18F-FDG-PET in the follow-up of thyroid cancer

Differentiated thyroid cancer is a rare tumour with an incidence of 4 - 9/100 000/year. For preoperative assessment of thyroid nodules, ultrasonography (US) and US-guided fine needle aspiration biopsy are the methods of choice to detect thyroid cancer. The value of preoperative fluorine-18 fluorodeoxyglucose positron emission tomography (F-18-FDG-PET) in differentiating malignant from benign nodules, especially in cases of follicular proliferation, has not yet been evaluated. After thyroidectomy and radioiodine remnant ablation, several methods are used to follow patients with differentiated thyroid cancer, including serum thyroglobulin, ultrasonography of the neck, iodine-131 (I-131) whole body scintigraphy (WBS) and scintigraphy with nonspecific tracers such as technetium-99 m (Tc-99m) Tetrofosmin or Sestamibi. Whereas the specificity of I-131-WBS is high, sensitivity is low, especially if one takes into account that only two-thirds of recurrences or metastases store iodine. With the introduction of F-18-FDG in oncology, it is also used for the detection of local recurrences and metastases of differentiated thyroid cancer. Elevated thyroglobulin but negative I-131-WBS belongs to the 1a indications for F-18-FDG-PET in oncology according to the German Consensus Conference 2000. The sensitivity for detecting I-131-negative metastases with F-18-FDG-PET can be increased by elevated thyroid-stimulating hormone (TSH) after withdrawal of thyroid hormone therapy or after intramuscular injection of recombinant TSH. Most of the I-131-negative metastases demonstrate F-18-FDG uptake, which represents rapid tumour growth and poor differentiation, whereas most of the I-131-positive metastases are F-18-FDG negative. The combination of I-131-WBS and F-18-FDG-PET leads to an increase in the detection rate to more than 90 - 95 % in cases of elevated thyroglobulin, because well- and less-differentiated cancer cells may be present in one patient. In rare cases, a recurrent tumour or metastasis may accumulate I-131 as well as F-18-FDG. In these patients, it may be possible that well- and less-differentiated cells are present in one metastasis. The early use of F-18-FDG-PET in patients with elevated thyroglobulin, especially in the case of negative I-131-WBS, changes the therapeutic strategy in up to half of the patients (surgery, external radiation).