**5. Radioiodine therapy (RAIT)**

The ability of thyroid follicular cell to take up and concentrate iodide becomes the basic fundamental of RAI therapy. Selecting the appropriate method for RAI treatment requires careful evaluation of postoperative status. However, no universally accepted recommendations exist to assess postoperative disease status [34]. Consensus in 2019 of the American Thyroid Association (ATA), the European Association of Nuclear Medicine and Molecular Imaging (EANM), the Society of Nuclear Medicine and Molecular Imaging (SNMMI), and the European Thyroid Association (ETA) state that the goal of the administration of RAIT in differentiated thyroid cancer is for remnant ablation, adjuvant treatment for irradiation of resumed foci of tumor cells to reduce the recurrence risk, or treat persistent or recurrent disease [1, 11, 12]. Remnant ablation is done after near/total thyroidectomy to destroy presumably benign residual thyroid tissue to eliminate thyroglobulin (Tg) production, facilitating follow-up of remnant ablation [1, 11, 12]. The adjuvant treatment goal is to eliminate potential microscopic foci of thyroid cancer tissue after complete resection of thyroid metastatic (locoregional, distant, or both). The condition can minimize the risk of recurrence, improving disease-specific and progression-free survival. Treatment of persistent or recurrent disease aims to improve progression-free, disease-specific, and overall survival at radioiodine-avid DTC, where the diagnosis is based on anatomical detection or biochemical evidence [1, 11, 12].

A joint of the ATA, the SNMMI, the EANM, and the ETA published a statement acknowledging the absence of high-quality evidence against using RAIT for remnant ablation post-total thyroidectomy for low-risk patients, and RAIT decisions should be taken on an individual basis, depending on tumor features (risk of recurrence), patient-related factors (comorbidities, motivation, emotional concerns), healthcare setting (availability and quality of thyroid surgeons, ultrasonography, RAI scintigraphy, Tg assays), and the local management preferences [11]. The benefit of RAIT should outweigh the risks associated with its administration, which include adverse events and diminished quality of life (QoL) [1, 11]. Application of RAIT activity dose has three broad approaches [4]: (1). Empiric dose (based on convention, experience, and patient-related factors. (2). A maximum permissible dose (determined by the upper bound limit of whole-body blood [bone marrow] dosimetry (WBBD)). (3). Target/lesion dosimetry. ATA guidelines do not endorse RAI dose activity one over the other [2, 12]. The level of risk for persistent/recurrent disease will determine the RAI dose activities. A 131I-whole-body scan (131I-WBS) must follow RAI administration to document the RAI avidity and stage of the disease [12]. 131I-WBS post-therapy scans may show additional metastases in 10–26% of patients compared with pre-therapy scans [35, 36]. Extensive disease noted on 131I-WBS post-therapy may alter the clinical stage in about 10% and clinical management in 10–15% of DTC patients [2].



**Table 2.**

*Risk stratification and recommendation of radioiodine therapy.*
