Prenatal Exposure Limit

[BFLOOD@SLAC.Stanford.EDU (Bob Flood)]

I'd like to point out that there has been some misuse of the concept of the
prenatal exposure limit in this forum recently. The dose limit in the
regulations (DOE and NRC) applies to the fetus, not the mother. The
regulators conveniently allow us dosimetrists to use the external deep dose
equivalent to the mother as a fair estimate of the external deep dose
equivalent to the fetus. Since most dose arises from external exposure, this
leads to a tendency to see the mother's dose and the fetal dose as the same
thing. However, when the exposure involves internal radioactivity (as in
P-32 at the NIH), the accepted practice is to either calculate the true
fetal dose, a feat destined to cause either lasting fame or insanity, or use
the committed dose equivalent to the mother's uterus (adjusted to the
gestation period) as the fair estimate of the fetal dose. In such a case, an
intake just under that allowed for *occupational* exposure of the mother
(and resulting in 4.8 CEDE) will have much more important dose implications
for the fetus, since the dose limit is an order of magnitude lower and the
dose quantity of interest is no longer CEDE but CDE to the uterus.
Bob Flood
Unless otherwise noted, all opinions are mine alone.
(415) 926-3793
bflood@slac.stanford.edu