Answer to Question #7314 Submitted to "Ask the Experts"

My father-in-law has had some thyroid issues and he just went in to have his thyroid destroyed. They gave him 566 MBq of iodine-131. Can you tell me if there is anything his wife should be worried about as far as his exposure is concerned? They told her to stay "10 feet" away for three days. I can't quite figure out why three days, since iodine-131 has a half-life of eight days. They also told them to wipe the shower down with antiseptic.

When I asked what his whole-body dose would be the doctor only told him, "a whole lot." Can you estimate what his whole-body dose would be?

He is supposed to go in for a lung scan in just a few weeks and the iodine-131 may not be fully out of his system. I'm just looking to minimize his exposure, or am I worried for naught?

The story you relate is somewhat disconcerting because I fear you may have been given some inaccurate information. In the first place, if your mother-in-law stays 1 meter from her husband six hours a day, and significantly less the rest of the day, and sleeps alone for about a week, her dose will be about 0.7 millisievert (mSv), which is small and of no concern. Most people are not that close to each other through the day for that long, and her dose may well be lower. This dose is equivalent to about 70 days of normal background radiation.

While your father-in-law's thyroid dose will likely be large enough to ablate the gland, the dose to the rest of his body will not be particularly large. It will be somewhere around 100 mSv, which is not associated with any ill effects. A radiation worker may receive 50 mSv a year, every year, from work-related activities. There is no evidence that hyperthyroid patients treated with iodine-131 have higher levels of a radiosensitive cancer like leukemia from the radiation as opposed to patients treated surgically.

Wiping down the shower with antiseptic makes no sense to me. Antiseptics kill microbes. They have no effect on radioactive material. The iodide is soluble in water, and that which comes out in his sweat and is on his body surface should go down the shower drain.

As to the lung scan, the doses from whatever kind of lung scan he is getting are not large, but the iodine-131 could interfere with a nuclear medicine lung scan or a chest x ray, so be sure to tell the radiologist about the iodine-131 before he has the procedure.

You should not be concerned. The really great thing about hyperthyroid therapy with iodine-131 is that its major target is the thyroid and there are no other problems to the patient from it. We have been doing this since the end of World War II and have virtually completely replaced surgery for this disease because iodine-131 therapy is cheaper, more convenient, and safer for the patient.

Carol S. Marcus, PhD, MD
Professor of Radiation Oncology and of Radiological Sciences, UCLA