Answer to Question #4507 Submitted to "Ask the Experts"

Q

If a patient has had his/her thyroid removed and is about to receive a follow-up dose of 100 mCi of ¹³¹I and is on dialysis three days a week, what precautions should be used? Will the dialysis be effective to rid the body of the iodine? How long after an iodine treatment should a facility wait to perform the dialysis?

A

There are a small number of dialysis patients who have been treated with ¹³¹NaI for thyroid cancer. In my limited experience involving just a couple of cases, I decreased the administered activity to the patient. Instead of a biological half-life of about 12 hours as it is in patients who are allowed to become hypothyroid by stopping synthroid therapy, the biological half-life is closer to the frequency between dialysis treatments, e.g., one day if dialysis is used daily. Also, the bone marrow dose is much higher than usual. I started dialysis 24 hours after administration, and continued it daily for several days. The first dialysis took out about half the radioiodine, and the next couple of dialysis sessions took out about one-third of the remaining radioiodine each time. Before I did my first case, I did internal dosimetry assuming daily dialysis and a half-life of about 24 hours, and used about half the administered activity I would have normally used if the patient had normal renal function. If a patient's dialysis is every other day, then the biological half-life is approximately twice as long and that has to be taken into consideration, i.e., by decreasing the administered activity will be by more than a factor of two.

I am unaware of any "standard of medical practice" associated with this procedure. But in my opinion, a physician should either perform the dosimetry calculations, or get a good medical physicist to do so, before determining the appropriate administered activity to use.

Carol S. Marcus, PhD, MD