¹⁸F-FDG PET/CT Changes Therapy Management in High-risk DTC After First Radioiodine Therapy

Purpose: Advanced tumour stage and initial metastases are associated with reduced general and tumour-free survival in patients with differentiated thyroid carcinoma. Optimal initial therapy is mandatory for a positive patient outcome, but can only be performed if all non-iodine-avid tumour lesions are known before planning treatment. We analysed the benefit of (18)F-FDG PET/CT at initial diagnosis in patients with high-risk differentiated thyroid carcinoma and determined whether the (18)F-FDG PET/CT results led to a deviation from the standard procedure, which consists of two consecutive radioiodine treatments with thyroid hormone suppression in between and no additional imaging, with individual patient management.

Methods: The study group comprised 90 consecutive patients with either extensive or metastasized high-risk differentiated thyroid carcinoma who received (18)F-FDG PET/CT after the first radioiodine treatment approximately 4 weeks after thyroidectomy under endogenous TSH stimulation. We carried out PET/CT imaging with low-dose CT without contrast medium, which we only used for attenuation correction of PET images.

Results: (18)F-FDG PET/CT was positive in 26 patients (29%) and negative in 64 patients (71%). Compared to the results of posttherapeutic (131)I whole-body scintigraphy, the same lesions were PET-positive in 7 of the 26 patients, different lesions were PET-positive in 15 patients, and some PET-positive lesions were the same and some were different in 4 patients. TNM staging was changed due to the PET results in 8 patients. Management was changed in 19 of the 90 patients (21%), including all patients with only FDG-positive lesions and all patients with both FDG-positive and iodine-positive lesions. Age was not a predictive factor for the presence of FDG-positive lesions. FDG-positive and iodine-positive lesions were associated with high serum thyroglobulin. However, at low serum thyroglobulin values, tumour lesions (iodine- and/or FDG-avid) were also diagnosed. Thus, the serum thyroglobulin value prior to the first radioiodine treatment cannot be used as a predictor of the presence of FDG-positive lesions.

Conclusion: (18)F-FDG PET/CT resulted in a change of therapeutic procedure in 11 of 90 patients and in a change of patient management through additional diagnostic measures in 8 of 90 patients, and is consequently very helpful in initial staging. At our hospital, (18)F-FDG PET/CT in high-risk patients with differentiated thyroid carcinoma has been established as an initial staging modality.