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25 July 2008

Answer to Question #3451 Submitted to "Ask the Experts"

Category: Radiation Effects — Biological Effects of Radiation

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

I am a 36-year-old woman who just received sodium iodide, iodine-131, 15 millicuries to treat an overactive thyroid with a hot nodule. My left side was three times its normal size and my left side was not functioning at all. My concern, however, revolves around my twin two year olds. Their safety is foremost. How long should I stay away from my children to avoid harmful radiation exposure to them? When would it be safe, in regards to the degradation of iodine-131, to "expose" them to me without any harmful effects? I would appreciate any advice that you could share with me. I miss my children terribly, but want to keep them safe.

You are correct to be concerned about exposing your children to radiation from your radioiodine therapy. Your dose of 15 millicuries should result in an initial dose rate at 1 meter (39.37 inches, about 1 yard) from your body of about 30 microgray per hour. This is a slight overestimate because it neglects for attenuation of the gamma rays within your body. This dose rate is about 10 times natural background, the radiation to which all of us are exposed all the time from natural sources (that is, excluding all man-made radiation). Two properties of the radiation are important to you:

The intensity of the radiation is inversely proportional to the square of the distance between you and your child. Thus, doubling the distance to about 2 yards decreases the dose rate by a factor of 4 to about 7.5 microgray per hour. Conversely, halving the distance to 18 inches multiplies the dose by 4 to about 120 microgray per hour. It is therefore important to keep your distance from your children.
The intensity of the radiation decreases by one half every 8.1 days. This is called the half-life of iodine-131. The intensity will decrease to undetectable levels after 5 half-lives, or about 40 days. So after 40 days you should not be concerned about dosing your children.

All of this is a very conservative approach. It is extremely unlikely that brief exposure of your children in close proximity to your body will harm them in any way.

S. Julian Gibbs, DDS, PhD

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