Appropriate time for post-therapeutic I-131 whole body scan.

Iodine metabolism and kinetics in thyroid carcinoma cells may be different from that in normal thyroid tissue. The optimal time for performing post-therapeutic scans to detect metastatic lesions is still controversial.We retrospectively analyzed the images of 239 patients who received I-131 therapy at our hospital from January 2006 to May 2008. The therapeutic dose ranged from 1.1 GBq (30 mCi) to 7.4 GBq (200 mCi) and the patients received 3 sequential whole body scans on the third to fourth day, fifth to sixth day, and 10th-11th day after I-131 administration. We scored the I-131 avid lesions by visual assessment into 3 grades: 2=clearly visible; 1=visible; and 0=not visible. We also compared the thyroglobulin levels and FDG uptake among the lesions with probably different kinetics of iodine metabolism.In this study, there was a trend of decreasing visualization of lesions in the sequential images. Twenty-eight percent of lymph node metastases, 17% of lung metastases, and 16% of bone metastases were missed on the late images on 10-11th day. On the other hand, only 5% of the remnants were missed. The ratio of early washout was different between remnants and metastatic lesions. The serum thyroglobulin levels and FDG uptake did not correlate with early washout of the I-131 avid lesions. We concluded that earlier imaging is necessary for detection of metastatic lesions.