Use of fractionated doses of iodine-131 for ablation of thyroid remnants.

Background: Remnants of thyroid tissue are always present after surgery performed for thyroid cancer, and their ablation with I-131 is associated with decreased recurrence rates and probably increased survival. Administration of a single dose in excess of 50 mCi of I-131 requires hospitalization. In an attempt to obviate the necessity for hospitalization, the ablative dose was divided into three fractions given at weekly intervals. The purpose of this study was to assess the effectiveness of this approach.

Methods: We retrospectively analyzed the success rate of ninety-nine patients from 1995 to 1999 with thyroid carcinoma who received fractionated dose of ablation. Iodine-131 whole body scan was done 6 months after ablation to evaluate treatment efficacy. In addition, measurement of serum thyroglobulin (Tg) was obtained in 81 patients.

Results: "Successful ablation" has been defined as the absence of all residual thyroid uptake in the I-131 whole body scan. Ablation of thyroid remnants was successful in 71 of 99 (71.7%) patients (papillary cell type 89%; follicular cell type 53%). In 54 patients with negative I-131 whole body scan, 39 patients had undetectable serum Tg, and 11 patients were with serum Tg level below 10 ng/ml. In 27 patients with positive I-131 whole body scan, 15 patients had undetectable serum Tg.

Conclusions: We suggest that the use of fractionated doses of 1-131 is a reasonable approach to ablation therapy. Tg measurement appears to be superior to I-131 whole body scan in follow-up of patients with differentiated thyroid carcinoma. However, Tg measurement can certainly not replace I-131 whole body scan. A combined application of I-131 whole body scan and Tg radioimmunoassay is thus advisable.