Could [18]F-fluorodeoxyglucose PET/CT Change the Therapeutic Management of Stage IV Thyroid Cancer with Positive (131)I Whole Body Scan?

Aim: Stage-IV differentiated thyroid cancer (DTC) patients may present elevated serum thyroglobulin (Tg) levels associated with positive [(131)I] whole-body-scan (WBS). Nevertheless some patients in whom WBS does not reveal new sites of disease show increased Tg levels. This finding prompts thorough restaging in order to exclude the presence of metastases unable to concentrate iodine. The aim of our study was to evaluate the impact of [(18)F]FDG-PET/CT in both the assessment of overall extent of the disease and the therapeutic management in a group of stage-IV DTC patients.

Methods: On suspicious of non-iodine concentrating additional metastases, 20 stage-IV DTC patients with increasing Tg levels and stable positive post-therapy WBS were enrolled. Conventional imaging (CI) procedures, including neck ultrasonography, bone-scintigraphy and computed tomography (CT) were performed before [(18)F]FDG-PET/CT.

Results: [(18)F]FDG-PET/CT was positive in 16 out of 20 patients (80%). In 9 patients (45%) [(18)F]FDG PET/CT detected a larger number of tumour recurrences/metastatic sites than WBS+CI. [(18)F]FDG PET/CT findings prompted modification of the management of 11 patients (55%), in whom surgery or external radiotherapy were eventually considered more appropriate than radioactive iodine therapy. These further therapies improved the quality of life in several patients but did not change their survival rate.

Conclusion: Our results showed that [18F]FDG-PET/CT can detect new radioiodine-negative metastases in advanced DTC patients with unchanged positive WBS and increasing Tg levels. [(18)F]FDG-PET/CT may constitute a useful tool in the choice of the best therapeutic strategy in such difficult cases.