*DOI: http://dx.doi.org/10.5772/intechopen.108481 Aspects Considered in Differentiated Thyroid Cancer for Radioiodine Therapy*

The RAI activity consists of low and high doses. A low dose is usually given for remnant ablation at 1.1 GBq (30 mCi) [1, 11]. A high dose for treatment at > 1.1 GBq– 5.55 GBq (>30 mCi–150 mCi), is recommended for high-risk recurrence conditions as shown in **Table 2** [1, 11, 12, 37]. The RAIT is not recommended for particular lowrisk/very low-risk conditions [tumor with a small size nodule (< 1cm intrathyroidal) and without locoregional metastases] [11]. The administered RAI activities higher than 5.55 GBq (150 mCi) are unnecessary in intermediate-risk patients. Limiting RAI dose activities to a maximal 5.55 GBq (150 mCi) mainly considers the risks of side effects [1]. Regarding the treatment of known DTC, the ATA guidelines recommend RAI dose up to 7.40 GBq (200 mCi) and not to exceed 5.55 GBq (150 mCi) in patients ≥70 years old, to avoid the risk of toxicity [12]. In patients with prolonged radioiodine clearance, the RAI dose is reduced by up to 50% [38, 39].


#### **Table 3.**

*Treatment responses of differentiated thyroid cancer treated with total thyroidectomy and radioiodine therapy.*

#### *DOI: http://dx.doi.org/10.5772/intechopen.108481 Aspects Considered in Differentiated Thyroid Cancer for Radioiodine Therapy*

In low-risk DTC patients, RAIT is influenced by any adverse feature that modulates recurrence risk and patient preference. The ATA 2015 guidelines recommend a low RAI dose (1.11 GBq/30 mCi) for remnant ablation of low-risk or intermediate-risk DTC with low-risk features [2, 11, 12]. Radioiodine therapy after total thyroidectomy should be considered in intermediate-risk DTC and is routinely recommended in ATA highrisk DTC. The therapy consideration is to balance treatment efficacy with unwanted side effects [1]. However, patient preference plays a crucial role in decision-making. Therefore, the activity of RAI ought to be specifically prescribed for each patient [12]. Moreover, on the 2015 ATA guidelines, RAI for adjuvant therapy is considered for DTC having a low-to-intermediate risk for recurrence with (1). Tumor with a dimension > 4 cm without nodal or distant metastases (T3a N0 or Nx M0 or Mx) by the AJCC 8th edition, TNM classification. (2). Any tumor size with microscopic extra-thyroidal extension but without nodal or distant metastasis (3). Tumors (T1–T3a) with nodal but no distant metastasis (T1–T3a N1 M0 or Mx) (4). Tumors (T1–T3a) with vascular invasion and aggressive pathological subtype [1, 11]. A microscopic residual disease increases in intermediate-risk DTC, as evidenced by higher recurrence rates in most intermediate categories compared with low-risk. A study that reported adjuvant RAIT in intermediate-risk patients with the exclusion of aggressive variants and multiple primaries showed a survival benefit in patients <45 years old, and improved overall survival was also shown in patients with aggressive variants of PTC [40–42]. The adjuvant therapy of RAI in high-risk DTC without distant metastasis shows improved outcome and without controversial decision. RAI adjuvant therapy is recommended in patients with T3b, T4a, and T4b, any N, M0, and high-risk DTC, including cervical nodes (≥3 cm in largest dimension) and/or with extranodal extension [1]. Treatment responses after total thyroidectomy and RAIT are defined as excellent, biochemical incomplete, structural incomplete, or indeterminate based on findings (neck ultrasound and serum Tg and anti-Tg anti- body (TgAb) levels, as shown in **Table 3**.
