Uncertainties in electron-absorbed fractions and lung doses from inhaled beta-emitters

The computer code LUDUC (Lung Dose Uncertainty Code), developed at the University of Florida, was originally used to investigate the range of potential doses from the inhalation of either plutonium or uranium oxides. The code employs the ICRP Publication 66 Human Respiratory Tract model; however, rather than using simple point estimates for each of the model parameters associated with particle deposition, clearance, and lung-tissue dosimetry, probability density functions are ascribed to these parameters based upon detailed literature review. These distributions are subsequently sampled within LUDUC using Latin hypercube sampling techniques to generate multiple (e.g., ∼1,000) sets of input vectors (i.e., trials), each yielding a unique estimate of lung dose. In the present study, the dosimetry component of the ICRP-66 model within LUDUC has been extended to explicitly consider variations in the beta particle absorbed fraction due to corresponding uncertainties and biological variabilities in both source and target tissue depths and thicknesses within the bronchi and bronchioles of the thoracic airways. Example dose distributions are given for the inhalation of absorption Type S compounds of ⁹⁰Sr (T_max = 546 keV) and ⁹⁰Y (T_max = 2,284 keV) as a function of particle size. Over the particle size range of 0.001 to 1 μm, estimates of total lung dose vary by a factor of 10 for ⁹⁰Sr particles and by a factor of 4 to 10 for ⁹⁰Y particles. As the particle size increases to 10 μm, dose uncertainties reach a factor of 100 for both radionuclides. In comparisons to identical exposures scenarios run by the LUDEP 2.0 code, Reference Man doses for inhaled beta-emitters were shown to provide slightly conservative estimates of lung dose compared to those in this study where uncertainties in lung airway histology are considered.