Re: pregnancy post I-131 NaI treatment

[John Cormack <rajc@flinders.edu.au>]

>I am looking for reference material concerning how long a patient
>should wait before trying to become pregnant following I-131NaI
>treatment of hyperthyroidism or thyroid cancer.  The guidance I have
>come across ranges from one month to one year.
>
>Thanks for your help.
>
>Jim Mikowski

Dear Jim

Here's an extract from a QA sheet on pregnancy and nuclear medicine
procedures we use for our medical staff here at Flinders Medical Centre.
There are some references here which might be useful as well. If you (or
anyone else) would like a full copy of the full document (it's in Word 6.0
format) I can send it as an attachment.

Cheers

John Cormack

___________________________________________________
What precautions should be taken concerning radionuclide therapy and pregnancy?

Radionuclide therapy is not indicated in pregnancy and should be avoided.
The radiation dose to the embryo/fetus may result in an increased risk of
childhood cancer, and in certain applications may approach threshold levels
for deterministic radiation effects such as depression of fetal thyroid
function. Many physicians adopt the precaution of performing a pregnancy
test immediately prior to the administration of therapy doses to women who
could be pregnant.  At the initial interview, the patient should be made
aware of the need to avoid pregnancy until treatment is completed,
preferably until the dose to the conceptus would be less than 1 mSv (see
below). There have been a number of instances where 131I treatment of
hyperthyroidism was performed in patients who happened to be pregnant at
the time, one review [13] showing no increase in fetal loss, congenital
abnormalities, or childhood cancer.  However hypothyroidism following
pre-natal exposure to 131I has been reported, and the need for prompt
intervention to prevent cretinism has been noted.[14]  The fetal thyroid
concentrates iodide from about the 13th week of gestation, with the fetal
thyroid dose estimated to peak during the fifth month of pregnancy at about
580 mGy/MBq of 131I administered to the mother.[15]  The maximum fetal
whole body dose from 131I treatment of maternal hyperthyroidism has been
estimated at 8 x 10-2 mGy/MBq, occurring during the first month of
pregnancy.[16]


How long should patients delay conception following a nuclear medicine
study or radionuclide therapy?

Patients (both male and female) attempting to conceive children often wish
to know how long conception should be delayed following administration of
radioactive materials for nuclear medicine examinations. There are two
possible issues to consider:

*	Radiation exposure of a future conceptus from residual activity of
a long-lived radionuclide administered to a female patient.
*	Pre-conception radiation exposure of the the germ cells of either
parent resulting in inherited genetic damage.

For diagnostic nuclear medicine procedures, gonad doses are relatively
small and there is no need to delay pregnancies on account of the genetic
risk.  A delay is only required following administration for diagnostic
purposes of a few radiopharmaceuticals for diagnostic purposes because of
potential exposure of a developing embryo: 59Fe for iron metabolism (6
months); 75Se-selenocholesterol for adrenal imaging (12 months); 131I-MIBG
for tumour imaging (1 month); 131I-iodide for thyroid metastases (6 months).
 For those therapy procedures which involve a significant radiation dose to
the gonads, the minimum delay should be 8 to 10 half-lives of the
radionuclide. Manifestation of oocyte damage appears to be less likely if
more than seven weeks intervene between radiation and fertilisation,[11]
and repair of spermatogonial damage also occurs over time.  NCRP
guidelines[17] suggest that it is prudent to postpone pregnancies for at
least several months in order to permit the repair of such genetic damage
as may have occurred. To be conservative, it is often suggested that
nuclear medicine therapy patients wait at least six months before
attempting conception.
For female patients with disseminated thyroid cancer it is advisable to
defer pregancy until there is no evidence of disease requiring further 131I
therapy, which may require a delay of at least 12 months.[12]
Further details relating to delay of conception following administration of
radiopharmaceuticals may be obtained from the ARSAC and NCRP
guidelines.[17, 18]

12.	Casara D, Rubello D, Saladini G, et al. Pregnancy after high
therapeutic doses of iodine-131 in differentiated thyroid cancer: potential
risks and recommendations. Eur J Nucl Med 1993; 20 :192-194

13.	Sakar S, Beierwaltes W, Gill S, Cowley B. Subsequent fertility and
birth histories of children and adolescents treated with 131I for thyroid
cancer. J Nucl Med 1976; 17 :460-464

14.	National Council on Radiation Protection and Measurements.
Protection of the thyroid gland in the event of releases of radioiodine.
Report Number 55. National Council on Radiation Protection and
Measurements. Bethesda, MD: NCRP. 1977

15.	Watson EE. Radiation absorbed dose to the human fetal thyroid. In:
Watson EE, Schlafke-Stelson AT, eds. 5th International Radiopharmaceutical
Dosimetry Symposium.      Oak Ridge, TN: Oak Ridge Associated Universities,
1992 :pp 179-187

16.	Stabin MG, Watson EE, Marcus CS, Salk RD. Radiation dosimetry for
the adult female and fetus from Iodine-131 administration in
hyperthyroidism. J Nucl Med 1991; 32 :808-813

17.	National Council on Radiation Protection and Measurements. Nuclear
Medicine - Factors Influencing the Choice and Use of Radionuclides in
Diagnosis and Therapy. Report Number 70. National Council on Radiation
Protection and Measurements. Bethesda, MD: NCRP. 1982

18.	Administration of Radioactive Substances Advisory Committee
(ARSAC). Notes for guidance on the administration of radioactive substances
to persons for purposes of diagnosis, treatment or research. Department of
Health and Social Security. London: HMSO. 1993


_____________________________________________________
John Cormack
Chief Medical Scientist
Division of Medical Imaging
Flinders Medical Centre
Bedford Park
South Australia 5042

Ph:08-8204-4642 National, 618-8204-4642 International
Fax:08-8204-5450 National, 618-8204-5450 International


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