CHANTILLY, VA. — Thyroid cancer patients who have received radioactive iodine ablation after thyroidectomy and who later have a negative whole body scan for thyroid cancer probably do not need to receive radioactive iodine therapy if their stimulated thyroglobulin levels stay below 10 ng/mL, Dr. R. Michael Tuttle said at the annual meeting of the Mid-Atlantic Chapter of the American Association of Clinical Endocrinologists.
Whether a patient with a very low stimulated thyroglobulin level should be treated again with radioactive iodine (RAI) “is probably one of the most common phone calls I get from around the country,” said Dr. Tuttle of Memorial Sloan-Kettering Cancer Center, New York.
The way thyroid cancer is treated today is “so remarkably different now than it was 10 or 15 years ago,” he said, adding that the detection of persistent thyroid cancer has improved to the point where thyroglobulin (Tg) levels of 1, 0.5, or 0.2 ng/mL can be detected and 3-mm lymph nodes in the tracheal-esophageal groove can be seen with ultrasound in patients who were thought to be cured 20 years ago.
“I can't tell you the number of patients who leave my office mad at me because I've explained to them that I cannot cure their thyroid cancer. Their thyroglobulin level of 0.3 ng/mL is probably going to be there for awhile and taking out that 2-mm lymph node beside their recurrent laryngeal nerve is probably only going to help the surgeon and pathologist because they can bill and code for it, and it is unlikely to help our patient.”
About 80% of full body scans after repeat administration of therapeutic RAI appear to show some uptake by thyroid cells in the neck, but about 40% of these may be lung metastases, based on experience with all forms of thyroid cancer. If physicians give 100–150 mCi of RAI to patients with a high Tg level but a negative body scan, “you're very likely to find real disease,” Dr. Tuttle said, “and I think it's that finding of the lung metastasis that made us all do this.”
Rationale for RAI Changes Over Time
One rationale for using a therapeutic level of RAI when stimulated Tg levels remain high despite a negative diagnostic whole body scan was to localize the disease, not necessarily to treat it, Dr. Tuttle pointed out. Today's imaging modalities of helical CT and PET scans and neck ultrasounds can give the same information that RAI scanning would, without the possibility of side effects.
There is no evidence that repeat RAI treatment for patients who have a high stimulated Tg level after an initially negative whole body scan increases disease-specific or overall survival, Dr. Tuttle said.
Most clinicians would give another course of therapeutic RAI if the stimulated Tg level rose above 10 ng/mL, and most would not use repeat RAI if the stimulated level was below 2 ng/mL. RAI treatment for Tg levels of less than 2 ng/mL offers little potential benefit, while potentially causing complications, including damage to salivary glands, taste buds, and tear ducts. “We don't know what to do between 2 and 10,” Dr. Tuttle said.
Multiple RAI treatments appear to clear small-volume papillary thyroid cancer from pediatric to 30-year-old patients with positive lymph nodes by consistently decreasing stimulated Tg levels and clearing evidence of the tumor on CT scans and x-rays. These patients, who do not have distant metastases, are either cured or go into long-term remissions.
But older patients with thyroid cancer extending beyond the thyroid are more commonly seen than are younger patients with less aggressive, localized disease. Older patients may have a negative whole body scan and the same level of Tg as a younger patient, but the tumor is likely to be visible in the lungs on x-ray, CT, and PET scans. RAI will not help these patients if the lung lesions are poorly differentiated.
In a soon-to-be published study, Dr. Tuttle found that of about 400 thyroid cancer patients who received
Observe or Treat?
Another study involved 70 patients with thyroid cancer who had detectable Tg levels despite having a total thyroidectomy, RAI ablation, a negative whole body scan, and no structural evidence of disease. Of the 28 patients with a median stimulated Tg level of 9.5 ng/mL who were observed instead of being treated with RAI, 19 (68%) had an undetectable stimulated Tg level after 12 years of follow-up. Among the 42 patients with a median Tg level of 55 ng/mL who received RAI, only 12 (29%) had an undetectable stimulated Tg level after 7 years of follow-up (J. Clin. Endocrinol. Metab. 2001;86:4092–7).