When to evaluate the final success rate of thyroid remnant ablation following low dose I-131 administration?

Objectives This work challenges the applicability of the usual procedure guideline on the recommended time period of 6-12 months for post-thyroidectomy post-ablation imaging to evaluate the ablation success rate, in case when a very low activity of I-131 is administered for thyroid remnant ablation in well-differentiated thyroid cancer patients.

Methods One-hundred and ten patients were included in this study. The inclusion criteria were: [A] total thyroidectomy for papillary pT1-2N0M0 thyroid cancer, [B] TSH>30 mU/L, measured Tg and negative Tg antibodies off T4, [C] radioiodine treatment for thyroid remnant with 0.9 GBq I-131, [D] one post-ablation and 4 follow-up sets of I-131 scans. The successful ablation of thyroid remnant was defined as negative thyroid bed uptake on the whole-body and neck/chest scans with 185 MBq I-131.

Results The patents were given 900±14 MBq I-131 at TSH of 86±37 mU/L. All post-ablation scans were positive regardless of skipped post-thyroidectomy pre-ablation diagnostic radioiodine scanning. At the first, second, third and fourth follow-up examination performed 8±3, 21±3, 34±4 and 48±4 months post-ablation, the complete ablation of thyroid remnants was achieved in 59%, 74%, 76% and 80% of patients, and in 73%, 83%, 85%, 87% of lesions, respectively, without any additional radiation treatment in the meantime. Similar trends have, interestingly, been observed after the administration of moderate and high ablation activities of I-131 as well (Kusacic Kuna S. Nucl Med Commun 2009).

Conclusions The ever lowest activity of I-131 used for more than ten years in a large serie of post-surgical thyroid cancer patients (>500 of our patients so far) appears justified, optimized and dose-compliant for high but delayed complete ablation rate. Thyroid remnant ablation process itself seems to last significantly longer than usually expected. All this has already caused large, if not dramatic, changes in overall postsurgical treatment procedure for well-differentiated thyroid cancer in our patients