Answer to Question #1964 Submitted to "Ask the Experts"

Q

I was just given radioactive iodine to treat an overactive thyroid gland due to toxic goiter. I was given 20 (I am not sure how it is measured, but know it is 20). I am a 31-year-old female. Over the past six months I have had four x rays, one to include a mammogram. I was never asked about the amount of radiation I had received over the year before receiving this ¹³¹I treatment. Now, after the fact, I have been doing some research and it seems as though with all of this exposure I have greatly increased my risk of cancer. Is it not standard to ask how much radiation one has received before administering x rays or other radioactive treatment? Should I be concerned for my own health now? How many x rays a year is too many? I was told at the time of the mammogram that because of my age the risk of exposure was higher. How much higher? I have two small kids and fear I have made some uninformed decisions regarding my own safety. I guess I am hoping for some good news . . . am I now at high risk for cancer?

A

It is likely that you were prescribed 20 millicuries of ¹³¹I for thyrotoxicosis of the thyroid. This is an appropriate and well-established treatment for your age, condition, size of thyroid gland, and surgical status. You should be confident that this treatment was necessary to prevent the life-threatening effects of toxic goiter—if left untreated. Although cancer is associated with radiation exposure, the risk to you is very small and it is highly unlikely that the combination of ¹³¹I and x-ray exams that you received will lead to cancer later on in life.

Your major radiation dose was from the ¹³¹I treatment, not from the mammogram or other x rays. Scientists have conducted follow-up studies in adults treated with ¹³¹I. The safety of ¹³¹I therapy has been shown with average follow-up periods of 12 to 15 years in adults. These studies showed little tendency in treated patients to develop thyroid cancer, leukemia, or reproductive abnormalities (Holm et al. 1980). There was a slight incidence in cancer of the small bowel, but the absolute risk was very small. Children born to treated adults did not show any increase in congenital defects or evidence of thyroid damage (Franklyn et al.). Thus, you have not greatly increased your risk of cancer.

The normal risk of cancer during a lifetime from all causes combined is actually quite high (about 20 to 25 percent, just less than heart disease), and your total risk won't change substantially as a result of these specific medical treatments. We don't have a number of acceptable x-ray exams per year. As a rule, we try to limit the exposure to medical x rays to as low as possible and as few as possible. Diagnostic x-ray exams should be prescribed only when needed for your best care and, when properly prescribed, the benefits of such outweigh the associated risks of cancer.

References

• Holm LE, Dahlqvist I, Israelsson A, Lundell GM. Malignant thyroid tumors after 131-iodine therapy. N Engl J Med 303:188-191; 1980.
   
• Franklyn JA, Maisonneuve P, Sheppard M, Betteridge PB. Cancer incidence and mortality after radioiodine treatment for hyperthyroidism: A population based cohort study. Lancet 353:2111-15; 1999.

Darrell R. Fisher, PhD