Thallium-201 and iodine-131 scintigraphy in differentiated thyroid carcinoma

The purpose of this study was to determine the concordance and discordance between diagnostic I-131 and (TI)-T-201 whole-body scintigraphy in patients with differentiated carcinoma of the thyroid. Methods: Following thyroidectomy for differentiated thyroid carcinoma, 50 patients underwent whole-body I-131 and (TI)-T-201 scanning (60 pairs of scans in total). Fifteen pairs of studies were obtained before ablative therapy, 30 pairs after ablative therapy and 15 pairs after I-131 therapy for metastatic disease. Serum thyroglobulin levels were concurrently determined by radioimmunoassay. Results: Thirty-six I-131 whole-body scans (in 34 patients) showed residual uptake in the neck, but only six (17%) of the corresponding whole-body thallium studies had detectable uptake in the neck. Fourteen I-131 scans (in nine patients) identified multiple metastatic lesions, whereas the thallium scans were interpreted as either negative, nonspecific or showing fewer lesions. In four study pairs, the thallium scans showed solitary lesions that were not detected by the corresponding radioiodine scans. In 16 scans, the thallium studies gave false-positive results. Conclusion: Iodine-131 scintigraphy for differentiated thyroid carcinoma is more sensitive and more specific than (TI)-T-201 scintigraphy for detection of distant metastases and residual activity in the neck following thyroidectomy.