**2.1.1 Gamma camera or SPECT**

The efficacy of a system for imaging I-131 is dependent on the thickness of the crystal of camera and the collimator (Mazzaferri, 1986). I-131 whole body scan was usually done

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

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

(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

c. RxWBS has better detection performances of detecting remnant or metastatic lesions

d. Performing scan after therapy does not require additional radiation exposure, like other

The efficacy of a system for imaging I-131 is dependent on the thickness of the crystal of camera and the collimator (Mazzaferri, 1986). I-131 whole body scan was usually done

remnant thyroid tissues in the thyroid bed. B. On RxWBS obtained 2 days after

metastases on RxWBS (61% and 39%, respectively) (Iwano et al., 2009).

b. Presence of distant metastasis can change treatment strategies.

shown) lesions suspicious of metastatic lymph nodes.

There are several reasons of performing RxWBS. a. Metastasis is not extremely rare in thyroid cancer.

than DxWBS.

radiologic evaluation.

**2.1.1 Gamma camera or SPECT**

**2. Protocols of RxWBS** 

**2.1 Imaging protocol** 

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 metastatic lesions are newly detected in both the lung fields.

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

Post-Therapeutic I-131 Whole Body Scan in Patients with Differentiated Thyroid Cancer 235

Fig. 4. Diffuse pulmonary metastases. The metastatic uptake is not seen on I-123 DxWBS (A) and early RxWBS (on the third day after therapy) (B). It only appears on the RxWBS which was performed on the seventh day after the administration of 7.4 GBq (200 mCi ) I-131 (C).

Fig. 5. Multiple osseous metastases. The metastatic lesion in the upper thoracic vertebra (arrow) is only visible on the RxWBS obtained seventh day after administration of 7.4 GBq (200 mCi) I-131 (B). These lesion is not visible on RxWBS obtained third day after therapy

(A) or Tc-99m HDP WBS (C).

### **2.1.2 SPECT/CT**

Whole body imaging with SPECT/CT requires long scan time. Therefore, SPECT/CT is usually performed for specific site after whole body scan. The field of view (FOV) of SPECT/CT is usually determined by nuclear medicine physicians based on the planar image findings. So far, many companies have their models of SPECT/CT. This is the one of the usual protocol of SPECT/CT (Infinia Hawkeye 4) of our institution. First, emission SPECT images are acquired with counts from the 10% energy window at 364 KeV, with a matrix size of 128 x 128. A total image of 64 frames is acquired over 360 with an acquisition time of 30 s/frame, angular step of 6, and zooming factor of 1. After SPECT acquisition, a CT scan is acquired with a low-dose, helical CT scanner. The CT parameters are 140 KeV and 5 mAs, and no intravenous iodinated contrast is administered. The CT data are used for attenuation correction. The Images are reconstructed with a conventional iterative algorithm, ordered subset expectation maximization (OSEM). A workstation providing multiplanar reformatted images are used for image display and analysis.
