Thyroglobulin and low-dose iodine-131 and technetium-99m-tetrofosmin whole-body scintigraphy in differentiated...

Determination of thyroglobulin (Tg) levels, determined under endogenous thyroid-stimulating hormone stimulation after withdrawal of L-thyroxin treatment (off-T4), has been proven to be the most sensitive method for evaluation of patients with recurrent malignancy or distant metastases. This study uses a comparative approach between low-dose 131I scan and the previously reported highly sensitive 99mTc-tetrofosmin whole-body scintigraphy, using Tg-off-T4 as a basis for comparison.

Fifty-eight consecutive patients of our follow-up program with primary thyroid carcinoma ablated with thyroidectomy and radioiodine therapy were examined after L-thyroxin withdrawal over 3-4 wk with 131I (185 MBq) and 99mTc-tetrofosmin whole-body scintigraphy and Tg determination (off-T4) within 5 days. Patients with Tg levels above 0.5 ng/ml were defined as Group A (n = 29). Group B (n = 29) comprised patients who had Tg levels (off-T4) below 0.5 ng/ml.

Iodine-131 revealed only 19 of 44 tumor sites (43.18%). Additionally, three remnants could be demonstrated. Sensitivity showed decreasing values for local recurrences (4 of 7, 57.1%), bone lesions (7 of 13, 53.85%) and mediastinal (2 of 4, 50%), lung parenchymal (3 of 7, 42.85%) and lymph node (2 of 9, 22.2%) metastases. Whole-body scintigraphy with 99mTc-tetrofosmin revealed a total of 39 of 44 malignant lesions (88.6%). Sensitivity was superior for lung parenchymal metastases (9 of 9, 100%), mediastinum (4 of 4, 100%) and lymph nodes (9 of 10, 90%) and inferior for bone metastases (11 of 13, 84.6%). Local recurrences could be detected in 6 of 7 patients (85.7%), and thyroid remnants were detected in 2 cases (2 of 11, 18.2%). One liver metastasis could not be detected because of the physiologic tracer distribution of 99mTc-tetrofosmin. Thyroglobulin-off-T4 detected malignant recurrence or metastases in 18 of 19 patients (94.7%) when a cutoff of 3 ng/ml was used and in 16 of 19 patients (84.2%) when a cutoff of 10 ng/ml was used. Specificity was calculated as 71.8% when a cutoff of 0.5 ng/ml was used, 89.7% when a cutoff of 3 ng/ml was used and 100% when a cutoff of 10 ng/ml was used.

Scintigraphy with 99mTc-tetrofosmin showed clear advantages concerning sensitivity in most metastatic lesions when compared with low-dose 131I scan. Despite a slight lower specificity, 99mTc-tetrofosmin whole-body scintigraphy has, therefore, been proven to be a useful tool in the assessment of metastatic lesions in differentiated thyroid carcinoma.