Answer to Question #10420 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

Q

I received a treatment/ablation dose of 1,850 mBq for low-risk papillary thyroid cancer. Is there any way of expressing approximately what magnitude of radiation exposure this dose actually constitutes?

The physicians who treated me seem unable to answer this question. I keep hearing about how safe radioiodine treatment is for the patient. If this is so, why is there so much concern about the very low doses that those whom you come into contact with would receive? It seems as though the patient, in relative terms, must be getting a huge dose. Why is this considered "safe" for ablation when the cancer itself is said to be "low risk"? Is there any way that someone could provide an estimate of how much radiation I actually received?

A

Your effective dose equivalent is about 133 mSv. To calculate this, I used the Annals of the International Commission on Radiological Protection, Publication No. 53, page 275; 1988.

The problem with a finding of "low-risk" thyroid cancer is that about 8 percent of patients have lymph node spread (NOT "low risk"). The 131I therapy will hopefully burn out those malignant thyroid cells, even though you don't even know that they're there. If you don't get the 131I therapy, you need careful "watchful waiting" and repeated ultrasounds and possibly more needle biopsies to make sure that you are not one of the 8 percent. Your thyroglobulin measurement will not be too helpful because you will likely have normal thyroid remnant present, and this tissue makes thyroglobulin, obscuring the thyroglobulin contribution of a thyroid cancer.

While data show that low dose 131I is as good as high dose (3,700 mBq) 131I for burning out normal thyroid remnants, we do not have data showing equivalence in terms of avoiding thyroid cancer recurrence. That will take several decades.

We have been treating thyroid cancer patients with 131I since 1946, and there do not appear to be any adverse effects of typical doses of 131I (3,700 mBq or lower). Dry mouth can be seen after higher doses (several therapies of 3,700 mBq), and there have been a couple of reports of leukemia after doses greater than 37,000 mBq. Your 131I dose is safe.

As to the permissible radiation dose to others, the cut off at 5 mSv is about a factor of 10 too low, in my estimation. Radiation workers are permitted 50 mSv per year, every year, from occupational activities, and studies generally show less cancer and less heart disease in these workers relative to the general public.

Your questions are perceptive and I hope I have answered them to your satisfaction.

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

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