Background: Objectives were to analyze the relationship between a positive (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET) result and clinical and tumor factors in patients treated for differentiated thyroid cancer (DTC) and under suspicion of recurrence or metastasis, and to determine the diagnostic validity of PET in DTC patients with elevated serum thyroglobulin (Tg) and negative (131)I whole-body scan ((131)I-WBS).
Methods: We studied 50 DTC patients with elevated serum Tg and negative WBS treated with total thyroidectomy and (131)I ablation. Thyroxin treatment was withdrawn and patients were on iodine-free diet before WBS. Tg, anti-Tg antibodies, and thyroid-stimulating hormone (TSH) were determined. Patients with negative WBS and elevated Tg underwent PET study 1 week later. PET findings were verified by pathology findings or other imaging techniques [computed tomography (CT), magnetic resonance imaging (MRI), ultrasound (US)] and/or 12-month follow-up. The relationship between PET findings and tumor (histological type, size, multifocality, thyroid capsular invasion, lymph-node and/or metastatic involvement) and clinical (age at diagnosis, sex, Tg, accumulated iodine dose, and recurrence time) variables was analyzed.
Results: PET was positive in 32/39 patients with confirmed disease (82% sensitivity) and negative in 7/11 of disease-free cases (64% specificity), a positive predictive value (PPV) of 89%. Tumor size (P < 0.05) and thyroid capsular invasion (P < 0.05) were significantly associated with positive PET study. The relationship of PET findings with Tg levels and age at diagnosis was close to significance.
Conclusion: (18)F-FDG-PET study offers a high sensitivity and positive predictive value (PPV) in patients with negative WBS and Tg positive. The use of FDG-PET is strongly recommended in DTC patients with large tumors, thyroid capsule invasion or poor-prognosis variants.