Interpretation of Urinary Excretion Data From Plutonium Wound Cases Treated With DTPA: Application of Different Models and Approaches

Modelling DTPA therapy following Am contamination in rats

A major challenge in modelling the decorporation of actinides (An), such as americium (Am), with DTPA (diethylenetriaminepentaacetic acid) is the fact that standard biokinetic models become inadequate for assessing radionuclide intake and estimating the resulting dose, as DTPA perturbs the regular biokinetics of the radionuclide. At present, most attempts existing in the literature are empirical and developed mainly for the interpretation of one or a limited number of specific incorporation cases. Recently, several approaches have been presented with the aim of developing a generic model, one of which reported the unperturbed biokinetics of plutonium (Pu), the chelation process and the behaviour of the chelated compound An-DTPA with a single model structure. The aim of the approach described in this present work is the development of a generic model that is able to describe the biokinetics of Am, DTPA and the chelate Am-DTPA simultaneously. Since accidental intakes in humans present many unknowns and large uncertainties, data from controlled studies in animals were used. In these studies, different amounts of DTPA were administered at different times after contamination with known quantities of Am. To account for the enhancement of faecal excretion and reduction in liver retention, DTPA is assumed to chelate Am not only in extracellular fluids, but also in hepatocytes. A good agreement was found between the predictions of the proposed model and the experimental results for urinary and faecal excretion and accumulation and retention in the liver. However, the decorporation from the skeletal compartment could not be reproduced satisfactorily under these simple assumptions.

URINARY EXCRETION OF PLUTONIUM IN MAYAK WORKERS DURING AND AFTER CA-DTPA ADMINISTRATION

Chelation therapy is sometimes used after potential exposures to plutonium to increase urinary excretion of the radionuclide to improve the accuracy of bioassay measurements. The purpose of this report is to describe the enhancement of urinary excretion of plutonium during and after the administration of the trisodium salt of calcium diethylenetriaminepentaacetate (Ca-DTPA) daily for 3 d to a group of male and female plutonium workers from the Mayak Production Association in Ozyorsk, Russia. One-hundred and two cases (18 females and 84 males) were selected where urinary contents of plutonium, prior to chelation, exceeded the detection threshold. Daily urine collections were obtained during the 3 d of Ca-DTPA treatments. In addition, 58 of these cases had urine bioassays at 1-45 d after chelation. The daily enhancement over baseline values excretion of plutonium was found to be 50.4×/1.4 (geometric mean and geometric standard deviation); 58.9×/1.2; 72.9×/1.4 in the first, second and third days of Ca-DTPA administration. The mean enhancement for the 3-d period was 60.1×/1.7. The rate of plutonium excretion from 1 to 45 d after chelation decreased with a half-period of 3.9 d and the chelation enhancement factor (Кenh-i) is described by the function Кenh-i = (0.79 ± 0.24) + (42.9 ± 1.2) × e-(0.18 ± 0.01) × day.

DTPA Treatment of Wound Contamination in Rats with Americium: Evaluation of Urinary Profiles Using STATBIODIS Shows Importance of Prompt Administration

ABSTRACT

In the nuclear industry, wound contamination with americium is expected to increase with decommissioning and waste management. Treatment of workers with diethylenetriaminepentaacetic acid (DTPA) requires optimization to reduce internal contamination and radiation exposure. This work aimed at evaluating and comparing different DTPA protocol efficacies after wound contamination of rats with americium. Wound contamination was simulated in rats by depositing americium nitrate in an incision in the hind limb. Different routes, times, and frequencies of DTPA administration were evaluated. Individual daily urinary americium excretion and tissue retention were analyzed using the statistical tool STATBIODIS. Urinary profiles, urinary enhancement factors, and inhibition percentages of tissue retention were calculated. A single DTPA administration the day of contamination induced a rapid increase in americium urinary excretion that decreased exponentially over 7 d, indicating that the first DTPA administration should be delivered as early as possible. DTPA treatment limited americium uptake in systemic tissues irrespective of the protocol. Liver and skeleton burdens were markedly reduced, which would drive reduction of radiation dose. Local or intravenous injections were equally effective. Inherent difficulties in wound site activity measurements did not allow identification of a significant decorporating effect at the wound site. Repeated intravenous injections of DTPA also increased americium urinary excretion, which supports the use of multiple DTPA administrations shortly after wound contamination. Results from these statistical analyses will contribute to a better understanding of americium behavior in the presence or absence of DTPA and may aid optimization of treatment for workers.

Dose Assessment Following a 238Pu-contaminated Wound Case with Chelation and Excision

The urinary excretion and wound retention data collected after a Pu-contaminated wound were analyzed using Markov Chain Monte Carlo (MCMC) to obtain the posterior distribution of the intakes and doses. An empirical approach was used to model the effects of medical treatments (chelation and excision) on the reduction of doses. It was calculated that DTPA enhanced the urinary excretion, on average, by a factor of 17. The empirical analysis also allowed calculation of the efficacies of the medical treatments-excision and chelation averted approximately 76% and 5.5%, respectively, of the doses that would have been if there were no medical treatment. All bioassay data are provided in the appendix for independent analysis and to facilitate the compartmental modeling approaches being developed by the health physics community.

Development of a New Chelation Model: Bioassay Data Interpretation and Dose Assessment after Plutonium Intake via Wound and Treatment with DTPA

The administration of chelation therapy to treat significant intakes of actinides, such as plutonium, affects the actinide's normal biokinetics. In particular, it enhances the actinide's rate of excretion, such that the standard biokinetic models cannot be applied directly to the chelation-affected bioassay data in order to estimate the intake and assess the radiation dose. The present study proposes a new chelation model that can be applied to the chelation-affected bioassay data after plutonium intake via wound and treatment with DTPA. In the proposed model, chelation is assumed to occur in the blood, liver, and parts of the skeleton. Ten datasets, consisting of measurements of C-DTPA, Pu, and Pu involving humans given radiolabeled DTPA and humans occupationally exposed to plutonium via wound and treated with chelation therapy, were used for model development. The combined dataset consisted of daily and cumulative excretion (urine and feces), wound counts, measurements of excised tissue, blood, and post-mortem tissue analyses of liver and skeleton. The combined data were simultaneously fit using the chelation model linked with a plutonium systemic model, which was linked to an ad hoc wound model. The proposed chelation model was used for dose assessment of the wound cases used in this study.

Chelation Modeling: The Use of Ad Hoc Models and Approaches to Overcome a Dose Assessment Challenge

Chelating agents are administered to treat significant intakes of radioactive elements such as plutonium, americium, and curium. These drugs may be used as a medical countermeasure after radiological accidents and terrorist acts. The administration of a chelating agent, such as Ca-DTPA or Zn-DTPA, affects the actinide's normal biokinetics. It enhances the actinide's rate of excretion, posing a dose assessment challenge. Thus, the standard biokinetic models cannot be directly applied to the chelation-affected bioassay data in order to assess the radiation dose. The present study reviews the scientific literature, from the early 1970s until the present, on the different studies that focused on developing new chelation models and/or modeling of bioassay data affected by chelation treatment. Although scientific progress has been achieved, there is currently no consensus chelation model available, even after almost 50 y of research. This review acknowledges the efforts made by different research groups, highlighting the different methodology used in some of these studies. Finally, this study puts into perspective where we were, where we are, and where we are heading in regards to chelation modeling.

Evaluating Plutonium Intake and Radiation Dose Following Extensive Chelation Treatment

A voluntary partial-body donor (US Transuranium and Uranium Registries case 0785) was accidentally exposed to Pu via inhalation and wounds. This individual underwent medical treatment including wound excision and extensive chelation treatment with calcium ethylenediaminetetraacetic acid and calcium diethylenetriaminepentaacetic acid. Approximately 2.2 kBq of Pu was measured in the wound site 44 y after the accident. Major soft tissues and selected bones were collected at autopsy and radiochemically analyzed for Pu, Pu, and Am. Postmortem systemic retention of Pu, Pu, and Am was estimated to be 32.0 ± 1.4 Bq, 2,172 ± 70 Bq, and 394 ± 15 Bq, respectively. Approximately 3% of Pu whole-body activity was still retained in the lungs 51 y after the accident indicating exposure to insoluble plutonium material. To estimate the intake and calculate radiation dose, urine measurements not affected by chelation treatment, in vivo chest counts, and postmortem radiochemical analysis data were simultaneously fitted using Integrated Modules for Bioassay Analysis Professional Plus software. The currently recommended International Commission on Radiological Protection Publication 130 human respiratory tract model and National Council on Radiation Protection and Measurements Report 156 wound model were used with default parameters. The intake, adjusted for Pu removed by chelation treatment, was estimated at approximately 79.5 kBq with 68% resulting from inhalation and 32% from the wound. Inhaled plutonium was predominantly insoluble type S material (74%) with insoluble plutonium fragments deposited in the wound. Only 1.3% reduction in radiation dose was achieved by chelation treatment. The committed effective dose was calculated to be 1.49 Sv. Using urine data available for this case, the effect of chelation therapy was evaluated. Urinary excretion enhancement factors were calculated as 83 ± 52 and 38 ± 17 for initial and delayed calcium ethylenediaminetetraacetic acid treatments, respectively, and as 18 ± 5 for delayed calcium diethylenetriaminepentaacetic acid. The enhancement factor decreases proportionally to an inverse cubic root of time after intake. For delayed calcium ethylenediaminetetraacetic acid treatment, with five consecutive daily administrations, the enhancement factor increased from day 1 to 4, followed by approximately a 50% drop on day 5. The half-time of plutonium ethylenediaminetetraacetic acid complex removal in urine was evaluated to be 1.4 d.

Some Considerations for Chelation Treatment and Surgical Excision Following Incorporation of Plutonium in Wounds

After a plutonium-contaminated wound, the role of an internal dosimetrist is to inform the patient and the physician of the dosimetric considerations. The doses averted due to medical treatments (excision or chelation) are higher if the treatments are administered early; therefore, the internal dosimetrist needs to rely on limited information on wound counts and process knowledge for advising the physician. Several wound cases in the literature were reviewed to obtain estimates of the efficacies of surgical excision and chelation treatment after plutonium-contaminated wounds. The dose coefficients calculated by coupling the NCRP 156 wound model with the systemic model were used to derive the decision guidelines that may indicate medical treatment based on 1) the concept of saved doses proposed by the NCRP 156 wound model, 2) the limits recommended by the CEC/DOE guidebook, and 3) the Clinical Decision Guidelines proposed in NCRP Report No. 161. These guidelines by themselves, however, are of limited use for several reasons, including 1) large uncertainties associated with wound measurements, 2) exposure to forms of radionuclides that cannot be assigned to a single category in the NCRP 156 framework, 3) inability of the NCRP 156 model to explain some of the wound cases in the literature, 4) neglect of the local doses to the wound site and the pathophysiological response of the tissue, 5) poorly understood relationship between effective doses and risks of late health effects, and 6) disregard of the psychological aspects of radionuclide intake.

ANALYSIS OF URINARY EXCRETION DATA FROM THREE PLUTONIUM-CONTAMINATED WOUNDS AT LOS ALAMOS NATIONAL LABORATORY

The National Council on Radiation Protection (NCRP)-156 Report proposes seven different biokinetic models for wound cases depending on the physicochemistry of the contaminant. Because the models were heavily based on experimental animal data, the authors of the report encouraged application and validation of the models using bioassay data from actual human exposures. Each of the wound models was applied to three plutonium-contaminated wounds, and the models resulted in a good agreement to only one of the cases. We then applied a simpler biokinetic model structure to the bioassay data and showed that fitting the transfer rates from this model structure yielded better agreement with the data than does the best-fitting NCRP-156 model. Because the biokinetics of radioactive material in each wound is different, it is impractical to propose a discrete set of model parameters to describe the biokinetics of radionuclides in all wounds, and thus each wound should be treated empirically.

Application of NCRP 156 Wound Models for the Analysis of Bioassay Data from Plutonium Wound Cases

The NCRP 156 wound model was heavily based on data from animal experiments. The authors of the report acknowledged this limitation and encouraged validation of the models using data from human wound exposures. The objective of this paper was to apply the NCRP 156 wound models to the bioassay data from four plutonium-contaminated wound cases reported in the literature. Because a wide variety of forms of plutonium can be expected at a nuclear facility, a combination of the wound models-rather than a single model-was used to successfully explain both the urinary excretion data and wound retention data in three cases. The data for the fourth case could not be explained by any combination of the default wound models. While this may possibly be attributed to the existence of a category of plutonium whose solubility and chemistry are different than those described by the NCRP 156 default categories, the differences may also be the result of differences in systemic biokinetics. The concept of using a combination of biokinetic models may be extended to inhalation exposures as well, where more than one form of radionuclide-particles of different solubility or different sizes-may exist in a workplace.