A new trial compared two thyrotropin-stimulation methods and two 131I doses for postoperative ablation in patients with low-risk thyroid cancer. Rates of ablation were similar in all treatment groups. Doses lower than those currently recommended may be adequate for this condition.
In patients with low-risk thyroid cancer, it is unclear whether the administration of radioiodine provides any benefit after a complete surgical resection, and radioiodine is not recommended in patients with disease that is categorized as consisting of a tumor less than 1 cm in diameter and clinical stage N0. Therefore, radioiodine should be used with great care in order to minimize harm, administer the minimal amount of activity, and involve the best-tolerated methods.
• Why is radioiodine administered to patients with thyroid cancer after total thyroidectomy?
Radioiodine (131I) is administered to patients with thyroid cancer after total thyroidectomy for three reasons: first, to eradicate normal-thyroid remnants (ablation) to obtain an undetectable serum thyroglobulin level; second, to irradiate any neoplastic focus to decrease the risk of recurrence; and third, to perform a 131I total-body scan to detect persistent carcinoma. Successful ablation is defined by the combination of undetectable serum thyroglobulin levels after thyrotropin stimulation and normal results on neck ultrasonography 6 to 12 months after 131I administration.
• What are the two methods used for thyrotropin stimulation prior to the administration of radioactive iodine for ablation?
The two methods used for thyrotropin stimulation are the use of recombinant human thyrotropin and thyroid hormone withdrawal. Each is administered after surgery and before radioiodine administration. The method used is a matter of debate. The use of recombinant human thyrotropin maintains quality of life, is cost-effective, and reduces the radiation dose delivered to the body as compared with the amount delivered with thyroid-hormone withdrawal. Recombinant human thyrotropin and thyroid-hormone withdrawal provide similar ablation rates when a radiation activity of 3.7 GBq is administered. Furthermore, whether the 3.7-GBq dose is necessary has been questioned.
Morning Report Questions
Q: What were the primary results of this study, which compared different doses of postoperative radioactive iodine in patients with low-risk thyroid cancer?
A: The primary outcome of this study was thyroid ablation. Ablation was assessed at a mean (+/-SD) of 8+/-2 months after radioiodine administration with the use of neck ultrasonography and determination of the level of recombinant human thyrotropin-stimulated serum thyroglobulin or a diagnostic 131I total-body scan in patients with detectable antithyroglobulin antibody. For the 684 patients who could be evaluated in this study, a follow-up study was performed between 6 and 10 months (average, 8.3+/-1.6 months) after 131I administration, and no significant difference was found between the 1.1-GBq group and the 3.7-GBq group.
Q: What were the findings in this study with respect to use of thyroid hormone withdrawal versus recombinant human thyrotropin prior to radioactive iodine?
A: The proportion of patients with symptoms of hypothyroidism was significantly higher in the groups undergoing thyroid-hormone withdrawal than in the groups receiving recombinant human thyrotropin. Thyroid-hormone withdrawal was associated with deterioration of the quality of life, as compared with recombinant human thyrotropin. Radioiodine may induce lacrimal and salivary-gland disturbances, depending on the amount of radioactivity administered. The incidence of salivary problems did not differ significantly between groups, but lacrimal dysfunction (runny eyes) was more frequent among patients undergoing thyroid-hormone withdrawal along with low (19%) or with a high (25%) 131I activity than among patients receiving recombinant human thyrotropin (10%).