**3.1 False positive RxWBS**

False positive RxWBS occurs for nonthyroidal I-131 concentration, including external contamination by the saliva, nasal secretions and sweat containing I-131, internal contamination through nasopharyngeal secretion, as well as physiologic uptake in nonthyroidal tissue such as the choroid plexus, salivary glands, gastric mucosa, and urinary tract. Carliscle et al. summarized false positive findings (Carlisle et al., 2003). I-131 uptake can be seen in the nose, salivary glands, mouth, thyroid bed, lactating breast, liver, gall bladder, stomach, esophagus and sweat (Figure 6). Physiologic uptake of I-131 in the salivary glands, nasal mucosa, gastric mucosa, colon, mammary glands and choroid plexus is due to the NIS presence in these tissues (Carlisle et al., 2003; Riedel et al., 2001). Thymic uptake is rare, there is a report that the incidence of thymic uptake was 1-1.2% of cases (Davidson & McDougall, 2000) (Figure 7).

Fig. 6. Diffuse perspiration. (A) On the RxWBS obtained on the third day after I-131 therapy with 3.7 GBq (100 mCi), mild perspiration is noted in both the axillae. (B) On the RxWBS obtained on the seventh day after I-131 therapy, diffuse perspiration is present in both the axillae, chest, upper arms and hands.

Thyroid hormone replacement may be resumed on the second or third day after therapy

The following data are needed to be reported: the name of patient, ID, age, the date of I-131 therapy, I-131 dose, date of RxWBS. When it is available, the serum level of thyroglobulin, thyroid stimulating hormone and anti-thyroglobulin antibody are to be reported. The sites

False positive RxWBS occurs for nonthyroidal I-131 concentration, including external contamination by the saliva, nasal secretions and sweat containing I-131, internal contamination through nasopharyngeal secretion, as well as physiologic uptake in nonthyroidal tissue such as the choroid plexus, salivary glands, gastric mucosa, and urinary tract. Carliscle et al. summarized false positive findings (Carlisle et al., 2003). I-131 uptake can be seen in the nose, salivary glands, mouth, thyroid bed, lactating breast, liver, gall bladder, stomach, esophagus and sweat (Figure 6). Physiologic uptake of I-131 in the salivary glands, nasal mucosa, gastric mucosa, colon, mammary glands and choroid plexus is due to the NIS presence in these tissues (Carlisle et al., 2003; Riedel et al., 2001). Thymic uptake is rare, there is a report that the incidence of thymic uptake was 1-1.2% of cases

Fig. 6. Diffuse perspiration. (A) On the RxWBS obtained on the third day after I-131 therapy with 3.7 GBq (100 mCi), mild perspiration is noted in both the axillae. (B) On the RxWBS obtained on the seventh day after I-131 therapy, diffuse perspiration is present in both the

(Cooper et al., 2009; Luster et al., 2008).

(Davidson & McDougall, 2000) (Figure 7).

axillae, chest, upper arms and hands.

of significant I-131 uptake are needed to be mentioned.

**3. Findings of RxWBS** 

**3.1 False positive RxWBS**

Fig. 7. Thymic uptake. A 12-year-old girl underwent therapy with 3.7 GBq (100 mCi) I-131. RxWBS were obtaind on the third day (A) and seventh day (B) after therapy. Hot uptakes are seen in the right lower neck and upper chest. I-131 SPECT/CT (C) and F-18 FDG PET/CT images (D) confirmed I-131 uptake in the upper chest to be physiologic uptake in the thymus.

Diffuse hepatic uptake of I-131 is rarely found due to occult hepatic metastases but more commonly due to the hepatic de-ionization and conjugation of I-131 which was not incorporated into the thyroid hormone (Carlisle et al., 2003). Chung et al. investigated hepatic uptake on DxWBS or RxWBS. They analyzed scans of 399 patients. They reported that hepatic uptake is more often when higher dose of I-131 was administered. They also reported that the more uptakes appeared in the residual thyroid, the more it appeared in the liver. However, they found that 15 patients showed diffuse hepatic uptake without uptake by the remnant thyroid or metastatic lesion. They followed these patients and metastatic lesions were found in 7 of 15 patients. So, they insisted that diffuse liver uptake indicated functioning thyroid remnant or metastasis (Chung et al., 1997) (Figure 8).

There are some reports I-131 false positive uptake in other pathologic condition unrelated to thyroid cancer: tracheostomy site, bronchiectasis, pulmonary inflammatory disease, pleural effusion, salivary gland tumor, some other carcinoma such as adenocarcinoma, squamous carcinoma, Barrett's esophagus, Meckel's diverticulum (Ain & Shih, 1994; Berquist et al., 1975; Caplan et al., 1987; Carlisle et al., 2003; Fernandez-Ulloa et al., 1976; Hoschl et al., 1988; Misaki et al., 1994; Mitchell et al., 2000; Muratet & Giraud, 1996), and so on. (Figure 9)

Post-Therapeutic I-131 Whole Body Scan in Patients with Differentiated Thyroid Cancer 239

The visualizing functioning metastasis as well as remnant thyroid tissue is related to the dose of I-131. Waxman et al. reported that more lesions were detected when the activity administered was increased from 74 to 370 MBq (2 to 10 mCi) and when even higher yields at 1,110 to 3,700 MBq (30 to 100 mCi) (Waxman et al., 1981). Spies et al. reported that with higher therapeutic dose, RxWBS demonstrated additional findings or more accurate localization compared with a diagnostic dose of 185 MBq (5 mCi) in 46% of cases (Spies et

The diagnostic accuracy of RxWBS is also related with the time interval from the date of administration of I-131 and that of scanning. Khan et al. and Chong et al. reported that earlier scanning might miss the lesions and the effectiveness of the delayed scan on the seventh day from the administration of I-131 (Khan S et al., 1994). Khan et al. performed RxWBS on the second day and seventh day post-therapy (Khan et al., 1994). They reported that the seventh day scan is more sensitive than third day scan. Chong et al. reported that 22% of lung metastasis and 33% of bone metastases that were not shown on the third day scan, though they were detected on the seventh day scan (Chong et al., 2010) (Figure 10).

Fig. 10. Vertebral metastasis mimicking remnant thyroid tissue. A 61-year-old female underwent RxWBS after administration of 7.4 GBq (200 mCi) I-131. RxWBS shows multiple uptakes in the abdomen and right pelvic area suggesting distant metastases. Focal uptake in the anterior neck was supposed to be usual remnant thyroid tissue. However, on additional

thyroidectomy site. SPECT/CT also reveals I-131 uptake on thelumbar vertebra (L3, D) and

SPECT/CT (B, C), the uptake is detected in the cervical vertebra (C5), not in the

**3.2 Diagnostic value of RxWBS**

al., 1989).

right iliac bone (E).

Fig. 8. Diffuse hepatic uptake. A 59-year-old female underwent I-131 therapy with 370 MBq (100 mCi) after total thyroidectomy. On the RxWBS obtained on the third day after therapy, mild focal uptakes in the anterior neck and right lower anterior neck are shown (A). On the RxWBS obtained on the seventh day after therapy, diffuse hepatic uptake appears in addition to the cervical uptakes (B).

Fig. 9. I-131 accumulation in the benign pulmonary disease. A 51-year-old female underwent RxWBS on the third day (A) and the seventh day (B) after the administration of 6.66 GBq I-131. Besides the hot uptake in the neck, slightly increased, focal, linear uptake (arrows on A and B) is shown. On SPECT/CT (C), it is on the medial segment of the right middle lobe. Chest CT (D) shows peribronchial cicatrical consolidation and adhesive atelectasis containing mild traction bronchiectatic change in this lesion.
