2023 HPS68 | U.S. Transuranium and Uranium Registries

^{U.S. Transuranium and Uranium Registries} _{Conference Contributions}

Health Physics Society Meeting, National Harbor, MD, July 23-26, 2023

Four USTUR faculty members were authors on six presentations at the 67^th Annual Meeting of the Health Physics Society in National Harbor, MD on July 23-26, 2023. A wide variety of topics were presented, including the long-term retention of plutonium in respiratory tract tissues, uncertainty in estimates plutonium activity in the skeleton, plutonium in Rocky Flats workers, the micro-distribution of ²²⁶Ra in radium dial painter bones, and topics related to the Million Person Study.

Joey Y. Zhou (DOE), Maia Avtandilashvili (USTUR), Martin Šefl (USTUR), George Tabatadze (USTUR), Sergey Y. Tolmachev^† (USTUR)

At the United States Transuranium and Uranium Registries (USTUR), 87% of deceased Registrants are partial-body tissue donors with two to six bones commonly collected at autopsy. The most collected bone samples are rib, sternum, vertebral body, patella, clavicle, and femur middle shaft. The most frequent 2-bone combinations are rib/sternum (157) and rib/ vertebral body (155). These bone samples are relatively easier to collect postmortem and, therefore, they are non-random convenience samples not representative of the entire skeleton. To estimate plutonium activity concentration in the skeleton (C_skel) from measured concentrations in individual bone samples (C_bone), the USTUR has developed a latent bone modelling (LBM) approach based on principal component regression (PCR) using C_skel values for 13 non-osteoporotic whole-body tissue donors, as well as C_bone data for 90 samples from each donor. LBM was implemented by using Python into a simple graphical user interface, EasySkelLBM, which calculates C_skel and associated uncertainty from two or more bone sample measurements. Although LBM has been shown to provide a better estimate of C_skel compared to the average of C_bone values, the associated uncertainty is based on convenience sample only and does not represent the true uncertainty in the C_skel estimate. This study developed a method to improve uncertainty estimates of C_skel based on all possible C_bone combinations of representative bones. To select representative bones, a loading plot of principal component analysis (PCA) was used to identify clusters or highly correlated bones, and 13 bones were selected from the original 90 bones. The PCR was then applied to 13 cases and 13 selected bones. For a given number of bones, PCR models were fit for all possible bone combinations. There are 78, 286, 715, 1,287, and 1,716 combinations for 2, 3, 4, 5 and 6 bones, respectively. The standard deviation of the residuals (SD_res) of all combinations for each of the 13 cases was used to determine the uncertainties associated with the estimated C_skel. Linear regression was used to derive a relationship between SD_res and C_skel for a given number of bone samples. The results showed that the most collected bone samples were clustered on the PCA loading plot, and, therefore, underestimated uncertainties. In general, the higher uncertainties were associated with a lower C_skel and a smaller number of analyzed bones. The coefficient of variation (SD_res/C_skel) was stable at C_skel ≥ 10 Bq kg^-1. [USTUR-0638-23A] ^†Presenter

Martin Šefl (USTUR), Maia Avtandilashvili (USTUR), Joey Y. Zhou (DOE), Sergey Y. Tolmachev (USTUR)

Radiation epidemiology typically relies on dose predictions based on bioassay monitoring data, most commonly, in-vitro urinary excretion measurements and, in some cases, in-vivo examinations of the whole body or organ activities. At the United States Transuranium and Uranium Registries (USTUR), bioassay data and post-mortem tissue radiochemical analyses are used for actinide biokinetic modeling and estimation of radiation doses. To evaluate uncertainties in radiation dose estimates from internally deposited ²³⁹Pu, a group of 25 former nuclear workers from Rocky Flats Plant was selected from the USTUR health physics database. The selected workers had at least 5 positive urine samples (more than the contemporary minimum detectable activity) and did not undergo an extensive chelation treatment. A preliminary analysis was performed on nine individuals from the group exposed to ‘high-fired’ PuO₂ aerosols in the same incident. For six workers, this was the only intake, for three others, an additional wound intake was considered. The measured ²³⁹Pu activities were between 9.4 and 123 Bq in the liver, between 9.2 and 215 Bq in the skeleton, and between 92.9 and 7,540 Bq in the lungs. Post-mortem activities in the lungs and liver+skeleton were compared to the predictions based on bioassay measurements (urine, chest counts). For the liver+skeleton, the predicted activity was on average 40.7% higher than the measured activity; for the respiratory tract, the predicted activity was on average 16.2% lower than the post-mortem measurements. Committed effective doses (E50) calculated using only bioassay data were compared to the doses calculated using bioassay together with post-mortem tissue analysis results (liver+skeleton and lungs). The results show that using post-mortem tissue analysis results increased the estimated E50 on average only by 4.2%. [USTUR-0639-23A]

Presentation Slides

George Tabatadze (USTUR) and Sergey Tolmachev (USTUR)

The United States Transuranium and Uranium Registries (USTUR) studies actinide biokinetics and tissue dosimetry by following up occupationally exposed individuals. Estimation of the micro-scale distribution of radionuclides in tissues is an important task to support dose assessment. Previously, an ionizing radiation quantum imaging detector (iQID) system was used to study micro-distribution of 239 Pu and 241 Am. In this study, 226 Ra micro-distribution was mapped in bones of a radium dial painter (RDP) who worked for 6 years, had estimated 226 Ra uptake of 58.9 MBq, and died at age 24. These samples were obtained from the National Human Radiobiology Tissue Repository (NHRTR), which is a part of the Registries. The NHRTR holds collection of tissue materials obtained from various radium worker studies, including histological bone slides and tissue blocks from RDPs. Two plastic embedded bone sections selected from left femur middle shaft and left side of thoracic vertebra were imaged with iQID. Regions of interest (ROI) for cortical bone (CB) and trabecular bone (TB) were segmented in each bone section and surface activity was quantified within each ROI. The surface activity (A s ) ranges from 1.3 to 56.9 mBq/mm² (average surface activity = 17.2 mBq/²) in CB and from 0.6 to 27.5 mBq/² ( = 14.5 mBq/²) in TB. This study showed that iQID imaging approach is an effective method for micro-scale heterogeneous distribution studies. [USTUR-0642-23A]

Presentation Slides
Real-time iQID video of alpha detection

Deepesh Poudel (LANL), John A Klumpp (LANL), Maia Avtandilashvili (USTUR), Sergey Tolmachev (USTUR)

The Human Respiratory Tract Model provides some mechanisms to account for retention of inhaled plutonium that can be subject to little to no mechanical transport or absorption into the blood. One of these mechanisms is ‘binding’, which refers to a process by which a fraction (the ‘bound fraction’) of the dissolved material chemically binds to the tissue of the airway wall. Our earlier analyses showed that such chemical binding alone is 1) unable to explain post-mortem data on regional retention of Pu in the respiratory tract, and 2) inconsistent with some observations in the literature. A literature review points to the presence of – and a significant retention of – plutonium in the scar tissues of the respiratory tract. Accordingly, an alternate model with scar-tissue compartments was proposed which was able to explain the data. However, it is possible that other mechanisms may also be responsible for plutonium retention in the respiratory tract. One such mechanism may be systemic uptake. This presentation discusses post-mortem regional retention in the respiratory tract of an individual with a wound intake to draw inferences on how much plutonium, if any, the lungs uptake from the blood. [USTUR-0636-23A]

Caleigh E. Samuels (CRPK), Richard W. Leggett (CRPK), and Sergey Y. Tolmachev (USTUR)

The Hanford site began operations in 1943 as part of the Manhattan Project. The site was first used to produce plutonium for the bomb that helped end WWII. After the war, production of plutonium was ramped up to meet the challenges of the Cold War. Research and development efforts were gradually expanded to include non-defense projects such as development of heat sources and production of medical isotopes. Over the years, large quantities of many different radionuclides were handled in site laboratories, and large volumes of radioactive wastes were produced in reactor operations. Bioassay data indicate relatively high intakes of a variety of radionuclides by Hanford workers. As part of the Million Person Study (MPS), ORNL is performing internal dose reconstructions for workers at the Hanford Site. Dose reconstructions are based on the latest biokinetic models of the International Commission of Radiological Protection (ICRP) or site-specific variations of those models. The Hanford database includes over 300,000 bioassay measurements for over 20,000 workers, and over 2,000 incident reports. Additional information comes from bioassay data and post-mortem tissue analyses of plutonium and americium for 28 individuals who worked at Hanford and voluntarily donated their bodies (partially or entirely) to the United States Transuranium and Uranium Registries (USTUR). The early stages of the dose reconstructions have been aimed at identifying the most important internal emitters to which the workers were exposed, and performing scoping exercises to identify workers with highest intakes of those radionuclides. Conclusions regarding the dosimetrically dominant internal emitters at Hanford will be discussed, and our methods of reconstructing doses from those radionuclides will be described. [USTUR-0637-23A]

Nicole E. Martinez (Clemson Univ., ORNL), Dereck W. Jokisch (Francis Marion Univ., ORNL), Caleigh Samuels (ORNL), Richard W. Leggett (ORNL), Sergey Tolmachev (USTUR), Maia Avtandilashvili (USTUR), George Tabatadze (USTUR), Ronald Goans (MJW Corporation), Larry Dauer (Memorial Sloan Kettering Cancer Center), John D. Boice Jr. (Vanderbilt-Ingram Cancer Center, NCRP)

Current progress in radium dial painter (RDP) dosimetry within the Million Person Study is discussed along with our initial community engagements and public collaborations. Although dose reconstruction is often approached as a ‘pure’ science, the importance of people, relationships, and stories should be center stage. Progress in RDP dosimetry is a true collaborative effort between multiple generations of scientists as well as the RDPs themselves and their family and friends. Current work builds on the innovation and foresight of those who came before us some 100 years ago and draws on the recorded experiences of the dial painters coupled with community knowledge. The person-centered approach adopted in this work is two-fold: (1) recognition and incorporation of prior personal experience and scientific accomplishments and (2) improving individualized dose determination as practical and possible. For the latter, approaches include use of individual measurements at multiple times following chronic intake of radium; applying reference models corresponding to an individual’s age at exposure and length of exposure; improvements to age and sex dependent models (particularly for females); evaluation over the lifetime of workers; consideration of intake rate by workplace and workplace practice; and potential incorporation of biodosimetry. The overarching goal is to enhance the understanding of lifetime risk from intakes of radionuclides for this classic epidemiologic study (Martinez et al. IJRB 2022)* which has been foundational for radiation protection guidance from the Manhattan Project to the present. The humanity of the young girls and women studied as early as 1915 will not be forgotten. [USTUR-0641-23A]

*Financial support for the RDP study is provided by the US Department of Energy (Grants DE-AU0000042 and DE-AU0000046) and the National Aeronautics and Space Administration (Cooperative Agreement 80NSSC17M0016).