¹⁸F-FDG PET/CT changes therapy management in high-risk DTC after first radioiodine therapy

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 ¹⁸F-FDG PET/CT at initial diagnosis in patients with high-risk differentiated thyroid carcinoma and determined whether the ¹⁸F-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.

The study group comprised 90 consecutive patients with either extensive or metastasized high-risk differentiated thyroid carcinoma who received ¹⁸F-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.

¹⁸F-FDG PET/CT was positive in 26 patients (29%) and negative in 64 patients (71%). Compared to the results of posttherapeutic ¹³¹I 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.

¹⁸F-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, ¹⁸F-FDG PET/CT in high-risk patients with differentiated thyroid carcinoma has been established as an initial staging modality.