Strontium in the Fetus

["MacLellan, Jay A" <jay.a.maclellan@pnl.gov>]

I retrieved the following from NUREG/CR-5631 "Contribution of Maternal
Radionuclide Burdens to Prenatal Radiation Doses," Rev.1 March 1992.

Page B-6. Strontium

a. Biological Aspects

...The ICRP and other organizations employ adult models that consider the
skeleton as the major compartment.  We also presented the role of the developing
skeleton, which becomes increasingly important throughout gestation on placental
transfer kinetics and embryo/fetal concentrations and activities.  Through 2
months of gestation, therefore, it may be assumed that after tissue
redistribution has been completed, a maximum of 10% of the woman's burden will
be distributed throughout soft tissue, including the uterus.  This distribution
pattern does not pertain through the embryo/fetus/placenta compartment at t=0,
as will be discussed below.

b. Biokinetics

A generally accepted gestational stage-related model for entry of strontium into
the embryo/fetus is not available.  A simplified model was used to illustrate a
reasonable calculational approach that might be consistent with the regulatory
needs.  Accordingly, calculations are based on the assumption that the
concentration of strontium in the embryo/fetus becomes the same as the
concentration of strontium in the soft tissue of the mother, immediately upon
introduction into the first maternal transfer compartment.  The activity is
assumed to remain constant in the embryo/fetus, but the strontium concentration
will decrease because to the growth of the conceptus.

Using our dosimetry models, I estimate 2% of the Sr ingested goes to the bone,
and about 10% of the annual intake would be present in the bone at equilibrium.
About 1-2% of the original quantity of Sr deposited in the mother's bone from
fallout would be present 30-years later, and any material in the soft tissue
would be from redistribution from the bone (material originally deposited in the
soft tissue would be gone).  From ICRP 23 I get a recycling factor of about 2%
per year, and the material from fallout now available for uptake by the fetus is
less than 0.0008% of the woman's annual intake during the fallout years (0.02
uptake x 0.02 retention x0.02 recycled).  If 10% of the current intake is
available for uptake by the fetus, the fallout intake would have to be 10,000
times to present levels to have an equal contribution.  Therefore, uptake of
fallout Sr by the mother during the 60's is not a significant contributor to
Sr-90 in baby teeth today.  Sr-90 measured in baby teeth today, comes from Sr-90
in the environment today.  I probably made some errors with my assumptions, but
I don't think my conclusion is wrong.
Jay A. MacLellan, CHP
Phone:  509-376-7247
Email:   jay.a.maclellan@pnl.gov

The caluculations above are mine alone, and haven't been check by anyone else,
so don't blame my employer or friends.   

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