Comparison of 800 and 3700 MBq iodine-131 for the postoperative ablation of thyroid remnant in patients with low-risk differentiated thyroid cancer

Abstract

The initial treatment of differentiated thyroid cancer is thyroidectomy, followed by remnant ablation with iodine-131 (I-131) in some patients. However, controversy exists concerning the appropriate radioiodine dose. The aim of the study is to compare the success rate of low and high activities of I-131 for postoperative remnant ablation. A total of 108 nonmetastatic low-risk patients (mean age: 46, 85% women) with papillary and follicular carcinoma had I-131 ablation for the postoperative thyroid remnant. Fifty-three patients received a low dose (L) (800 MBq) and 55 patients received a high dose (H) (3700 MBq) of I-131. After total thyroidectomy, thyroid bed I-131 uptake (RAIU) and neck ultrasonography (USG) were performed to determine the remnant volume and the iodine avidity, which were used to calculate the dose delivered to the remnant tissue. The success rate of I-131 ablation was assessed with four different criteria based on serum thyroglobulin (Tg) and USG with and without the utilization of I-131 diagnostic whole-body scintigraphy (DxWBS). Ablation was considered to be successful if patients fulfilled all of the following criteria. (a) Strict criteria based on three tests: (i) USG negative, (ii) no tracer uptake or less than twice the background activity in the thyroid bed on DxWBS and/or up to 0.2% RAIU, and (iii) Tg < 0.2 ng/ml; (b) lax criteria based on three tests: (i) USG negative, (ii) no tracer uptake or less than twice the background activity in the thyroid bed on DxWBS and/or ≤ 0.2% RAIU, and (iii) Tg < 2 ng/ml; (c) strict criteria based on two tests: (i) USG negative and (ii) Tg < 0.2 ng/ml; (d) lax criteria based on two test: (i) USG negative and (ii) Tg < 2 ng/ml. When three tests were used to define successful ablation, in group L, 32 out of 53 (60%) and 43 out of 53 (81%) patients were successfully treated versus 35 out of 55 (64%) and 42 out of 55 (76%) for group H on the basis of strict and lax criteria, respectively (P=NS). The differences were not statistically significant between the two groups when only two tests were used to define ablation success (62 vs. 69% with strict and 89 vs. 87% with lax criteria, respectively). Our findings suggest that remnant thyroid tissue in patients with low-risk, well-differentiated thyroid cancer after total thyroidectomy can be ablated with 800 MBq of I-131. The success rate is not different from that obtained with 3700 MBq I-131.