18F-FDG-PET scans and high dose of iodine-131 (I-131) treatment in patients with elevated thyroglobulin and negative iodine-131 scan differentiated thyroid carcinoma

2111 Objectives To investigate the usefulness of 18F-FDG-PET and therapeutic effects of high dose I-131 in differentiated thyroid cancer (DTC) with elevated serum thyroglobulin (Tg) but negative radioactive iodine scan (RAI). Methods One hundred and twenty nine patients who had FDG-PETs for elevated Tg and negative RAI from February 2002 to December 2009 were included after total thyroidectomy followed by radioiodine ablation. Results 69 patients had positive findings on FDG-PET, true positive in 58 patients, false positive in 11 patients, true negative and false negative in 32, 28 patients, respectively. The overall sensitivity, specificity, and accuracy of FDG-PETs were 67%, 74%, and 70%, respectively. FDG-PET had a high specificity (94%) in patients with positive post-therapy scan. Clinical management changed for 58 of 129 (45%) patients, including surgery, radiation therapy, or chemotherapy. Of 129 patients included, 101 patients received I-131 treatment. The post-therapy scans were positive in 54/101 (53%) patients, and 51/101 (50%) patients had 50% decrease in stimulated Tg. The change of stimulated Tg before (15.2, range from 2 to 1349) and after (5.55, range from 0.4 to 7666) I-131 treatment was statically significant after 26 ±22.1 months period of follow-up (p=0.001). In 13 patients without any treatment, 8 patients (8/13, 62%) had 50% decrease in Tg. Conclusions FDG-PET scan is useful in the diagnosis of recurrence and metastases of DTC with elevated Tg and negative scan, and have a high specificity in patients with positive post-therapy scan. Iodine-131 treatment achieved 53% positive post-therapy scan and 50% decrease in stimulated Tg, which suggests that I-131 therapy has a therapeutic effect for half of patients when the Tg level is considered an index of tumour burden. However, the spontaneous decrease of Tg in limited patients (8/13, 62%) with relatively low Tg without any treatment indicated that iodine-131 therapy should be individualised according to clinical characteristics. Research Support Alberta Cancer Foundation, Canada Foundation for Innovatio