Radioactive Iodine for Differentiated Thyroid Cancer

Transcript: Marcia S. Brose, MD, PhD: Radioactive iodine is often given after surgery for 2 purposes. The first purpose is to actually ablate any thyroid tissue that might be remaining after surgery. Later on, you’ll want to use radioactive iodine to detect any recurrence.

Diagnostically, do we see any areas of possible cancer outside of the surgical area? We can use radioactive iodine to detect metastasis in the bone, or, perhaps, metastasis in the lungs, that may have gotten away; or even recurrence in the neck.

The new phrase for this is called “theragnostic.” In other words, it’s both therapeutic and diagnostic. You give a dose that is over 100 millicuries. At the same time that you’re detecting where the cancer might be, because it’s radioactive and is really selectively taken up by these thyroid cancer cells, they’re obliterated. They can die. So it’s initially used to get rid of the remnants of the thyroid. It’s used to diagnose the cancer, and is also used to treat it.

In the beginning, all patients who have differentiated thyroid cancer will usually get a therapeutic dose and we’ll be able to evaluate all of those things. The hope is that we won’t have any recurrence, but that doesn’t always happen. Sometimes we’ll see thyroglobulin levels, which we then monitor. We may use ultrasounds to monitor the neck. We’ll follow along. After a year or 2, if it starts to increase again, the patient may actually get another dose of radioactive iodine to treat the recurrence. Radioactive iodine really works best when the cells are caught at a very early stage—sometimes when the cells are small enough that only the iodine scan can see them. Sometimes we can’t even see them on an ultrasound or on a CT [computed tomography] scan. That’s when radioactive iodine works best—when the number of cells are lowest.

And then it’s given up to a few times. Hopefully they’ll be cured after 1 dose. If they do have a recurrence, it will be used for as long as it’s felt that there’s a clinical benefit. If you give somebody radioactive iodine and then the thyroglobulin shoots up again within 3 months, it really hasn’t worked. That tells you that even if there are cells that are taking up iodine, if you can still see some cells on the scan, it suggests that there are cells that are not taking up the radioactive iodine.

Because we now have the ability to treat these cases with systemic therapies, we are trying to get physicians to order a CAT [computerized axial tomography] scan after every iodine treatment. Radioactive iodine will actually detect the cells that take up the iodine. That’s wonderful, and these are the ones that are actively treated. But sometimes there are cells or nodules right next to them that are separate that are growing and are not taking up iodine at all. Unless you actually do a CAT scan, you won’t be able to see that. So you need to get that imaging done.

Sometimes they’ll use a PET [positron emission tomography] scan when iodine is not being taken up. You want to do some sort of imaging right after the radioactive iodine is given to make sure that every spot you see on the CAT scan is being taken up by the iodine. As long as that’s happening, you have clinical benefit. At a certain point—after approximately 600 or 700 millicuries—you’re unlikely to cure a patient. At that point, unless you really have gotten a good 2-year interval or something with radioactive iodine, you would then hold back and not use that modality anymore. It’s really meant to cure any microscopic metastases and then, maybe, to control some recurrence.

Transcript Edited for Clarity