The Center for Radiation Protection Knowledge was established in 2010 by a memorandum of understanding (MOU) signed by Oak Ridge National Laboratory and the Department of Energy, Department of Defense, the Environmental Protection Agency, the Nuclear Regulatory Commission, and the Occupational Safety and Health Administration. Keith Eckerman and Rich Leggett of the Human Health Risk & Environmental Analysis Group of ORNL Environmental Sciences Division led this effort to establish this center for the preservation of expertise in radiation dosimetry.
Dr. Nolan Hertel, Professor of Nuclear and Radiological Engineering at Georgia Institute of Technology, accepted a Joint Faculty Appointment in the Environmental Sciences Division in the Risk and Regulatory Analysis Team in the Human Health Risk and Environmental Analysis Group. Nolan will spend approximately 80% of the remaining calendar year in residence at ORNL, and now serves as the acting director of the Center for Radiation Protection Knowledge. He will assist in providing direction to the radiation protection dosimetry program as ORNL seeks to preserve and expand this important activity, which has a 50-year history of excellence in the development and use of models and methodology for radiation protection. The Center is responsive to the participating agencies and is administered by DOE. This MOU is intended to help maintain and preserve U.S. expertise in radiation dosimetry and to ensure that Federal radiation programs are based on the best available information, applied in a consistent manner. For those interested, the MOU can be found here.
ORNL Dosimetry Research Program History
The ORNL Dosimetry Research Program was started in the 1950s by K. Z. Morgan, Director of ORNL's Health Physics Division and an early recipient of the Swedish Royal Academy Gold Medal for Radiation Protection. Since 1979 this program has been led by Keith Eckerman, recipient of the most recent (12th) Swedish Royal Academy Gold Medal.
Since its inception the ORNL Dosimetry Research Program has provided the national and international scientific communities with models and data required to estimate doses from exposure to radionuclides and establish exposure guidelines for radionuclides. The models developed in this program generally have become international standards. The program also has traditionally served as a center for archival and computer implementation of biokinetic and dosimetric models.
Models and Data Developed by ORNL
The estimation of radiation doses to human tissues from exposure to radionuclides involves application of biokinetic and dosimetric models. Biokinetic models describe the biological behavior (i.e., the time-dependent distribution, retention, and excretion) of radionuclides deposited in the human body through inhalation, ingestion, wounds, or injection. Dosimetric models describe radiation transport from biokinetic model-predicted sites of nuclear transformation ('decay') of radionuclides in the body to sites of deposition of the energy of the ionizing radiations emitted during decay. Dosimetric models are also used to estimate tissue doses from external sources of ionizing radiation such as medical imaging equipment or radionuclides in the environment or workplace. Since the 1950s the ORNL Dosimetry Research Program has been one of the primary international sources for biokinetic and dosimetric models for radionuclides.
The framework of a dosimetric model is an anatomical 'phantom' representing the approximate masses, shapes, and spatial relationships of organs and tissues of a reference (typical) person of a given sex and age. A dosimetric phantom is used together with nuclear decay data (described later) for radionuclides to predict the sites of deposition of energy in the body from penetrating ionizing radiations emitted either from internally deposited radionuclides or from external sources.
ORNL's first anatomical phantom was developed in the 1960s and represented a reference adult hermaphrodite to allow calculation of doses to both male and female sex organs. For computation convenience (e.g., for Monte Carlo calculations of cross-irradiation of tissues) the organs and tissues and external body surfaces of the ORNL phantom were represented by union and intersection of regular geometric shapes such as ellipsoids and cylinders. The impetus for this phantom was the concern expressed by the Medical Internal Radiation Dose (MIRD) Committee (representing the nuclear medicine community) that dosimetry methods used at that time for time photon-emitting radiopharmaceutical sources were not credible because they did not account for cross irradiation of tissues. The ORNL phantom was adopted by MIRD for applications in nuclear medicine and later by the International Commission on Radiological Protection (ICRP) for use in a series of reports that provided guidance on intake of radionuclides by workers (ICRP Publications 30, 1979-1988; ICRP Publication 68, 1994). In the late 1970s ORNL added analogous phantoms for five pre-adult ages: infant, 1, 5, 10, and 15 years. In the mid-1980s the age-specific phantoms were modified and an adult female phantom was added. The modified age-specific phantoms were adopted by the ICRP for use in a series of reports on age-specific doses to members of the public from intake of radionuclides (ICRP Publications 56, 67, 69, 71, and 72, published from 1989-1996). ORNL's age-specific anatomical phantoms are illustrated in Figure 1.
ORNL's biokinetic models have evolved considerably over the years. Biokinetic models developed from the 1950s to early 1980s generally were simple mathematical formulae ('retention functions') derived as curve fits to observed behavior of element tracers in human subjects or laboratory animals. Biokinetic models developed through the early 1980s applied only to a reference adult. Work on a set of age-specific biokinetic models was initiated in 1982. Due to the sparsity of radiobiological data for children, the development of age-specific biokinetic models required a significant departure from the empirical curve-fitting approach traditionally used. A process-oriented 'physiological systems' modeling approach was designed to facilitate the extrapolation of age-specific biokinetic data from laboratory animals to humans and to make the best use of information on changes with age in the physiological processes judged to govern the distribution, retention, and translocation of an internally deposited substance.
Two categories of physiologically based biokinetic models have been developed by ORNL since the early 1980s: (1) models of basic physiological systems such as blood circulation or transit of material through the gastrointestinal transit, and (2) element-specific models, i.e., models that address the characteristic behavior of individual elements in the body.
An example of a basic physiological systems model is the dynamic blood flow model developed by ORNL. The model structure is shown in Figure 2. Age- and sex-specific parameter values describing the distribution of cardiac output, blood perfusion rates of tissues, blood volumes of tissues, and transport times between compartments were developed from an extensive review of the literature on blood flow. The model provides a detailed description of the distribution of cardiac output and of regional blood volumes and can be used to follow the early movement of a bolus of activity beginning at the point of injection. This blood flow model has been used to derive dose estimates for ultra-short-lived radionuclides used in nuclear medicine, and as a model framework for development of element-specific biokinetic models for elements whose initial distribution is determined largely by the distribution of cardiac output (e.g., potassium, cesium, and radon).
An example of a physiologically based element-specific biokinetic model developed at ORNL is the age-specific model for lead. The model structure is shown in Figure 3. Age- and sex-specific parameter values describing the time-dependent distribution, retention, and paths of excretion of lead were developed from a comprehensive review of the literature on the fate of lead in the human body. The model is applied in ICRP documents to derive dose coefficients for radioisotopes of lead, by the EPA to assess the potential chemical toxicity of lead sources to children, and by the state of California to set standards for occupational exposure to lead as a chemical toxin.
The ICRP has now adopted ORNL's element-specific biokinetic models for a large portion of the periodic chart. Many of these models are also applied by other organizations for a variety of purposes, including nutrition studies of essential elements, assessment of chemical toxicity (as illustrated above for lead), and radiation dose reconstruction for epidemiological studies. In an interesting criminal case, ORNL's biokinetic model for polonium was used as a forensic tool to estimate the amount of polonium-210 that had been used to poison a former Russian spy.
Since the 1950s the ORNL Dosimetry Research Program has also served as a center for development of data bases required in radiation protection. Two of the most important and widely used types of data are (1) nuclear decay data and (2) reference anatomical and physiological data.
The nuclear decay data for a given radionuclide include its radiological half-life, the types of radiations emitted (e.g., beta particles, alpha particles, gamma rays), and the energies and yields of the emitted radiations. In the early 1970s ORNL compiled nuclear decay data for radionuclides of interest in nuclear medicine, as part of ORNL's long-term cooperative efforts with the MIRD Committee. From the mid-1970s to the early 1980s ORNL compiled nuclear decay data for over 800 radionuclides of potential importance in the workplace. The compilation was published by the ICRP in its Publication 38 (1983). Recently ORNL led the development of ICRP Publication 107 (2008), which updates the nuclear decay data of ICRP Publication 38 and extensions the database to 1252 radionuclides.
From the mid-1960s to the mid-1970s ORNL led an ICRP effort to define typical anatomical and physiological features of a 'Reference Man' in order to provide a common international basis for biokinetic and dosimetric models for an adult. From the early 1990s to early 2000s ORNL led an ICRP effort to develop typical anatomical and physiological features of a "Reference Family" that would provide a common international basis for development of age-specific biokinetic and dosimetric models, for improved dose estimates for the general public.
Contributions to National and International Committees
International Commission on Radiological Protection (ICRP): Members of the ORNL Dosimetry Research Program have served on many ICRP committees or task groups since the 1950s. At present we are represented on ICRP Committee 2 (radiation dosimetry), Task Group DOCAL on dose calculations, and Task Group INDOS (biokinetic modeling). ORNL has produced or been a major contributor to a variety of ICRP reports issued since the late 1950s, including reports on limits on occupational intake of radionuclides, compilations of nuclear decay data, reference anatomical and physiological features of the human body, and biokinetic models. For several years ORNL has served as the lead organization for development of the ICRP's biokinetic models for systemic (absorbed) radionuclides for workers and members of the public.