**3.2 Diagnostic value of RxWBS**

238 12 Chapters on Nuclear Medicine

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

Fig. 9. I-131 accumulation in the benign pulmonary disease. A 51-year-old female

atelectasis containing mild traction bronchiectatic change in this lesion.

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

addition to the cervical uptakes (B).

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 al., 1989).

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 SPECT/CT (B, C), the uptake is detected in the cervical vertebra (C5), not in the thyroidectomy site. SPECT/CT also reveals I-131 uptake on thelumbar vertebra (L3, D) and right iliac bone (E).

Post-Therapeutic I-131 Whole Body Scan in Patients with Differentiated Thyroid Cancer 241

Fluorodeoxyglucose (FDG) is a glucose analogue. This is taken into the cell and phosphorylated by the same mechanism as glucose. In thyroid cancer, "Flip-flop phenomenone" is reported (Feine et al., 1996; Khan N. et al., 2003). It means the alternating uptake pattern of I-131 and FDG by the differentiated papillary or follicular thyroid cancer, which thought to be related to the differentiation of tumor (Figure 11 and 12). Bertagna et al. reported that F-18 FDG PET/CT positive results correlated with the serum thyroglobulin level in patients with negative I-131 whole body scan and high serum thyroglobulin level. They also reported that F-18 FDG PET/CT showed highest accuracy when the patient's thyroglobulin level was higher than 21 ng/mL (Bertagna et al., 2009). In addition to that, they also revealed that the levothyroxine therapy regimen does not influence F-18 FDG PET/CT results. Yoshio et al. evaluated 55 cases with differentiated thyroid cancer with F-18 FDG PET/CT and reported that FDG-avid lesions are resistant to radioactive iodine therapy with or without I-131 uptake (Yoshio et al.,

Fig. 11. Flip-flop phenomenone. A 44-year-old male underwent total thyroidectomy due to thyroid papillary carcinoma. I-123 DxWBS (A) shows several hot uptakes in the anterior

Tc-99m MIBI, a lipophilic cationic molecules, was originally developed as a myocardial perfusion imaging agent. Today Tc-99m MIBI is also used for the study of many neoplastic diseases. The mechanism of Tc-99m MIBI accumulation in tumor has been reported to depend on the size of a tumor, the blood flow and the richness of mitochondria in the tumor

neck, whereas F-18 FDG PET/CT (B) is negative.

**4.2 Tc-99m methoxyisobutylisonitrile (Tc-99m MIBI)**

**4. Other imaging modalities** 

2011).

**4.1 F-18 Fluorodeoxyglucose PET/CT**

Hung et al. conducted different protocol, RxWBS on the third to fourth day, fifth to sixth day and tenth to eleventh day post-therapy (Hung et al., 2009). They reported that there is a trend of decreasing visualization of I-131 uptake in sequential images and that 17% of lung metastasis and 16% of bone metastases were missed on the tenth to eleventh day scans. Lee et al. conducted RxWBS on the third and tenth day post-therapy (Lee et al., 2011). They also reported significant reduction in visual analysis scores and uptake ratios of I-131 avid lesions on the delayed RxWBS.

In addition, diagnostic accuracy of RxWBS is related to the past-history of previous I-131 therapy. Oh et al. reported the sensitivity of RxWBS in detecting distant metastasis after first I-131 therapy is 75% (Oh et al., 2011). In contrast, in patients with history of multiple radioiodine therapy, sensitivity of RxWBS in detecting distant metastasis is only 35%. The sensitivity of RxWBS and SPECT/CT is reported to be similar. However, the specificity of RxWBS is lower than SPECT/CT (Table 1). The sensitivity of FDG PET/CT is lower than RxWBS in detecting distant metastasis and the specificity of FDG PET/CT is much higher than RxWBS or SPECT/CT.


Table 1. Diagnostic performance of WBS, SPECT/CT and PET/CT in detecting distant metastasis (patient-based analysis) (adapted and modified from Oh et al., 2011). *Sens*, Sensitivity; *Spec*, Specificity; *DA,* diagnostic accuracy; *PPV,* positive predictive value; *NPV,* negative predictive value

Previous radioiodine scanning might be the reason of decreased sensitivity (Rawson et al., 1951). Thyroid stunning has been reported as the temporary impairment of thyroid tissue after a 111 MBq (3 mCi) or greater diagnostic I-131 dose that decreases the final absorbed dose in ablative therapy. However, Rosario et al. reported that diagnostic scanning using a 185 MBq (5 mCi) of I-131 dose does not interfere with uptake of the ablative dose or with treatment efficacy when ablation is performed within 72 hours (Rosario et al., 2005). Dam et al. reported that even though, stunning might occur but there was no significant difference in treatment success rates (Dam et al., 2004).
