Can Thyrogen-Stimulated FDG-PET Detect Early Recurrence of Thyroid Cancer? Does Recombinant Human Thyrotropin-Stimulated Positron Emission Tomography With [18F] Fluoro-2-Deoxy-d-Glucose Improve Detection of Recurrence of Well-Differentiated Thyroid Carcinoma in Patients With Low Serum Thyroglobulin?

Background: When whole-body scintigraphy (WBS) is negative despite elevated serum thyroglobulin (Tg) levels, the value of FDG-PET in the diagnosis of recurrent and metastatic thyroid cancer is well documented. An experimental study by Petrich and colleagues (2002) showed that thyrotropin (TSH) levels >0.3 μg/L with equivocal or negative WBS showed an increased level of FDG uptake after recombinant human TSH (rhTSH; Thyrogen®) administration in locally recurrent and distally metastatic lesions. Furthermore, a study by Chin et al (2004) has shown that rhTSH improves detection of occult thyroid metastases with PET-FDG compared to scans performed with TSH suppression. These studies were performed on patients with elevated serum Tg. Objective: To determine in a controlled prospective study whether rhTSH stimulation improves the detectability of occult thyroid metastases with PET-FDG with low but detectable serum Tg. Participants/Methods: Prospective study of 28 females and 16 males with Tg >2 μg/L while on T4 suppression (mean, 7 μg/L) and negative or noncontributory radioiodine WBS. Of patients, 38 had papillary and 6 had follicular thyroid carcinoma; all patients had previously undergone thyroidectomy and postoperative iodine ablation. Sixty-one Thyrogen-stimulated PET/CT-FDG scans were performed in these 44 patients. Results: PET/CT-FDG was positive in 20 patients and negative in 24. Of 61 PET/CT-FDG scans, 25 were positive. Of 25 positive PET scans, 19 had Tg values <10 μg/L while on T4 suppression. Among 24 negative patients, Tg continued to rise in 13, and 1 had surgically proven recurrence; these were regarded as false negatives. In the other 10 negative patients, Tg levels remained steady or fell over the 1.5 years. There was no difference in PET/CT-FDG results (positive vs negative) compared to serum Tg concentrations, nor was there correlation with PET/CT-FDG results and initial tumor size or node metastasis. Conclusions: The sensitivity of Thyrogen-stimulated PET/CT-FDG is low, and no correlation exists between PET/CT-FDG and Tg level. Positive PET-FDG, however, was found in a substantial number of patients with serum Tg levels <10 μg/L. Reviewer's Comments: While sensitivity of rhTSH-stimulated PET/CT-FDG may be low, in some situations, it is clearly still the best test available. Thyroid cancer patients who are post-thyroidectomy with successful ablation, but now have low but detectable serum Tg, are at high suspicion for early recurrence. If the wholebody radioiodine scan were negative, I would first try a high-dose I-131 treatment in hopes of treating a tumor I could not see with a diagnostic scan. If the post-treatment high-dose scan fails to reveal the source, and if the Tg continues to rise, PET/CT-FDG is probably the best method of localizing the source. (Reviewer-C. Richard Goldfarb, MD).