sizes. The radionuclides chosen for this study were those commonly used in nuclear medicine. No penetration of the radionuclide into the blood vessel was assumed nor was cross fire between the vessel assumed. The results are useful in assessing the dose to blood and blood vessel walls for different nuclear medicine procedures.