Clinical Challenges: Radioiodine Scans in Thyroid Cancer

Of the four main types of thyroid cancer, the vast majority are differentiated thyroid cancer (DTC). Radioiodine (I-131) therapy has been used in the management of patients with DTC since the first therapeutic dose was administered to a human in 1941.

Yet, today the use of I-131 therapy remains controversial as debates continue regarding its use in the imaging or therapy of patients with DTC.

According to Douglas Van Nostrand, MD, director of Nuclear Medicine Research at MedStar Health Research Institute and professor of medicine at Georgetown University Medical Center in Washington, D.C., the most contentious issue when it comes to imaging is the pre-ablation or pre-therapy scan.

As noted in a review in the World Journal of Nuclear Medicine, there are several arguments that have been made against pre-ablation whole-body radioiodine scans:

• Pre-ablation scans have little to no benefit

• There is the potential for stunning (in which the uptake of iodine seen in a post-therapy scan in certain tissues of the thyroid appears to have less uptake than predicted by the pre-therapy scan)

• All the information a physician needs can be obtained by other methods

• Post-therapy scans are more sensitive and provide much of the information that is needed

• It inconveniences patients and is expensive

• The patient will be treated regardless of diagnostic whole-body scan and/or uptake values

Many of these arguments, however, are wide of the mark, said Van Nostrand, explaining that there are strategies for resolving stunning and that in many instances the scans do provide valuable information.

For example, the American Thyroid Association guidelines state: "Pre-therapy scans and/or measurement of thyroid bed uptake may be useful when the extent of the thyroid remnant cannot be accurately ascertained from the surgical report or neck ultrasonography, or when the results would alter either the decision to treat or the activity of [radioiodine] that is administered."

For his part, Van Nostrand said he is "definitely of the school that if you're planning to have I-131 therapy, you should have a radioiodine scan before that."

"If I find on a post-therapy scan metastases that have spread distally, I want to know that before the therapy, not after I've already treated it -- that would alter my management," he said. "You really want to completely stage the patient before you do a treatment, not do a treatment and then find out that there is metastatic disease. And if you go to the literature you can find over 14 publications demonstrating that the scan performed before therapy will alter management in anywhere between 20% and 40% of the patients."

A study in the Journal of Clinical Endocrinology & Metabolism, for example, showed that in a trial of 320 patients, pre-ablation I-131 scans resulted in a change in the estimated recurrence risk in 15% of patients, and a change in management in 31% of patients, compared with what was originally determined on histopathology alone.

So, if a pre-ablation radioiodine scan does have value, who should get them?

Van Nostrand postulated a case of a young patient, in otherwise good health, who had a small nodule removed (with all of the cancer within the nodule), with no evidence of spread outside the nodule or in any lymph nodes: "Then there is very little risk for recurrence, and I-131 therapy is not necessarily indicated. So, it's reasonable not to scan that patient," he said.

"At the other end of the spectrum you could have a patient with a known abnormality on an x-ray, and it's highly likely its cancer. The surgery is done, it's thyroid cancer, and you are going to do an I-131 treatment. That patient should clearly have a pre-therapy radioiodine scan. And then with the intermediate-risk and some low-risk patients, if you are going to give I-131 therapy it is reasonable to do a staging radioiodine scan," Van Nostrand said.

While there is generally no controversy concerning the necessity of a post-therapy scan in the days after the administration of radioiodine therapy, there is some disagreement regarding the utility of long-term surveillance scans.

"In the past the practice had been to do a surveillance scan after 1 year to re-screen for metastatic disease," he said. "The guidelines now generally state that a surveillance scan is no longer valuable. I'm in agreement it is no longer valuable in the sense of identifying recurrence, because if the patient is doing well and the blood tumor tests are negative, it is unlikely that one will pick up anything useful from that scan. The yield is too low."

Instead, he said, a scan should be performed at 1 year on any patient who has been administered I-131 therapy -- not to do "surveillance" in the sense of seeing whether there has been recurrence, but to establish a baseline that could be used in the future should that patient exhibit indications of a recurrence.

"Let's say you have a patient who comes to you 3 years after I-131 therapy, and his thyroid globulin levels have started to rise, and it seems likely that the patient has a recurrence," Van Nostrand continued. "And you do a radioiodine scan and you see a spot in the thyroid bed, and the neck area. What you don't know is whether that's normal residual tissue that was left over and not completely destroyed back with the first I-131 therapy, or whether all the tissue was destroyed and now you've discovered where the recurrence is. So, you need that baseline scan."

The problem, he said, is that there is no study actually showing what the cost-benefit of getting a baseline scan is. However, until there is such a study, performing this scan makes "common sense."

"Certainly you do it with blood tests and chemistry," Van Nostrand said. "You need to do it with that scan, but not as a surveillance scan."

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