Potential improvements in brain dose estimates for internal emitters

Abstract

Background

Element-specific biokinetic models are used to reconstruct doses to systemic tissues from internal emitters. Typically, a systemic model for a radionuclide explicitly depicts only its dominant repositories. Remaining tissues and fluids are aggregated into a pool called Other tissue in which the radionuclide is assumed to be uniformly distributed. In the systemic biokinetic models used in radiation protection, the brain usually is addressed as an implicit mass fraction of Other tissue rather than an explicitly depicted repository. Due to increasing interest in radiation effects on the brain, efforts are underway to improve brain dosimetry for internal radiation sources.

Methods

We assessed potential improvements in brain dosimetry for internal emitters by explicitly modeling brain kinetics rather than treating the brain as a mass fraction of Other tissue. We selected 10 elements for which brain kinetics can be modeled using published biokinetic data. Injection dose coefficients were calculated for a relatively long-lived radioisotope of each element using each of two versions of the ICRP’s latest systemic biokinetic model for the element, the original version and a modified version differing only in the treatment of brain. If the ICRP model contained an explicit brain pool, the modified version depicted brain instead as a mass fraction of Other tissue. If the ICRP model included brain in Other tissue, the modified version included an explicit brain pool with kinetics based on best available brain-specific data.

Results

The result for a given radionuclide is expressed as a ratio A:B, where A and B are the dose coefficients based on the versions of the model with and without an explicit brain pool, respectively. The following ratios A:B were obtained for the 10 radionuclides addressed here: ²⁴¹Am, 0.13; ²⁰⁷Bi, 0.57; ²³⁴U, 0.81; ²³⁹Pu, 0.96; ²⁰³Hg (vapor), 1.4; ¹³⁴Cs, 1.5; ⁵⁴Mn, 1.7; ²¹⁰Po, 1.7; ²²⁶Ra, 1.9; ²¹⁰Pb, 3.3. These ratios indicate that a dose estimate for brain based on a biokinetic model with brain implicitly contained in Other tissue may substantially underestimate or substantially overestimate a dose estimate that reflects best available brain-specific biokinetic data. Of course, the reliability of the latter estimate depends on the quality of the underlying biokinetic data.

Conclusions

Where feasible, the brain should be depicted explicitly in biokinetic models used in epidemiological studies addressing adverse effects of ionizing radiation.

Keywords:

• Radiation dose
• recondtruction
• biokinetics
• brain
• epidemiology

Disclosure statement

The authors declare no conflict of interest. The authors alone are responsible for the content and writing of the paper.

Additional information

Funding

This manuscript and research were supported by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The U.S. Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The work described in this manuscript was sponsored by the U.S. Department of Energy under WAS Project No. 2018-AU-2000MPS, the U.S. Department of Energy, Office of Domestic and International Health Studies (AU‐13), under grant award number DE-HS0000073 (USTUR funding), the Centers for Disease Control and Prevention (CDC) Office of Noncommunicable Diseases, Injury and Environmental Health, National Center for Environmental Health, under Interagency Agreement DOE No. 2220-Z051-16, under contract No. DE-AC05-00OR22725 with UT-Battelle, nd under grant No. 5NUE1EH001315 with the National Council on Radiation Protection and Measurements (NCRP). The authors also are grateful for the financial support received by the NCRP from the U.S. Department of Energy (Grant No. DE-AU0000042) and from the National Aeronautics and Space Administration (Grant No. 80NSSC17M0016).

Notes on contributors

Richard W. Leggett

Richard W. Leggett is a research scientist in the Environmental Sciences Division at Oak Ridge National Laboratory (ORNL). His main research interest is in physiological systems modeling, with primary applications to the biokinetics and dosimetry of radionuclides and radiation risk analysis. He is a member of Committee 2 of the International Commission on Radiological Protection (ICRP) and the ICRP Task Group on Internal Dose Coefficients (IDC). His physiological systems models of the human circulation, skeleton, and gastrointestinal transfer and his systemic biokinetic models for many elements are used by the ICRP as dosimetry and bioassay models. He is the author of ICRP Publication 70, Basic Anatomical and Physiological Data for Use in Radiological Protection: The Skeleton, and co-author of several other ICRP reports.

Sergei Y. Tolmachev

Sergei Y. Tolmachev is Associate Research Professor in the College of Pharmacy and Pharmaceuticals Sciences (COP), Washington State University, where he directs the U.S. Transuranium and Uranium Registries Research Center and the National Human Radiobiology Tissue Repository. His current research interests are in the area of radiation protection, including radionuclide biokinetic modelling, actinide decorporation treatment, retrospective biodosimetry, and study of radioactive nanoparticles as well as application of mass-spectrometric techniques for stable element determination in the human body.

John D. Boice

John D. Boice is President of the National Council on Radiation Protection and Measurements and Professor of Medicine at Vanderbilt University. He is an international authority on radiation effects and served on the Main Commission of the International Commission on Radiological Protection and on the United Nations Scientific Committee on the Effects of Atomic Radiation. He directs the Million Person Study of Low-Dose Health Effects.