Answer to Question #8244 Submitted to "Ask the Experts"

Q

I had a thyroidectomy and the pathologist report showed a tumor less than 1 cm diameter at the isthmus and two smaller tumors in the left lobe, both less than 0.3 cm in size. No other cancers were found. The recommendation is to follow up with iodine-131 treatments to be sure all cancer cells associated with the thyroid are killed off. The levels of dosage recommended are either 29 millicuries or 75 millicuries, OR take a smaller dosage and scan for any spreading of these cells and take a higher dosage later if necessary. What are the risks of another cancer type from exposure to the iodine-131 treatments? I am a 51-year-old male in good health, but my family history (mother's side) has colon and breast cancer.

A

Fortunately, these doses of iodine-131, and even higher doses, are not associated with the production of any kind of cancer. There was a claim many years ago, that is not well substantiated, that doses over 1,000 millicuries can cause leukemia. Maybe so, but your dose will be much lower than this.

The reason for the "mop-up" dose is mainly to get rid of normal thyroid tissue; small amounts are virtually always left behind after surgery in order to avoid damage to parathyroid arteries and the recurrent laryngeal nerves. Once these normal remnants are gone, you can be followed with a blood test for thyroglobulin, which is made by most differentiated thyroid cancers. (Normal thyroid tissue makes this as well, which makes the test difficult to interpret. That is why you want to get rid of all the normal thyroid tissue.)

I assume that your lymph nodes were negative for thyroid cancer. If that is the case, it is highly unlikely that you have any other thyroid cancer cells around to get rid of, as your cancers were very small. (The person's lymph nodes were negative for thyroid cancer.)

You have an excellent prognosis.

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