Progression free survival related to 18F-FDG PET/CT uptake and 131I uptake in lung metastases of differentiated thyroid cancer.

OBJECTIVE: The lungs are the distant organ most frequently having metastases from differentiated thyroid cancer (DTC). Positive fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) scan can detect pulmonary metastases (PM) and thus suggest prognosis in DTC patients. The prognostic value of such a positive scan in DTC patients has not been specified. In this paper we studied the prognostic value of 18F-FDG PET/CT scan uptake and also of iodine-131 (131I) in DTC patients with PM. SUBJECTS AND METHODS: Out of 4500 DTC patients we retrospectively studied 83 patients having PM and treated with thyroidectomy and 131I ablation. Clinical data were also studied. Therapeutic response assessment was based on serum thyroglobulin (Tg) levels, Tg antibodies (TgAb) and tumor size on CT before and after 131I treatment. The mean follow-up period after the diagnosis of PM was 111.9±91.6 months (range: from 15 to 159 months). Sixty two (62/83) patients with PM were diagnosed soon after 131I ablation,8 patients at 6 months, 6 at 12 months and 4 at 18 months after 131I ablation. The remaining 3 patients were diagnosed at 30, 36 and 60 months after 131I ablation, respectively. The progression-free survival was estimated by the Kaplan-Meier method. RESULTS: Out of the 83 patients, 25 showed 18F-FDG uptake in PM of DTC with elevated Tg. Weak significant difference in the primary tumor size was found between 18F-FDG- positive and negative PM of DTC (P=0.05). After 131I ablation 57/83 patients had positive to 131I PM and also positive Tg. These patients were not statistically related to patients with positive or negative 18F-FDG PM according to CT and Tg levels (P=0.35, 0.47).The presence of 131I uptake in the lung lesions and the absence of 18F-FDG uptake in these lesions were independently related to a better progression-free survival (P=0.00). CONCLUSION: So, we conclude that 18F-FDG avidity predicates poor therapeutic effect on tumor size, high risk of disease progression and less favorable prognosis. Iodine-131 avidity remains the key factor suggesting a positive therapeutic effect on PM.