Answer to Question #5989 Submitted to "Ask the Experts"

How many times can a person be treated with high doses of iodine-131? I have had three treatments of 200 millicurie each in a year, and papillary thyroid cancer recurred 10 months after the last scan showed no activity. Can I be retreated with iodine-131?

While there is no hard and fast limit as to how many iodine-131 treatments may be given, as the total iodine-131 approaches 1,000 millicuries many physicians are concerned that the radiation might cause another malignancy, especially a leukemia. If your cancer is taking up iodine, then it is still a good idea to treat with iodine-131. With each treatment you need to have an elevated thyroid stimulating hormone (TSH), either by stopping your synthyroid for about a month or receiving recombinant human TSH. This will maximize iodine-131 uptake by your tumor. I assume that this was done the previous three times.

With regard to your question, the following needs to be considered. How do you know you have recurrence? If it was seen on an iodine-131 metastatic survey, this means the tumor takes up iodine-131. If it was inferred from an elevated serum thyroglobulin level, then the tumor may no longer be taking up iodine-131. If it was found on physical examination, we do not know if it will take up iodine-131 or not. Where is the recurrence? If it is in a surgically resectable position, such as the lymph nodes of the neck, then often surgery that removes the lymph node chain on that side is a highly successful treatment.

While the usual iodine-131 metastatic survey generally gives poor resolution, it is possible that a fluorine-18-fluorodeoxyglucose (F-18-FDG) positron emission tomography (PET) scan combined with a computed tomography (CT) scan will give more precise localization information. A PET/CT scan is indicated if the tumor takes up little or no iodine-131.

My last bit of advice is to make sure that your physicians have a significant amount of experience treating recurrent papillary thyroid carcinoma. Usually it is best to have a physician who is board certified in nuclear medicine and who works in a large medical center where many such cases are seen and there is a good deal of experience managing it.

Carol S. Marcus, PhD, MD