Answer to Question #13192 Submitted to "Ask the Experts"

Category: Pregnancy and Radiation — Exposures not directly to embryo/fetus

The following question was answered by an expert in the appropriate field:

Q

I have a pregnant patient who underwent a full body scan with iodine-131 (131I) for follow up of a thyroid cancer nine years previous. She received 111 megabequerals (MBq) diagnostic dose. She subsequently became pregnant and her date of conception by early ultrasound was approximately six and a half weeks later. Any concerns for this pregnancy?

A

This is an important patient management question. Although the risk of congenital hypothyroidism may be low, I suggest that you get the assistance of the nuclear medicine physician who treated the patient to confirm that assumption is important.

The fetal thyroid gland begins to concentrate iodine at about 10 to 12 weeks conception age. By that time, the original 131I dosage (111 MBq) will have been reduced to about 0.37 MBq by physical decay. Radioiodine is also cleared by biological processes that are dependent upon adequate renal function. However, radioiodine crosses the placenta readily, and even a small amount circulating in the iodine pool can reach the fetal thyroid. Iodine-131 is a beta emitter, with a beta range of 0.8 mm. Due to the low range of the beta particles, their energy is almost totally absorbed in the tiny fetal thyroid. The resulting radiation dose to the thyroid (measured as energy absorbed per unit mass) could be very large.

I recommend contacting the nuclear medicine practitioners who performed the 131I scan and inform them of the situation. They may have resources to calculate fetal thyroid dose and recommend an intervention. If not, you could contact the radiation safety authorities in your jurisdiction and ask for their assistance in obtaining the services of a qualified expert to calculate the radiation dose to the fetal thyroid.

Robert E. Reiman, MSPH, MD
Associate Professor of Radiology
Duke Radiation Safety Division

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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