Influences of Parameter Uncertainties Within the Icrp 66 Respiratory Tract Model

Abstract

Risk assessment associated with the inhalation of radioactive aerosols requires as an initial step the determination of particle deposition within the various anatomic regions of the respiratory tract. The model outlined in ICRP Publication 66 represents to date one of the most complete overall descriptions of not only particle deposition, but of particle clearance and local radiation dosimetry of lung tissues. In this study, a systematic review of the deposition component within the ICRP 66 respiratory tract model was conducted in which probability density functions were assigned to all input parameters. These distributions were subsequently incorporated within a computer code LUDUC (LUng Dose Uncertainty Code) in which Latin hypercube sampling techniques are used to generate multiple (e.g., 1,000) sets of input vectors (i.e., trials) for all of the model parameters needed to assess particle deposition within the extrathoracic (anterior and posterior), bronchial, bronchiolar, and alveolar-interstitial regions of the ICRP 66 respiratory tract model. Particle deposition values for the various trial simulations were shown to be well described by lognormal probability distributions. Geometric mean deposition fractions from LUDUC were found to be within approximately +/- 10% of the single-value estimates from the LUDEP computer code for each anatomic region and for particle diameters ranging from 0.001 to 50 microm. In all regions of the respiratory tract, LUDUC simulations for an adult male at light exertion show that uncertainties in particle deposition fractions are distributed only over a range of about a factor of approximately 2-4 for particle sizes between 0.005 to 0.2 microm. Below 0.005 microm, uncertainties increase only for deposition within the alveolar region. At particle sizes exceeding 1 microm, uncertainties in the deposition fraction within the extrathoracic regions are relatively small, but approach a factor of 20 for deposition in the bronchial region. Deposition fractions for particles above 1 microm become very uncertain within the deeper regions of the lungs (bronchiolar and alveolar-interstitial).

Influenza viruses

“…First, due to the localization, mostly in the deeper regions of the respiratory tract of humans, target cells may not easily be reached by avian influenza viruses transmitted via the respiratory route of transmission. Only the smallest droplets deposit in the deeper regions of the human respiratory tract, 69 and the extent to which avian influenza virus particles can be transported and released in association with such droplets, and attain the deeper regions of the human respiratory tract remains unknown. Furthermore, the muco-ciliary escalator may trap and remove influenza virus particles depositing along the airways toward the upper regions of the respiratory tract, 70 preventing them from reaching their target cells.…”

Radiation dose assessment of exposure to depleted uranium

“…The U S ratios between our and DCAL results are in the range of 0.995-1.001. Bolch et al (2001Bolch et al ( , 2003 analyzed the influences of uncertainties of particle deposition and clearance parameters on the nuclear transformations in the ICRP HRTM. It is found that in all the regions of the respiratory tract for an adult male at light exertion, uncertainties in particle deposition fractions are distributed only over a range of about a factor of 2-4 for particle sizes between 0.005 and 0.2 mm.…”

Reliability of Current Biokinetic and Dosimetric Models for Radionuclides: A Pilot Study

“…Bolch and coworkers (Bolch et al, 2001Farfan et al, 2003;Huston et al, 2003) published a series of papers addressing uncertainties associated with predictions of the HRTM as discerned from parameter uncertainty analysis. They reviewed the original data on which 69 default parameter values of the HRTM were based, selected subjective distributions that they referred to as "probability distributions" for these parameter values, and propagated these distributions through the HRTM to derive distributions of lung dose.…”

“…They reviewed the original data on which 69 default parameter values of the HRTM were based, selected subjective distributions that they referred to as "probability distributions" for these parameter values, and propagated these distributions through the HRTM to derive distributions of lung dose. Uncertainties in the following model predictions were addressed in separate papers: particle deposition (Bolch et al, 2001) ; and identification of the input parameters to which their dose estimates for inhaled 239 PuO 2 were most sensitive . For "typical exposure scenarios" involving these radionuclides, the ratio of their derived 95% to 5% confidence levels was on the order of 8-10 for particles diameters 0.1-1.0 μm.…”

“…In the papers by Bolch et al, the term "uncertainty" refers to "both (1) a lack of knowledge of the model parameters, and (2) any biological variability these parameters would exhibit within the population of interest" (Bolch et al, 2001). This suggests that the results should be interpreted as reflecting the investigators' uncertainty in a model estimate for a healthy individual, based on the assumption that model parameter values represent the only sources of uncertainty.…”