Answer to Question #12768 Submitted to "Ask the Experts"

As a cat owner, I completely understand your desire to get your cat back to his normal routine as soon as possible. However, there are a number of reasons for the precautions that your veterinarian has provided you that I will discuss below. As to your question about "home-testing equipment" and "something else to monitor and determine (your) exposure," there really are no good options for that.

While my facility does not treat cats with radioactive ¹³¹I, I have consulted with a veterinary medicine program at a nearby university that does perform these types of procedures to get myself in the "cat-patient" mode of thought. (At my facility we treat people but many of the concepts are the same, it is just that patient compliance is easier to obtain with people.) While I do not know the amount of radioactive ¹³¹I that was administered to your cat, the recommendations that your veterinarian has given you are very standard for the industry. What is happening in this procedure is that a portion of the radioactive iodine is being taken up by the cat's thyroid to treat its condition (hyperthyroidism, I assume?). What is not taken up by the thyroid will be present in the cat's bodily fluids and will primarily be voided in the cat's urine (in those first four days at the vet hospital). What is taken up by the thyroid will be released slowly over the next days/weeks to the other bodily fluids which is why they have the precautions that you describe.

There are two types of radiation exposure that your vet is trying to minimize—external exposure from the radioactive material within the cat's body, and the radioactive materials in the cat's urine and other bodily fluids released by the cat that you may come into contact with and may end up being ingested or absorbed by you accidentally.  
         
Let's tackle the external exposure first as it is the simplest concept. The three ways that we control exposures to external sources of radiation are time, distance, and shielding. Reduce the time near a source, increase the distance from the source, or use some sort of radiation absorbing shielding between you and the source of radiation and you will reduce your radiation dose.  
         
Unfortunately, ¹³¹I has a rather high-energy gamma emission that is difficult to shield. While in radiology we use radiation shielding garments (lead aprons) to reduce gamma and x-ray exposure, the energy of the ¹³¹I gamma is four to five times higher and those garments are ineffective as a shield. So, we must use distance and reduction of time near the source when dealing with patients and pets treated with ¹³¹I. (For example, for human patients we provide instructions such as "sleep in separate beds" and "stay at least 1 meter away from others as much as possible.") Your veterinarian's instructions regarding separation for the two weeks are simply doing the same thing we do with people.  
         
One of the reasons the cat was held for four days was to allow for much of the radioactive ¹³¹I to be excreted in urine so that the amount of external radiation exposure is significantly reduced.

Now let's deal with the issue of radioactive iodine released by the cat, which I believe is actually the bigger concern. All bodily fluids in the cat will have trace amounts of radioactive material within them, but it will be most prevalent within the urine. I have a cat also and I know when she is happy and purring while sitting on my lap getting strokes, she drools. In a cat treated with radioactive iodine, the drool will contain radioactive materials. Cats also sweat through their paws and the sweat will have trace amounts of radioactive iodine. Further, the litter will be radioactive as most of the radioactive iodine will be excreted via this route. I am certain that your veterinarian probably told you to take care when handling the litter and to wash your hands thoroughly after handling litter, the cat, and items that the cat came into contact with. What they are trying to prevent is the intake (ingestion, etc.) of the radioactive iodine into your body because, just like the cat, any ¹³¹I intake you have will result in an uptake by your thyroid and in larger quantities could result in significant thyroid dose. I might even suggest using disposable gloves when handling the litter box duties and when caring for the cat during the isolation time.  

As for equipment for tracking radioactive contamination, there really is no readily available "home-testing" equipment. In the hospital (or veterinary hospitals) we would use a Geiger counter with a thin-window probe to find the radioactive contamination. But those instruments are quite expensive and are not readily available in stores.  
         
As for your dose, my primary concern—especially if you were allowing the cat to have direct, prolonged contact within the first week of release—would be the potentially contaminated materials that could result in your intake of radioactive ¹³¹I. The only method to estimate that dose is to measure the radioiodine within your own thyroid and the types of equipment to measure that uptake are very expensive and difficult to interpret. The "radiation badge" worn by x-ray techs for example is good for measuring external exposure to radiation but does nothing to measure intakes of radioactive materials which is the primary concern in the case of a cat treated with radioactive ¹³¹I.

Hopefully this answers your questions and good luck with your cat. Having lost my 18-year-old feline member of the family a few years ago, I completely understand the attachment and your desire to not stress the cat. But there are clear reasons for the precautions that the veterinarian has given you and I also recommend that you follow them. It is actually the best approach to take to minimize exposures to you and your family while allowing the cat to have post-treatment recovery time at home and not in a veterinary hospital.

Jeff Brunette, CHP