Clinical Outcome of Remnant Thyroid Ablation with Low Dose Radioiodine in Korean Patients with Low to Intermediate-risk Thyroid Cancer

Radioiodine activity required for remnant thyroid ablation is of great concern, to avoid unnecessary exposure to radiation and minimize adverse effects. We investigated clinical outcomes of remnant thyroid ablation with a low radioiodine activity in Korean patients with low to intermediate-risk thyroid cancer. For remnant thyroid ablation, 176 patients received radioiodine of 1.1 GBq, under a standard thyroid hormone withdrawal and a low iodine diet protocol. Serum levels of thyroid stimulating hormone stimulated thyroglobulin (off-Tg) and thyroglobulin-antibody (Tg-Ab), and a post-therapy whole body scan (RxWBS) were evaluated. Completion of remnant ablation was considered when there was no visible uptake on RxWBS and undetectable off-Tg (<1.0 ng/mL). Various factors including age, off-Tg, and histopathology were analyzed to predict ablation success rates. Of 176 patients, 68.8% (n = 121) who achieved successful remnant ablation were classified into Group A, and the remaining 55 were classified into Group B. Group A presented with significantly lower off-Tg at the first radioiodine administration (pre-ablative Tg) than those of Group B (1.2 ± 2.3 ng/mL vs. 6.2 ± 15.2 ng/mL, P = 0.027). Pre-ablative Tg was the only significant factor related with ablation success rates. Diagnostic performances of pre-ablative Tg < 10.0 ng/mL were sensitivity of 99.1%, specificity of 14.0%, positive predictive value of 71.1%, and negative predictive value of 87.5%, respectively. Single administration of low radioiodine activity could be sufficient for remnant thyroid ablation in patients with low to intermediate-risk thyroid cancer. Pre-ablative Tg with cutoff value of 10.0 ng/mL is a promising factor to predict successful remnant ablation. Graphical Abstract Keywords: Remnant Thyroid Ablation, Radioiodine, Differentiated Thyroid Carcinoma, Thyroglobulin INTRODUCTION Post-operative remnant thyroid ablation using radioiodine followed by total or near total thyroidectomy is a mainstay in the management of patients with differentiated thyroid carcinoma (DTC). Remnant ablation eliminating residual thyroid tissues has been credited for improving sensitivity of serum thyroglobulin (Tg) monitoring and reducing the likelihood of recurrence (1, 2). These clinical benefits of remnant ablation mainly stem from facilitating complete initial staging to detect previously unrecognized metastases via a post-therapy whole body scan (RxWBS); enabling early detection of recurrence via serum Tg monitoring with/without a diagnostic (DxWBS) or RxWBS; serving as an adjuvant therapy by destroying suspected but unproven metastatic disease (3, 4, 5). The appropriate amount of radioactivity for remnant ablation has been recommended by major guidelines for managing patients with thyroid cancer (3, 4). However, these guidelines provide only broad guidance with a wide range of administered radioactivity (1.1-3.7 GBq) because radioiodine ablation using 1.1 GBq and 3.7 GBq shows similar rates of successful remnant ablation and recurrence rates (6). Although higher radioiodine activities are likely to achieve higher ablation rates, more frequent side effects such as thyroiditis, sialoadenitis, and nasolacrimal duct obstruction are unavoidable (7, 8, 9, 10, 11). Moreover, recent randomized and prospective studies have reported that low radioiodine activity (1.1 GBq) is as effective as high radioiodine activity (3.7 GBq) to decrease serum Tg levels and destroy remnant thyroid tissue (12, 13). In particular, in patients with low-risk thyroid cancer, the use of minimum radioactivity should be required to achieve beneficial effects of remnant thyroid ablation as well as avoid unnecessary exposure to radiation. Controversies exist on determining the total amount of administered radioactivity and number of radioiodine therapy sessions to complete remnant ablation. Successful remnant ablation is defined as an absence of visible radioiodine uptake on a subsequent scan or undetectable serum Tg (3). It is not easy to predict the outcome of radioiodine therapy in consideration of differences in the amount of residual thyroid tissues left in situ after surgery, preparation status including thyroid stimulating hormone level (TSH) and low iodine diet, and the effective biological half-life of radioiodine. Fixed radioactivity of 1.1 GBq is preferred in most countries based on feasibility of safety regulations about the release of patients after radioiodine administration and similar results with that of a high activity radioiodine ablation. However, a considerable number of patients need more than two sessions of radioiodine to complete remnant ablation. It would be of great importance to utilize prognostic factors of radioiodine ablation for the management of thyroid cancer. In this regard, we analyzed clinical outcomes of remnant ablation with 1.1 GBq of radioiodine in patients with low to intermediate-risk DTC, to suggest an optimized remnant thyroid ablation protocol.