A Prototype Interactive
Risk and Dose Calculator (IRAD)
for Exposure to I-131 in the Oak Ridge Region
IRAD Version 1.2
The current
version of the Interactive Risk and Dose Calculator (IRAD v.1.2) is a prototype
web-based computer code developed to estimate dose and risk for an individual
exposed to I-131 in the Oak Ridge Region due to releases from the production
of radioactive lanthanum (1944-1956) at the X-10 facility (now known as Oak
Ridge National Laboratory), and from fallout from atmospheric weapons testing
at the Nevada Test Site (1952-1957).
The exposure to both sources of I-131 are combined using Monte Carlo
simulation (median Latin Hypercube sampling) to propagate uncertainties in
all model inputs, from the concentration of I-131 in milk to the estimate of
the absorbed dose in the thyroid gland. The potential health effect from
exposure is calculated as the excess lifetime risk of thyroid cancer, and the
excess relative risk above the background incidence of thyroid cancer.
For individuals with a diagnosed thyroid neoplasm, IRAD will estimate the
probability that their disease was caused by their exposure to I-131 from the
Oak Ridge Region (Probability of Causation).
Uncertainty in the calculated result is expressed as a 95% subjective
confidence interval. This interval provides a region of high confidence
within which the true, but unknown, value should occur. The term
"subjective confidence interval" is emphasized because professional
judgment is used to determine the state of knowledge for model inputs based
on direct and indirect evidence.
This prototype calculator estimates doses and risks for a subset (25
locations) of the 41 locations considered in Task 1 of the Oak
Ridge Dose Reconstruction (Apostoaei et al., 1999). The intent is to
expand this prototype model to include all locations within 38 km of the X-10
facility.
In this present version, the doses and risks estimated by IRAD are specific
for the milk ingestion pathway only. Milk is the predominant pathway of
exposure to I-131 for individuals consuming 2 or more 8 oz. glasses per day.
The types of milk considered by this program are regionally mixed commercial
milk (dairy cows), commercial milk obtained from a local dairy, milk obtained
from a family owned (backyard) cow, and milk obtained from a backyard goat.
Future versions of IRAD will include exposure due to inhalation, cottage
cheese, eggs, leafy vegetables, and fresh meat.
The current version of IRAD uses estimates of I-131 in milk as the source
term to the calculation of dose and risk. The information on milk
concentrations of I-131 comes from the Oak Ridge Dose Reconstruction Study (Apostoaei et al.,
1999) and from the National
Cancer Institute Fallout Study (1997).
Future versions of this program will use estimates of I-131 deposition due to
all significant sources released from the X-10 facility and all significant
sources of fallout from atmospheric weapons testing (including NTS, Pacific
tests, and tests in the former USSR). Thus, a single model is used to account
for the transport of I-131 from vegetation to milk to the thyroid gland of an
individual from several independent sources.
The estimates of thyroid dose as a function of age, gender, and amount of
I-131 ingested is calculated using the methodology documented in Apostoaei et
al. (1999). The calculation of the risk per unit dose uses: (1) the thyroid
cancer incidence rates obtained from the cancer registry (Tennessee
Department of Health) and (2) data from Land et al. (2000).
This prototype is for demonstration purposes only and the user should be
aware that frequent revisions and updates are anticipated. For more
information, please contact Oak Ridge Center for Risk Analysis, Inc.