Re: I-131 and pregnancy

[carol marcus <csmarcus@ucla.edu>]

At 04:28 PM 6/22/00 -0500, you wrote:
>Richard
>
>As a first pass, I suggest that you visit the Radiation Internal Dose
>Information Center's site at:
>www.orau.gov/ehsd/ridic.htm. The Compendia of Dose Estimates, there, is
>very useful. Also, couple of years ago, they published Mike Stabin's Fetal
>Dose Calculation Workbook (ORISE 97-0961). It's about seventy (70) pages,
>comb-bound, and costs about US$10. Their phone number is (865)576-3448.
>Bear in mind that they are on Eastern Time.
>
>Break a leg
>cja
>
>P.S. Mike Stabin: it may give you a kick to learn that my spell checker
>wants to change your name to *Stalin*.
>
>At 01:52 PM 6/22/00 -0600, you wrote:
>>We have a 28 yr old woman with thyroid cancer who would like to get
>pregnant in the near future. We will be giving her 150 mCi initially and   
>>3mCi on follow-up visits every 6 months for whole body scans.
>>
>>1. When dose the fetus develop thyroid tissue?
>>2. Does the I-131 cross the placenta to the fetal thyroidal tissue?
>>3. How long after giving I-131 should the patient wait to get pregnant?
>>
>>She is currently on hold and we will wait until her first follow-up in six
>months to give her any more advice.
>>
>> Thanks.
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The answers to the 3 questions are as follows.  The fetal thyroid begins
actively taking up iodine at about 12 weeks; some estimates of range include
as early as 9 weeks.  The I-131 crosses the placenta like crazy, and even
low quantities will give huge doses to the thyroid of the fetus.  NO I-131
MAY BE USED AT ALL DURING PREGNANCY.  I would wait at least 1 month after
the 150 mCi dose to try to get pregnant.  However, if successful, she cannot
have a 3 mCi scan during pregnancy.  And what is the point?  If the scan is
positive, she can't have a large I-131 dose during pregnancy, anyway.
Better to follow her with serum thyroglobulin levels.  If it starts rising,
the baby can be aborted, induced a month or two early, or the mother can
knowingly decide to destroy her baby's thyroid in order to treat her own
cancer.  (I'd make sure your hospital lawyers are heavily involved if she
elects to do this, so she can't come back later and sue you.  The baby can
be treated from birth with thyroxine and can be otherwise normal, except
that a large iodine dose would irradiate the whole baby as well, and there
might be an increased risk of cancer.) 

A lot depends on the kind of thyroid cancer she has and the stage of disease
found at surgery.  A small papillary or follicular cancer that is
encapsulated and has not invaded blood vessels or spread beyond the thyroid
has an excellent prognosis for cure with surgery and I-131 afterwards.  If
she has more extensive disease, I would put off pregnancy until she is
disease free and thyroid remnant free, which would take about a year to
ascertain.  She is only 28.  Waiting a year is no big deal.  It's much
better to make sure the mother is healthy than to jeopardize both mother and
baby.  

These considerations are not theoretical.  A few years ago a young woman
about 8 months pregnant came to our E.R. because of shortness of breath.
What was first thought to be miliary TB turned out to be lung mets from
thyroid cancer.  The baby was induced immediately.  The tumor was iodine
negative (did not concentrate any iodine) and the mother was dead within 2
weeks. 

Ciao, Carol

Carol S. Marcus, Ph.D., M.D.
<csmarcus@ucla.edu>

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