6th European Congress on Radiation Protection, Budapest, Hungary, May 30 – June 3, 2022
USTUR research was well-represented at the International Radiation Protection Agency’s (IRPA) European Congress on Radiation Protection in Budapest, Hungary. Maia Avtandilashvili presented her research on the biokinetics of enriched uranium in a female Registrant. She also presented slides from Deepesh Poudel, a collaborative researcher from Los Alamos National Laboratory. His research explores evidence that plutonium is physically bound in the respiratory tract, and a modification of the human respiratory tract model to include compartments representing scar tissue was presented. Additionally, Martin Šefl gave a presentation where he discussed the use of a principal component regression to estimate the concentration of plutonium in the skeleton from the concentrations in a subset of individual bones.
Biokinetics of highly enriched uranium in a female nuclear worker
Maia Avtandilashvili (USTUR) and Sergei Tolmachev (USTUR)
A female whole-body donor to the United States Transuranium and Uranium Registries, was employed at a nuclear defense facility for 27 years and was exposed to enriched uranium (EU) via inhalation. She died 31 y post-exposure at age 86. A total of 129 tissue samples from the right side of the body was analyzed for uranium by alpha spectrometry. 236U was measured in 20 tissue samples using mass spectrometry. Analysis of the lung tissues confirmed that the inhaled material was EU with 67.2% of 235U, 31.9% of 238U, 0.7% of 234U, and 0.2% of 236U by weight. At the time of death, 27.1±0.6 Bq of uranium was retained in the respiratory tract, 0.29±0.01 Bq in the kidneys, 0.056±0.005 Bq in the liver, and 26.7±0.1 Bq in the skeleton. Bioassay data including urine and chest measurements and post-mortem activities in the lungs, liver, skeleton, and kidneys were simultaneously fitted using IMBA Professional Plus® to estimate the intake and the radiation dose. A combination of chronic inhalation and two acute inhalation intakes best describes the bioassay data. This individual was a heavy smoker that reflected in compromised particle clearance from the lungs to the thoracic lymph nodes. The models recommended by the International Commission on Radiological Protection (ICRP), with the adjustment for smoking status, adequately describe the biokinetics of inhaled EU except retention in the liver and kidneys. ICRP systemic models are mostly based on data from males and may not reflect female physiology. The best fit (p = 0.739) to all data including post-mortem tissue retention was achieved when the transfer rate from the liver to blood was increased by 10 and that from the kidneys to blood decreased by 2.1. The total intake was estimated to be 48.3 kBq, and the committed effective dose was 225 mSv with 97% contributed by 234U. Of this dose, 96.8% was delivered to the respiratory tract tissues followed by red bone marrow (0.8%), bone surfaces (0.6%), and liver (0.4%). [USTUR-0550-20A]
Modified human respiratory tract model to describe the retention of plutonium in scar tissues
Deepesh Poudel (Los Alamos National Laboratory), Maia Avtandilashvili (USTUR), John A. Klumpp (Los Alamos National Laboratory), Luiz Bertelli (Los Alamos National Laboratory), Sergei Y. Tolmachev (USTUR)
The Human Respiratory Tract Model (HRTM) described in Publication 130 of the International Commission on Radiological Protection (ICRP) provides some mechanisms to account for retention of material 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 of the dissolved material chemically binds to the tissue of the airway wall. Because this parameter ‘bound fraction’ – the value of which is given as 0.2% for plutonium (Pu) in ICRP Publication 141 – has a significant impact on the radiation doses imparted to different parts of the respiratory tract. To properly evaluate – and quantify – bound fraction, one would need information on long-term retention of Pu in individual compartments of the respiratory tract. Such data on regional retention in the respiratory tract of four workers – who had inhaled materials with solubility ranging from soluble nitrate to very insoluble high-fired oxides – were obtained at the United States Transuranium and Uranium Registries. An assumption of bound fraction alone was found to be inconsistent with this dataset and also with a review of the literature. Several studies show evidence of retention of a large amount of activity in the scar tissues of humans and experimental animals, and accordingly, a model structure with scar-tissue compartments was proposed. The transfer rates to these compartments were determined using Markov Chain Monte Carlo analysis of the bioassay and postmortem data, taking into account the uncertainties associated with deposition, dissolution, and particle clearance parameters. The models predicted that a significant amount – between 20-100% for the cases analyzed – of plutonium retained in the respiratory tract was sequestered in the scar tissues. Unlike chemically-bound Pu that irradiates sensitive epithelial cells, Pu in scar tissues may not be dosimetrically significant because the scar tissues absorb most, if not all, of the energy from alpha emissions. [USTUR-0611-22A]
Estimation of plutonium concentration in skeleton from occupationally exposed individuals
Martin Šefl (USTUR), Maia Avtandilashvili (USTUR), Joey Y. Zhou (U.S. Department of Energy), Sergei Y. Tolmachev (USTUR)
Purpose: The skeleton is a major plutonium retention site in the human body. The estimation of the total plutonium activity in the skeleton is a challenging problem. For most tissue donors at the United States Transuranium and Uranium Registries, a limited number of bone samples is available. The total skeleton activity is calculated using plutonium activity concentration (Cskel) and skeleton weight. If limited number of was bone samples analyzed, Cskel could be estimated using multiple linear regression (MLR) of data from whole-body donors, where Cskel were estimated based on the analysis of the half of the skeleton. The caveat of MLR is that individual bone sample concentrations are correlated. Multicollinearity can be addressed by principal component regression (PCR).
Methods: A case with eight analyzed bone samples: vertebral arch, vertebral body, sternum body, patella, skull, femur middle shaft, femur distal end, rib, was used to demonstrate the application of PCR for prediction of Cskel. For each combination of these eight bone samples, PCR was performed using data from 14 non-osteoporotic whole-body donors, and Cskel was predicted for each combination. The predicted 95% confidence intervals were compared.
Results: The lowest relative width of confidence interval (6.5%) was achieved for the following four-bone combination: vertebral arch, vertebral body, patella, and skull. The widest confidence interval (90%) was observed for the combination of three bone samples: patella, skull, and femur middle shaft.
Conclusion: PCR was used to estimate Cskel for various bone sample combinations. Analysis revealed that a proper selection of bone samples significantly reduced the width of the 95% confidence interval of estimated Cskel. [USTUR-0593-21A]
*Presentation title was changed to “Latent bone modelling for estimation of plutonium concentration in skeleton of former nuclear workers” after the abstract was published.