Modelling of long-term retention of high-fired plutonium oxide in the human respiratory tract: importance of scar-tissue compartments

Forty-eight-year follow-up of a female worker exposed to highly enriched uranium via chronic and acute inhalation

The United States Transuranium and Uranium Registries (USTUR) is a unique resource of data and materials for studying biokinetics of uranium in the human body. In this study, bioassay data and post-mortem organ activities from a female whole-body USTUR donor who was exposed to highly enriched uranium were analyzed using the IMBA Professional Plus® software to derive the best estimate of the total intake. The resulting radiation doses delivered to this individual's whole body and major target organs were calculated from estimated intake based on case-specific dose coefficients derived using the AIDE® software. Both intake and dose calculations were carried out using the biokinetic and dosimetric models recommended by the International Commission on Radiological Protection (ICRP) in its Occupational Intakes of Radionuclides publication series. Different exposure scenarios including chronic and acute inhalation intakes were tested. A combination of a chronic inhalation intake and two acute inhalation intakes appears to best describe the bioassay data. To fit this female individual's autopsy data, the transfer rate from the liver to the blood was increased by a factor of 8 and the transfer rate from the kidneys to the blood was decreased by a factor of 2.2. This resulted in the best fit to all data (p = 0.519). The total intake was estimated to be 44.1 kBq, and the committed effective dose was 211 mSv with 96.8% contributed by 234U. 96.6% of the committed effective dose was contributed by the lungs. The remaining 3.4% of the committed effective dose was contributed by all systemic tissues and organs with the highest contribution (0.40%) from the red bone marrow. It is concluded that the current ICRP models, with the adjustment for smoking status, adequately describe uranium biokinetics for this individual except retention in the liver and kidneys. However, this study was based on a single case and may not be sufficient to identify any apparent sex-specific differences in uranium biokinetics.

Localized Instantaneous Dose Rates from Inhaled Particles of 239 Pu

In this work, the authors present instantaneous local dose rates from particles of plutonium-239 oxide ( 239 PuO) embedded in various regions of the respiratory tract. For comparison, a small number of simulations were performed in a representative region of the respiratory tract with other chemical compounds including pure metallic 239 Pu, 239 PuO 2 , 239 PuO 3 , 239 Pu 2 O 3 , and 239 Pu(NO 3 ) 4 . A small number of simulations were also performed with 238 PuO, weapons grade Pu, and Pu from a typical radioisotope thermoelectric generator (RTG) source for the same reason. The self-shielding effect is minor for very small particles but gradually becomes more significant as the particle size increases. For particles that are 0.1 μm and larger (excluding Pu nitrate), the calculated dose rate within several microns of the particle may be sufficient to damage lung cells, but the implications of damage to such a small volume of tissue are unclear. However, it is reasonable to assume that clinical effects will be observed if a large enough volume of tissue is damaged, as might happen when large numbers of particles are inhaled. The instantaneous dose rate around a particle may be predictive of deterministic effects, scar tissue formation, and biokinetics.

MODELING THE LONG-TERM RETENTION OF PLUTONIUM IN THE HUMAN RESPIRATORY TRACT USING SCAR-TISSUE COMPARTMENTS

The respiratory tract tissues of four former nuclear workers with plutonium intakes were radiochemically analyzed post mortem by the United States Transuranium and Uranium Registries. Plutonium activities in the upper respiratory tract of these individuals were found to be higher than those predicted using the most recent biokinetic models described in publications of the International Commission on Radiological Protection. Modification of the model parameters, including the bound fraction, was not able to explain the data in one of the four individuals who had inhaled insoluble form of plutonium. Literature review points to the presence of-and a significant retention of-plutonium in the scar tissues of the lungs. Accordingly, an alternate model with scar-tissue compartments corresponding to larynx, bronchi, bronchioles, alveolar-interstitium and thoracic lymph nodes was proposed. The rates of transfer to the scar tissue compartments were determined using Markov Chain Monte Carlo analysis of data on urinary excretion, lung counts and post-mortem measurements of liver, skeleton and individual respiratory tract compartments, as available. The posterior models predicted that 20-100%-depending on the solubility of the material inhaled-of the activities retained in the respiratory tract were sequestered in the scar tissues.

Plutonium in Manhattan Project workers: Using autopsy data to evaluate organ content and dose estimates based on urine bioassay with implications for radiation epidemiology

PURPOSE

Radiation dose estimates in epidemiology typically rely on intake predictions based on urine bioassay measurements. The purpose of this article is to compare the conventional dosimetric estimates for radiation epidemiology with the estimates based on additional post-mortem tissue radiochemical analysis results.

METHODS

The comparison was performed on a unique group of 11 former Manhattan Project nuclear workers, who worked with plutonium in the 1940s, and voluntarily donated their bodies to the United States Transuranium and Uranium Registries.

RESULTS

Post-mortem organ activities were predicted using different sets of urine data and compared to measured activities. Use of urinalysis data collected during the exposure periods overestimated the systemic (liver+skeleton) deposition of 239Pu by 155±134%, while the average bias from using post-exposure urinalyses was -4±50%. Committed effective doses estimated using early urine data differed from the best estimate by, on average, 196±193%; inclusion of follow-up urine measurements in analyses decreased the mean bias to 0.6±36.3%. Cumulative absorbed doses for the liver, red marrow, bone surface, and brain were calculated for the actual commitment period.

CONCLUSION

On average, post-exposure urine bioassay results were in good agreement with post-mortem tissue analyses and were more reliable than results of urine bioassays collected during the exposure.