Efficacy of low radioiodine activity versus intermediate-high activity in the ablation of low-risk differentiated thyroid cancer.

Purpose: The aim of our study was to compare the efficacy of thyroid remnant ablation using low (1.1 GBq) and intermediate-high radioiodine (RAI) activity (1.85-3.7 GBq) in low-risk differentiated thyroid carcinoma (DTC) and to evaluate the staging role of the whole body scan (WBS) in detection extrathyroidal disease.
Materials and Methods: We retrospectively included 277 patients who underwent total thyroidectomy and RAI for low-risk DTC and divided them in two groups according to RAI activity at ablation: group 1 (n = 174) treated with low activity (1.1 GBq), and group 2 (n = 103) with intermediate-high activity (1.85-3.7 GBq). To evaluate the successful ablation rate, the WBS 1 year after RAI was visually interpreted using a three-point scale: score 0 in case of absence of visible RAI uptake in thyroid bed; score 1 in presence of faint uptake in the thyroid bed; and score 2 in case of significant RAI uptake in thyroid bed.
Results: The success ablation rate was significantly higher in group 2 than group 1 (p < 0.001) with the presence of a positive WBS (score 1-2) in 65% low-activity group and 33% in intermediate-high group. Considering response to therapy categories, excellent response rate was significantly higher in group 2 (p = 0.020), while indeterminate response was higher in group 1 (p value = 0.005). Post RAI imaging revealed extrathyroidal uptake in 27 cases: 17 laterocervical nodal and 10 distant metastases. In both groups similar detection rate of nodal and distant metastases were recognized without any statistical difference.
Conclusions: The ablation rate with intermediate-high RAI activity (1.85-3.7 GBq) was better than with a low activity (1.1 GBq). First WBS may help to recognize nodal and distant metastases in about 10% of cases changing clinical stage and subsequent management.