Improved detection of lung or bone metastases with an I-131 whole body scan on the 7th day after high-dose I-131 therapy in patients with thyroid cancer.

Purpose: The purpose of this study is to compare post-therapy third day and seventh day I-131 whole body scans (3DWBS and 7DWBS) in detecting lung or bone metastasis from well-differentiated thyroid cancer.
Materials and Methods: We enrolled 52 patients with lung or bone metastasis out of 1,152 patients who were treated with high-dose I-131 therapy from January 2008 to June 2009. All patients underwent 3DWBS and 7DWBS. I-131 avidity was classified into three grades: no uptake, suspicious for uptake, and definite uptake. We compared the presence and grades of metastatic lesions on each scan. We categorized all cases into three groups based on I-131 uptake on each scan and compared several clinical parameters including FDG uptake and thyroglobulin (Tg) level among the groups.
Results: Sixty metastatic cases from 52 patients (45 lung and 15 bone metastases) were included. In 35 cases, I-131-avid metastatic lesions were detected by both 3DWBS and 7DWBS (group A). In 15 cases, metastatic lesions were missed on 3DWBS but detected on 7DWBS (group B). In 10 cases, I-131 uptake was not detected on either 3DWBS or 7DWBS (group C). Ten of 45 cases (22.2%) of lung metastasis that were negative on 3DWBS were detected on 7DWBS (p = 0.002). Five of 15 cases (33.3%) of bone metastasis that were negative on 3DWBS were detected on 7DWBS (p = 0.0625). The serum Tg level (TSH stimulated) was significantly different among groups A, B, and C (p = 0.0030). However, after exclusion of cases without a history of I-131 therapy, there was no significant difference in serum Tg level among the groups (p = 0.2330). The number of cases with a prior history of metastasis was higher in group A than in group B (p = 0.0069). However, there was no significant difference in prior history of metastasis between groups A and C (p = 0.8107).
Conclusion: 7DWBS showed more lung or bone metastatic lesions than 3DWBS. After high-dose I-131 therapy, 7DWBS should be considered regardless of the results of the 3DWBS for the diagnosis of lung or bone metastasis from well-differentiated thyroid cancer.