John W. Poston

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.

Principles of nuclear radiation detection

This book covers the transistorization of equipment and provides an introduction into practice of semiconductor and thermoluminescent detectors. It discusses the principles of radiation detectors most widely used in nuclear technology, medical practice and radiation protection. It stresses the alternative detectors available and discusses practical considerations in choosing and setting up detector systems for actual use. Traditional materials, including semiconductors, TLDs and modern data handling facilities are covered.

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.

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.

Long-lived impurities of 90Y-labeled microspheres, TheraSphere and SIR-spheres, and the impact on patient dose and waste management.

AbstractYittrium-90 microsphere brachytherapy procedures have increased in number due to their efficacy in treating some unresectable metastatic liver tumors. The discovery of long-lived impurities in two microsphere products, first reported between 2006 and 2007, has resulted in some radiation safety concerns. Since then, microsphere production processes have been refined, which reportedly lead to a reduction in detectable by-products. In this study unused vials of TheraSphere and SIR-Spheres, manufactured in early January 2011, were analyzed to identify and quantify the low-level radioactive impurities. Absorbed dose calculations were performed to assess the potential increased dose to the patient due to long-lived impurities. Results showed that while the SIR-Spheres vials contained no detectable impurities (contrary to other published results in the literature), the TheraSphere vials contained 17 radionuclides in one sample and 15 in the other. The dominant impurities were 91Y and 88Y, with specific activities ranging from 0.99 ± 3.40 × 10−4 kBq mg−1 to 6.30 ± 0.40 kBq mg−1 at vendor assay date. Other impurities were on the order of Bq mg−1. Based on Medical Internal Radiation Dose (MIRD) liver and lung dose estimates, the long-lived impurities would be expected to increase an administered dose by less than 0.1% from the prescribed dose.

Development of new two-dosimeter algorithm for effective dose in ICRP Publication 103.

The two-dosimeter method, which employs one dosimeter on the chest and the other on the back, determines the effective dose with sufficient accuracy for complex or unknown irradiation geometries. The two-dosimeter method, with a suitable algorithm, neither significantly overestimates (in most cases) nor seriously underestimates the effective dose, not even for extreme exposure geometries. Recently, however, the definition of the effective dose itself was changed in ICRP Publication 103; that is, the organ and tissue configuration employed in calculations of effective dose, along with the related tissue weighting factors, was significantly modified. In the present study, therefore, a two-dosimeter algorithm was developed for the new ICRP 103 definition of effective dose. To that end, first, effective doses and personal dosimeter responses were calculated using the ICRP reference phantoms and the MCNPX code for many incident beam directions. Next, a systematic analysis of the calculated values was performed to determine an optimal algorithm. Finally, the developed algorithm was tested by applying it to beam irradiation geometries specifically selected as extreme exposure geometries, and the results were compared with those for the previous algorithm that had been developed for the effective dose given in ICRP Publication 60.

Estimates of Absorbed Energy in Trabecular Bone Due to Beta-particles Or Electrons

A computer code, DAB-BE, has been written which can be used to calculate the absorbed energies of beta particles or electrons deposited inside human trabecular bone. The radiation source geometry can be either uniformly distributed in the whole bone or located at a fixed point inside the bone. Bremsstrahlung effects are not considered in these calculations. Results are presented and discussed for six monoenergetic sources of electrons distributed uniformly in a mathematical representation of the arm. In addition, a fixed point source configuration of 1.0 MeV monoenergetic electrons was studied.

90Sr content in 90Y-labeled SIR-spheres and Zevalin.

AbstractThree different 90Y internally-administered radionuclide therapies are currently used in both standard-of-care and clinical trial procedures at MD Anderson Cancer Center. TheraSphere and SIR-Spheres therapies utilize 90Y-labeled microspheres, while Zevalin is an 90Y-labeled radioimmunotherapeutic agent. Several publications have indicated radionuclidic impurities resulting from 90Y production methods. The 90Y in SIR-Spheres and Zevalin are produced from a 90Sr/90Y generator, which leaves measurable quantities of 90Sr in the final product. TheraSphere 90Y is produced in a nuclear reactor which results in a large number of impurities, most notably 88Y and 91Y. Product information sheets reference these impurities with specific limits given. These limits represent a tiny fraction of the total product activity, and in the case of TheraSphere and SIR-Spheres gamma-emitting impurities, this has been verified in the literature. An analysis of 90Sr impurities in SIR-Spheres and Zevalin is presented in this paper. Impurity quantities were found to be within the vendors’ documented limits.

How do we combine science and regulations for decision making following a terrorist incident involving radioactive materials

Approaches to safety regulations-particularly radiation safety regulations-must be founded on the very best science possible. However, radiation safety regulations always lag behind the science for a number of reasons. First, the normal scientific process of peer-review, debate, and confirmation must ensure that the conclusions are indeed correct, the implications of the research are fully understood, and a consensus has been established. Second, in the U.S., there is a well-established, all-inclusive political process that leads to changes in radiation safety regulations. This process can take a very long time, as was demonstrated when the process was initiated to change the Code of Federal Regulations more than 20 y ago in response to International Commission on Radiation Protection Publication 26 and other recommendations. Currently, we find ourselves in a situation where the possibility of a terrorist radiological attack may occur and where the existing body of regulations provides very little guidance. Many international and national bodies, including several federal agencies, have provided recommendations on the appropriate levels of exposure for first-responders and first-receivers, as well as for the general public. However, some agencies provide guidelines based on very conservative dose limits which are not appropriate in situations where there is a substantial chance for the loss of lives and critical infrastructure. It is important that an emergency response is not hampered by overly cautious guidelines or regulations. In a number of exercises the impact of disparate guidelines and training in radiological situations has highlighted the need for clear reasonable limits that maximize the benefit from an emergency response and for any cleanup after the incident. This presentation will focus first on the federal infrastructure established to respond to radiological accidents and incidents. It will review briefly the major recommendations, both international and national, for responders and will attempt, where possible, to establish the scientific foundation for these guidelines. We will also stress the need to clearly and openly communicate the recommendations to the first-responders and the public so that no unnecessary anxiety or associated actions on their part impedes the ability to respond to a disaster. Finally, the use of these guidelines and recommendations by decision-makers at all levels will be discussed.

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.