The therapeutic outcome after (131)I first ablative treatment in patients operated on for nonmedullary differentiated thyroid carcinoma was compared after both the currently used scanning dose of 111 MBq (131)I and a scanning dose of 37 MBq (131)I.
Methods: Two-hundred twenty-nine consecutive patients with no known metastases were retrospectively studied. They were divided in two populations according to the scanning dose (127 patients with 111 MBq and 102 patients with 37 MBq). All patients received 111 or 37 MBq (131)I for diagnostic purposes and 3.7 GBq (131)I for ablative therapy 9 days later. To assess the efficacy of the treatment, all patients were studied with (131)I and with thyroglobulin plasma assays 6-17 mo later.
Results: Successful outcome was significantly more frequent after a scanning dose of 37 MBq (131)I than after a scanning dose of 111 MBq (76% versus 50%, p < 0.001). The treatment efficacy was particularly enhanced after 37 MBq in patients with associated lymphocytic thyroiditis.
Conclusion: In patients with no known metastases, our data suggest that the impairment of the treatment efficacy observed after a scanning dose of 111 MBq (131)I is related to a stunning effect on the thyroid remnants. The threshold amount above which this effect begins to occur in thyroid remnants could be between 37 and 111 MBq (131)I. Consequently, a scanning dose of only 37 MBq (131)I could be recommended before first ablative treatment. The absence of metastatic patients in our study prevents any conclusion about the possible stunning of the neoplastic tissue. Nevertheless, we must suspect such an effect and try to avoid it, especially during follow-up after first radioiodine therapy. For instance, one may consider postponing radioiodine treatment several weeks or even months after scanning dose administration or using only thyroglobulin measurement for patients who are likely to receive a subsequent radioiodine treatment.