Abstract

We conducted a prospective study to define the sensitivity of 131I scintigraphy and 18FDG PET whole-body scanning in the detection of thyroid cancer and metastases.

METHODS: Forty-one patients with differentiated thyroid carcinoma who underwent thyroidectomy and 131I elimination of the remaining thyroid were studied by 18FDG whole-body PET in 52 examinations and by 131I whole-body scanning.

RESULTS: Combined 18FDG and 131I imaging resulted in a sensitivity of about 95%, with alternating uptake of 131I and 18FDG in the metastases: 131I trapping metastases with no 18FDG uptake and 18FDG trapping metastases with no 131I uptake. Five uptake types were differentiated. Alternating uptake was found in about 90% of the patients, which was nearly identical to the sensitivity of the combined 131I/18FDG investigation. In six patients with increasing human thyroglobulin levels, we found that 18FDG whole-body PET localized positive neck metastases of papillary thyroid carcinomas that were histologically confirmed after extirpation.

CONCLUSION: Combination 18FDG and 131I whole-body imaging protocol enables detection of local recurrence or metastases on whole-body scans that are often not shown by other imaging methods. Biochemical grading of thyroid cancer may also be possible with this method: Tumors with remaining functional differentiation for hormone synthesis and iodine uptake have low glucose metabolism in more than 95%; tumors without this functional differentiation of 131I uptake show high, glucose metabolism. Fluorine-18-FDG uptake seems to be an indicator of poor functional differentiation, and possibly higher malignancy, in thyroid cancer.