Answer to Question #8606 Submitted to "Ask the Experts"

How many times can a dose of ¹³¹I be given to a person? Are there limits as to how much and the number of times a dose may be given, both for persons with and without their thyroid?

This morning I gave a lecture about thyroid therapy with ¹³¹I at the University of California, Los Angeles, and amongst other things addressed your very question. Many years ago, I think in the 1950s, someone wrote a paper about two patients who received more than 37 GBq ¹³¹I and who got leukemia. For decades after that, the urban myth was that you couldn't safely give more than 37 Gbq (it didn't matter how many doses were given—just the total GBq mattered). But as years went by, physicians occasionally went beyond the 37 GBq limit, and no leukemia was seen. So, at present, I think that if there is thyroid tissue present that takes up ¹³¹I, it pays to give ¹³¹I to burn out the thyroid and any other thyroid cancer cells in the body. This is true even if the total dose is over 37 GBq. I think that treating the cancer that you have is more important than worrying about one you might create.

Chemotherapy agents and standard external beam radiation therapy can also produce cancer, but they successfully treat many, many more cancers than they cause. While it is conceivable that very high dose totals of ¹³¹I could produce a cancer, treating a real cancer is much more important.

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