Relationship between positive thyroglobulin doubling time and 18F-FDG PET/CT-positive, 131I-negative lesions

In patients with differentiated thyroid carcinoma (DTC), metastases can either show iodine-131 (I) uptake on whole-body scintigraphy or F-2-fluoro-2-deoxy-D-glucose (F-FDG) uptake on combined PET and X-ray computed tomography (PET/CT), or a mix of both. The present study investigates the relationship between uptake patterns and prognosis in DTC patients, using thyroglobulin doubling time (TgDT) as a surrogate marker of prognosis.

We retrospectively examined F-FDG PET/CT and I WBS in 65 DTC patients who were referred to our department of nuclear medicine for F-FDG PET/CT between May 2007 and June 2011.

Eight patients were excluded from analysis because of other diseases that caused intense F-FDG uptake or because of failure to show I WBS uptake. F-FDG uptake was seen in 30 out of 57 (53%) patients, of whom 14 showed some degree of I uptake. In these 30 positive scans, we identified a total of 181 F-FDG-positive lesions. Of these, 60 lesions (33%) showed concurrent I uptake on whole-body scintigraphy. Of the nine patients with a positive TgDT in the patient group eight had F-FDG-positive, I-negative lesions, indicating poorer prognosis for this group.

In this initial exploratory retrospective study there appears to be an association between a positive TgDT and F-FDG-positive, I-negative metastases, which should encourage further studies in order to establish whether F-FDG PET-CT is the preferred primary imaging modality in patients with a positive TgDT. Roughly two-thirds of patients with a negative TgDT will show at least some degree of I positivity, potentially enabling further I therapy.