Answer to Question #9684 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

Are there any guidelines published that comment on the maximum lifetime dose of 131I a patient can receive for the treatment of thyroid cancer?

A

This is an interesting question. Back in the 1950s, a couple of patients who received over 37 GBq of 131I for thyroid cancer came down with leukemia, and for decades the general wisdom was not to go above 37 GBq. As years went by, an association with second primary cancers was not reported,  so the idea was that if there was thyroid tissue that concentrated 131I, going over 37 GBq was still appropriate. One needs to treat the cancer that is there even if there is an increased chance of producing another later on. This is true of chemotherapy and external beam radiation therapy as well. They both can be effective cancer treatments, but are associated with an increased probability of producing a second cancer sometime in the future.

Recently, a published paper has shown no increase in second cancers after usual 131I administered activities, but a significant increase in second cancers above 37 GBq total administered activity (Fallahi B, Adabi K, Majidi M, et al.: Incidence of second primary malignancies during a long-term surveillance of patients with differentiated thyroid carcinoma in relation to radioiodine treatment. Clinical Nuclear Medicine 36(4):277-282; 2011). This lends some credence to the recommended 37 GBq limit, but again, you have to treat the cancer you have even if there is an increased risk of producing another cancer at some time in the future.

On the other hand, an increase in thyroglobulin level (a blood test usually signifying active thyroid-cancer growth) should not in itself trigger 131I treatment above 37 GBq. One needs to see active uptake in the tumor to justify treatment. Sometimes thyroid cancer becomes undifferentiated enough that the tissue no longer takes up 131I. Giving more 131I doesn't treat the tumor, but adds to radiation burden in other tissues and can increase risk of a second cancer.

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

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