INFLUENCE OF SCANNING DOSES OF I-131 ON SUBSEQUENT FIRST ABLATIVE TREATMENT OUTCOME IN PATIENTS OPERATED ON FOR DIFFERENTIATED THYROID-CARCINOMA

The therapeutic outcome after I-131 first ablative treatment in patien ts operated on for nonmedullary differentiated thyroid carcinoma was c ompared after both the currently used scanning dose of 111 MBq I-131 a nd a scanning dose of 37 MBq I-131. Methods: Two-hundred twenty-nine c onsecutive patients with no known metastases were retrospectively stud ied. They were divided in two populations according to the scanning do se (127 patients with 111 MBq and 102 patients with 37 MBq). All patie nts received 111 or 37 MBq I-131 for diagnostic purposes and 3.7 GBq I -131 for ablative therapy 9 days later. To assess the efficacy of the treatment, all patients were studied with I-131 and with thyroglobulin plasma assays 6-17 mo later. Results: Successful outcome was signific antly more frequent after a scanning dose of 37 MBq I-131 than after a scanning dose of 111 MBq (76% versus 50%, p < 0.001). The treatment e fficacy was particularly enhanced after 37 MBq in patients with associ ated lymphocytic thyroiditis. Conclusion: In patients with no known me tastases, our data suggest that the impairment of the treatment effica cy observed after a scanning dose of 111 MBq I-131 is related to a stu nning effect on the thyroid remnants. The threshold amount above which this effect begins to occur in thyroid remnants could be between 37 a nd 111 MBq I-131. Consequently, a scanning dose of only 37 MBq I-131 c ould be recommended before first ablative treatment. The absence of me tastatic patients in our study prevents any conclusion about the possi ble stunning of the neoplastic tissue. Nevertheless, we must suspect s uch an effect and try to avoid it, especially during follow-up after f irst radioiodine therapy. For instance, one may consider postponing ra dioiodine treatment several weeks or even months after scanning dose a dministration or using only thyroglobulin measurement for patients who are likely to receive a subsequent radioiodine treatment.