Relative value of thallium-201 and iodine-131 scans in the detection of recurrence or distant metastasis of well differentiated thyroid carcinoma

Radioactive iodine ¹³¹I has been found to be more sensitive and more specific than thallium-201 for the detection of distant metastases and thyroid remnants in the neck in cases of well-differentiated thyroid carcinoma. ²⁰¹T1 has been deemed particularly useful in localizing metastases or recurrence in patients with a negative ¹³¹I scan and abnormal levels of serum thyroglobulin (Tg). This study aimed to: (1) determine the value of ²⁰¹Tl imaging in localizing metastases or recurrence in patients with well-differentiated thyroid carcinoma, and (2) evaluate the false-positive and false-negative results of ¹³¹I and ²⁰¹Tl scintigraphy. Sixty-two thyroid remnant ablated patients who underwent simultaneous postoperative ²⁰¹Tl and ¹³¹I scans and and serum Tg determinations were evaluated. Fifty patients had papillary thyroid carcinomas and 12 had follicular thyroid carcinomas. ²⁰¹Tl imaging was performed before the ¹³¹I studies. Of the 62 patients who underwent ²⁰¹Tl imaging studies, 24 were found to have positive results, with local recurrence or distant metastases. Patients with positive results in the ²⁰¹Tl imaging studies tended to be older, were mor often male, had higher Tg levels and had a higher recurrence rate. Of these 24 patients, ten had negative diagnostic or therapeutic ¹³¹I scans. Concurrently, serum Tg levels were less than 5 ng/ml in five of these ten patients. Three patients were deemed false positive by ²⁰¹Tl scans; one had a parotid tumour, one a periodontal abscess and one lung metastasis. Among the 38 patients with negative ²⁰¹Tl scans, 11 had positive findings on ¹³¹I scans. Three had distant metastases: two with lung metastases and one with bone metastases. Patients with false-positive results on ¹³¹I scans included those with biliary tract stones, ovarian cysts, and breast secretion. Of the 27 patients with negative (2O1)Tl and ¹³¹I scans, 15 had elevated serum Tg levels. Among these, local recurrence followed by lung metastases was manifested in a 49-year-old male with papillary thyroid carcinoma. In conclusion, both ¹³¹I and ²⁰¹Tl scans are useful in the detection of recurrence or distant metastasis of well differentiated thyroid cancers. ²⁰¹Tl scan could in particular be used in patients with a negative ¹³¹I scan in conjunction with an elevated Tg level.