Answer to Question #712 Submitted to "Ask the Experts"

Q

There is a coworker in our relatively small office who will be undergoing injections (the stongest, largest doses) of radiation for thyroid cancer. I am pregnant and am concerned about the possible effects if baby is exposed. How long will the worker be unsafe to be around, and what are distance, etc., recommendations?

A

The current rules for when a patient is allowed to be around other people after an administration of radioactive material come from the U.S. Nuclear Regulatory Commission. Patients may be released when they would be expected to cause someone with whom they come in contact no more than 500 millirem. They are supposed to make it an overestimate, and I assume that you would be cautious (some suggestions are given below for situations to avoid) so that it would be unlikely for your fetus to receive even a fifth of that, or 100 millirem.

Is this amount of radiation safe during pregnancy? We could compare this number with background radiation levels that we receive just by living in different parts of the United States. According to the National Council on Radiation Protection and Measurements, a typical U.S. inhabitant receives, on average, 360 millirem per year. This can vary by more than 100 millirem per year in different regions of the country because of differences in soil composition, radon in buildings, altitude, etc. It varies by even more than that in some parts of the world because of unusual geology in which minerals that have high concentrations of uranium and thorium are present. These areas receive an additional 300-600 millirem per year from this source. Since no epidemiological studies have shown a link between bioeffects such as cancer or birth defects in high-background compared to low-background areas, it is assumed that doses of radiation that are the order of background radiation or even several times background are not hazardous. Current radiation protection philosophy is based upon the idea that we can't be certain that any amount of radiation is completely harmless. However, the probability of harm decreases as dose decreases, so at some point you reach probabilities that are far below the probability of day-to-day risks in life. Studies of groups such as pregnant radiation therapy patients who have elected to carry to term, atomic bomb survivors, etc., yield risk estimates, but we should emphasize that no one has seen actual incidence of harm at levels as low as 500 mrem.

For the record, the period of greatest sensitivity is 56-105 days after conception. The risk is calculated to be 0.04% chance of mental retardation if the fetus actually receives a dose of 100 millirem. Another way to say this is 4 chances in 10,000. The institution from which your office mate is receiving treatment is required to consider aspects of the patient's lifestyle which would cause him to be in contact with small children or pregnant women. So you might talk to that person before he goes in for the procedure. He is supposed to be advised, for example, that he should not take a long trip with others in a car immediately. The institution is also supposed to look at the possibility of cross-contamination; that is, if he touches an object and his body oils or sweat leaves a small amount of contamination behind. If you share a computer keyboard, phone, or other equipment with that person, the institution should be made aware of it. It may influence how long the patient is kept at the facility. Some institutions hold patients until the dose rate at a meter is less than 7 millirem per hour. If this is the case, and if you don't sit within a meter of your coworker very often during a day, then the risks would be quite small. So, a lot depends upon how much guidance the hospital or clinic gives your coworker and also, how well your coworker follows the advice.

Bruce Hasselquist, PhD and
E. Russell Ritenour, PhD