A Probabilistic Respiratory Tract Dosimetry Model with Applications to Alpha-Paraticle, Beta-Particle, and Gamma-Ray emitters

Dr. Eduardo Farfan
Nuclear Engineering Program
South Carolina State University

A complete respiratory tract model for predicting lung dosimetry of inhaled aerosols involves several component models, including models for particle deposition in airways, biokinetic clearance and radiological decay of deposited materials, and radiological dose to critical target tissues.  Each component depends on several parameters, which can vary among members of a population group.  The traditional approach has been to use reference values for parameters to generate a single, deterministic reference dose.  Based on conducting parameter uncertainty analyses, a methodology was developed in this study to incorporate parameter uncertainties into the respiratory tract modeling process.  The methodology allows lung dose predictions to e determined as probability distributions, which better reflect the potential spread in doses for members of population groups than a single reference dose.  The study involved compilation and critical evaluation of previous studies to recommend defensible distributions representative of parameter uncertainties.  Relationships were also identified to account for correlations between many model parameters.  An interactive computer program, LUDUC (for Lung Dose Uncertainty Code), was developed to implement the methodology.  LUDUC is able to handle alpha-particle, beta-particle, x-ray and gamma-ray emitters (233 different radionuclides) and aerosol diameters ranging from 0.001 to 10 micrometers. Doses resulting from inhalation of plutonium oxide, uranium oxide, and uranium octoxide (aerodynamic diameters ranging from 0.1 to 50 microns) were initially investigated with LUDUC to demonstrate the methodology.  The specific application of the methodology developed dose data, which support an ongoing dose reconstruction study of plutonium released by the Rocky Flats Plant in Colorado.  In addition, uranium was considered because of its importance in the nuclear fuel cycle.  Several beta-particle and gamma-ray emitters were also considered to further demonstrate the methodology.  In general, resulting dose distributions followed a lognormal distribution shape for all scenarios examined.  For many scenarios, the uncertainties in lung dose predictions were substantial: with geometric standard deviations approaching values of five.  Uncertainties in doses increased by about a factor of ten from smallest to the largest particle sizes.  Differences in predicted dose distributions were small when comparing different age and gender groups from 2 to 35 years of age.  Median doses for plutonium oxide, uranium oxide, and uranium octoxide generally agree with reference dose values, providing some level of confidence in the reference-man approach.  Recently, a computer code (IBUC - Intake and Uncertainty Code) based on LUDUC's methodology was created at the Savannah River National Laboratory to validate the current ICRP models using autopsy data of the US Transuranium and Uranium Registries.