**1.1 Rationale of RxWBS**

The purpose of RxWBS is for detection and localization or exclusion of functioning thyroid remnants, persistent or recurrent local disease or distant metastasis in patients receiving I-131 therapy (Luster et al., 2008). There are several reports regarding the higher detection ability of RxWBS than DxWBS. Therefore, the disease stage also can be changed after RxWBS. Fatourechi et al. reported that 13% of 117 patients of thyroid papillary cancer demonstrated abnormal foci on RxWBS, which were not seen on DxWBS (111 MBq (3 mCi), I-131) (Fatourechi et al., 2000). They also reported that RxWBS changed management strategy in 9% of 81 patients. Souza Rosario et al. reported that RxWBS on first ablation changed the disease stage in 8.3% of the patients and therapeutic approach in another 15% among total 106 patients (Souza Rosario et al., 2004). They also reported that RxWBS provided clinically relevant information for 26% of patients with 1 previous ablation. In their report, even when excluding cases whose lesions were known before scanning, the therapeutic approach was influenced by RxWBS in 15.6% of the patients.

There are also several reports regarding comparison of I-123 DxWBS and RxWBS. In the retrospective study by Donahue et al., they reported that RxWBS could find more lesions in 18% of 108 patients and clinical upstaging occurred in 10% of patients compared with DxWBS (I-123) (Donahue et al., 2008). In a study by Alzahrani et al., they showed 209 pairs of 238 pairs of RxWBS and DxWBS (I-123) were concordant (87.8% concordance rate) (Alzahrani et al., 2001). They also revealed that there were 29 discordant pairs, 13 RxWBS (5.5%) demonstrated additional foci of uptake at sites that were already positive on DxWBS

Post-Therapeutic I-131 Whole Body Scan in Patients with Differentiated Thyroid Cancer 233

Fig. 2. Multiple pulmonary metastases detected on RxWBS which were invisible on DxWBS. A 54-year-old woman who underwent total thyroidectomy for papillary thyroid cancer. A, I-123 DxWBS shows several hot uptake in the anterior neck and upper mediastinum. B, On RxWBS obtained 3 days after the administration of 7.4 GBq (200 mCi) I-131, multiple

using a large field of view gamma camera with a medium-energy, parallel-hole collimator, and the photo peak was 364 KeV with a 20% window. Continuous acquisition mode can be used with a scanning ratio of 9 - 13 cm/s with a 512 x 512 or 1,024 × 256 matrix. Anterior and posterior views of the whole body can be obtained simultaneously. Spot views of suspected sites of metastasis can be done additionally using a 256 × 256 matrix for a total of 500,000 counts. Additional image such as scanning after drinking a glass of water to wash

metastatic lesions are newly detected in both the lung fields.

131 (A). It disappeared after drinking a glass of water (B).

out physiologic uptake in the esophagus is sometimes needed (Figure 3).

Fig. 3. Physiologic retention of I-131 in the esophagus. The physiologic I-131 retention (arrow) in the esophagus on RxWBS obtained after administration of 3.7 GBq (100 mCi) I-

Fig. 1. Detection of metastatic lymph nodes on RxWBS, not on DxWBS. A 53-year-old woman with papillary thyroid cancer who underwent total thyroidectomy with left lateral neck dissection. A. I-123 DxWBS reveals a focal uptake in the anterior neck, suggesting remnant thyroid tissues in the thyroid bed. B. On RxWBS obtained 2 days after administration of 6.66 GBq (180 mCi) I-131, several focal hot uptake are noted in her left lateral neck and right highest mediastinal area. Neck ultrasonography revealed (image not shown) lesions suspicious of metastatic lymph nodes.

(I-123). Distant metastasis of thyroid cancer is not extremely rare. There have been studies reported the distant metastasis was found in 7-23% (Casara et al., 1991; Cooper et al., 2009; Mazzaferri, 1986; Samaan et al., 1985; Schlumberger et al., 1986). Iwano et al. reported that the concordance rates between DxWBS (I-123) and RxWBS were high for thyroid bed and bone metastases (89% and 86%, respectively), while they were low for lymph node and lung metastases on RxWBS (61% and 39%, respectively) (Iwano et al., 2009).

There are several reasons of performing RxWBS.

