Comparison of post-therapeutic sequential 131I whole-body scans in the detection of metastatic thyroid cancer.

BACKGROUND: The aim was to compare the detect ability of 3 sequential 131I whole-body scans (tri-WBS) on the second, third, and fourth day after 131I therapy for metastatic thyroid cancer.

METHODS: Differentiated thyroid cancer patients who received oral high-dose 131I therapy underwent routinely tri-WBS on the second, third, and fourth day after total or near-total thyroidectomy in our hospital. We enrolled 137 patients with 261 tri-WBSs in this study between January 2015 and November 2017. The inclusion criteria was that at least one metastasis was found in the tri-WBS. We classified radioactive uptake of metastatic lesions by visual assessment into 3 grades: grade 0 = no uptake, grade 1= suspicious uptake, and grade 2 = definite uptake. The fourth day 131I WBS images were also compared with concurrent pre-therapeutic 99mTc-pertechnetate WBS images when available. We also analyzed the serum Tg levels of probably statistical difference in the patients with lymph node, lung, bone, and multiple metastasis when they underwent the first radioiodine ablation.

RESULTS: A total of 722 metastatic accumulations were identified in the final decisions, including 293 lymph node metastases, 261 nodular pulmonary metastases, 49 diffuse bilateral pulmonary metastases, 106 bone metastases, and 13 other metastases. The differences of intensity of uptake in sequential three day images were significant in visualization of lymph node metastasis (x2=124.432, p<0.001), nodular pulmonary metastasis (x2=160.334, p<0.001), diffuse bilateral pulmonary metastasis (x2=41.710, p<0.001), and bone metastasis (x2=22.118, p<0.001) in our study. Compared to the second day scans, the fourth day scans detected 87 (29.70%) more metastatic lymph nodes, 111 (42.53%) more nodular pulmonary metastases, 26 (53.06%) more diffuse bilateral pulmonary metastases and 17 (16.95%) more bone metastases. The differences of intensity of uptake between 99mTc-pertechnetate WBS and the fourth day 131I WBS were significant in visualization of lymph node metastasis (x2=172.624, p<0.001), nodular pulmonary metastasis (x2 = 111.004, p<0.001), diffuse bilateral pulmonary metastasis (x2=17.400, p<0.001) and bone metastasis (x2=46.298, p<0.001). The means of RTg in the patients with only lymph node, lung, bone metastasis, and multiple metastases were 47.20, 76.58, 89.00, and 91.56, respectively. The differences of serum Tg levels in the patients with only lymph node, lung, bone matastasis, and multiple metastases were significant (x2=35.850, p<0.001).

CONCLUSIONS: The detect ability of tri-WBS was significantly different even for consecutive three day images on the second, third, and fourth day after 131I therapy for metastatic thyroid cancer. There was a linear trend of increasing 131I uptake from the second to fourth day 131I WBS. The pre-therapy 99mTc-pertechnetate WBS demonstrated a poor ability to detect metastatic thyroid cancer compared to 131I WBS. There was an increasing trend of the means of RTg in patients with more extensive metastases.