**5. Conclusion**

The I-123 DxWBS or I-131 DxWBS is sensitive and specific for treatable remnant or metastatic lesion. About 25% of RxWBS detect lesions missed by DxWBS. For this reason, following administration of a therapeutic I-131 dose, RxWBS should always be performed in patients with well differentiated thyroid cancer. The timing of RxWBS is currently still controversial. Based on several studies, the timing of post-therapeutic I-131 WBS is within 5th to 10th day, especially more opportune around 7th day after therapy. I-131 WBS with SPECT/CT may be the highly tailored approach for assessing distant metastatic lesions in patients who received a radioiodine therapy if SPECT/CT is available. RxWBS is especially likely to provide the useful information when DxWBSs are negative and the serum thyroglobulin levels are elevated. Other imaging modalities including F-18 FDG PET/CT is useful in detecting remnant thyroid tissues or metastatic thyroid cancer in patient with the elevated serum thyroglobulin level and negative iodine images. I-124 can be also useful in detection of the lesion and evaluation of lesional and whole-body dosimetry in patients with well-differentiated thyroid cancer.
