Answer to Question #10522 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 1,887 MBq of 131I for ablation after surgery for a low-risk papillary thyroid cancer (no lymph node spread and undetectable thyroid gland). The prior diagnostic scan using 37 MBq of 131I revealed an uptake to the thyroid bed of 0.12 percent.

My surgeon told me that an uptake of 0.12% would indicate that there was very little residual thyroid tissue following surgery. I have read that patients with hyperthyroidism or Graves' disease might typically receive 131I doses in the 370–1,110 mBq range which "destroys" their entire thyroid gland. If these lower doses are sufficient to destroy an entire thyroid gland, why would a higher dose of 1,850 mBq be required in order to destroy a small thyroid remnant?

I am concerned about having received more 131I than was actually required to ablate this residual tissue and would greatly appreciate your opinion.

A

Good question! First, the purpose of the therapy dose of 131I is usually not only to destroy the remnant, but to destroy any microscopic thyroid cancer tissue remaining. Thyroid cancer tissue has a very low 131I uptake, so you need a lot to be effective.

Second, in Graves' disease, the 131I uptakes are generally in the range of 40–90%. In addition, the thyroid cells are stimulated to make abnormally high amounts of 131I containing thyroid hormone relative to normal thyroid cells. If 131I is highly concentrated, then the thyroid cells will be easily killed. With such high uptakes and high concentrations of thyroid hormone production, you don't have to start with such a high amount of 131I. Normal thyroids have an uptake typically of only 10–25%.

I hope that this answers your question.

Carol S. Marcus, PhD, MD

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