Answer to Question #6269 Submitted to "Ask the Experts"

Our cat Rose just returned from a seven-day stay at the animal hospital after being treated with 3.5 millicuries of ¹³¹I for her hyperthyroid condition. State regulations require that she stay at the hospital until she measures less than 0.5 millirem/hr at a distance of one foot.

To minimize our exposure to radiation, our doctor recommends that during her first two weeks at home we maintain a distance of three feet or more as much as possible. The doctor advises that after two weeks we can resume our normal cat routine, which often includes Rose sleeping right next to my wife's head during the night.

Using the inverse square law, I calculate that if on day 0 Rose is emitting 0.5 millirem/hr at one foot, she also emits 8 millirem/hr at three inches, which is the approximate distance Rose would be from my wife during the evening. Assuming an eight-day half-life for ¹³¹I and, therefore, a ~9.17 percent decrease in radiation per day, after 14 days the radiation is only reduced to 2.3 millirem/hr at three inches. This seems like too much radiation per day considering eight hours of exposure every night.

According to my calculations, which admittedly do not include loss of radionuclides due to normal feline metabolic processes, it seems like waiting 90 days before resuming our normal cat routines would be more prudent. Do you agree? By the way, Rose is feeling much better.

Very nice! Your calculations are quite correct. I carried these calculations out to 50 days (see table below). You are also correct that there might be a bit of a faster clearance from the body due to metabolism, but not much. I'm not intimately familiar with feline iodine kinetics, but I expect that they mimic those in humans to a great degree. Activity not taken up in the thyroid in humans is rapidly excreted in the urine, mostly in the first day. Then, the activity trapped in the thyroid is only slowly excreted, having a half-time of about seven days, similar to the eight-day physical half-life of ¹³¹I. So, eight days is reasonable to use for conservatism. That is what my calculations reflect. And since the activity is all concentrated in the small thyroid, the use of a point source calculation is pretty reasonable.

However, the three-inch distance appears to be just to your wife's head—a good part of her body will be farther away, a half meter to a meter perhaps, so these doses will be lower to other regions, and the average dose to the whole body will be considerably lower. Depending on where we live, we all get 100 to a few hundred mrem of exposure annually to natural background radiation and other sources, so I personally would not worry about this additional exposure if it were me. However, by taking different segments of this spreadsheet calculation, you can see when you think an acceptable exposure rate is going to be experienced and permit the normal sleeping arrangements again. Thanks for the interesting question!

Michael G. Stabin, PhD, CHP