99mTc-tetrofosmin, 131I Whole Body Scan and Thyroglobulin in Detecting Differentiated Thyroid Carcinoma Metastases

ABSTRACT

PURPOSE: The purpose of this study was to assess the detectability of differentiated thyroid carcinoma metastases by 99mTc-tetrofosmin and to compare these results with a 131I whole body scan (131I WBS). The results of two scans were also compared with the T4 off-thyroglobulin (Tg) concentration. METHODS: A prospective study was performed on 43 patients (40 females, 3 males) with differentiated thyroid carcinomas (41 papillary, 2 follicular) having undergone a total thyroidectomy, and received 100~200 mCi (3,700~7,400 MBq) of radioiodine for ablation of residual thyroid tissue, or treatment of metastasis. All patients (n=43) had a 99mTc- tetrofosmin scan, and a 131I WBS following the discontinuation of thyroid hormone replacement. The T4 off-Tg level was checked immediately prior to the radioiodine therapy, with T4 off-Tg levels above 20 ng/ml defined as positive for metastasis or a remnant disease. RESULTS: Cervical metastases were considered in 6 patients and distant metastases in 7, based on the clinical, radiological, and histopathological findings. The 131I WBS (70.2%) was much more sensitive than the 99mTc-tetrofosmin scan (29.8%) in demonstrating the residual thyroid tissue following surgery. The 131I WBS revealed cervical metastases in 3 of the 6 patients, but only 2 of the 6 were revealed by the 99mTc-tetrofosmin scan. Of the 3 patients with negative 131I WBS, two were detected by the 99mTc-tetrofosmin scan. The sensitivities of the 131I WBS and 99mTc-tetrofosmin scan in diagnosing distant metastases were comparable (71%, and 57% respectively), but in 2 patients with negative 131I WBS, the 99mTc-tetrofosmin revealed distant metastases. The specificities of the 131I WBS and 99mTc-tetrofosmin scan were not comparable (100%, 97%, respectively) in the diagnosis distant metastases. The mean T4 off-Tg level of the patients with cervical or distant metastases was 317 ng/ml, with a sensitivity and specificity of 100%, and 83% respectively. In the patients with a T4 off-Tg level above 50 ng/ml, the two scans and clinical studies could not reveal any metastases in 3 of the patients. CONCLUSION: Although the specificity of the 99mTc-tetrofosmin scan was slightly lower than that of the 131I WBS, it is a useful tool for detecting cervical or distant metastases in differentiated thyroid carcinomas and does not require prior withdrawal from thyroid hormones. Therefore the concomitant use of a 99mTc-tetrofosmin scan, a 131I WBS and Tg, is more effective in detecting metastases in differentiated thyroid carcinomas.