Preoperative Diagnostic Value of [18F] Fluorodeoxyglucose Positron Emission Tomography in Patients With Radioiodine-Negative Recurrent Well-Differentiated Thyroid Carcinoma

Routine follow-up of patients with well-differentiated thyroid cancer (WDTC) includes whole-body 131I scintigraphy and serum thyroglobulin determination. Limitations in detecting locoregional recurrence or distant metastases occur when progressive dedifferentiation of thyroid carcinoma cells leads to a loss of iodine-concentrating capacity. In the course of the disease, false-negative radioactive iodine scans are seen in up to 20% of patients with original WDTC. In contrast to the otherwise encouraging overall prognosis of WDTC, this subpopulation of patients may have a poor outcome because of delayed detection of tumor relapse and consecutive surgical treatment. In addition, metastatic thyroid lesions that fail to concentrate radioactive iodine exhibit more aggressive biologic behavior. Several conventional imaging techniques, such as ultrasonography, computed tomography (CT), and magnetic resonance imaging (MRI), have been used to localize tumor deposits in the presence of negative 131I scans and clinical suspicion based on an increased serum thyroglobulin concentration in the absence of thyroglobulin antibodies or disturbance of the thyroglobulin recovery test. However, all these anatomic imaging tools can produce varied and inconsistent results, especially in patients undergoing repeated surgical procedures. In the neck, where metastases most frequently occur, the sensitivity and specificity of ultrasound are strongly dependent on subjective factors. 1 For CT and MRI, the overall error rate of assessment of cervical lymph node metastases is 7.5%. 2 The results of nonspecific scintigraphic examinations with 201Tl and 99mTc-sestamibi are unsatisfactory, with an average reported sensitivity of 60%. Recently, numerous reports have shown high utility of 2-[18F] fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) in the detection and staging of different malignant tumors: soft tissue sarcomas;3 cutaneous melanoma;4 pancreatic, 5 breast, 6 colorectal, 7 esophageal, 8 head and neck, 9 and non-small-cell lung cancer;10 neuroendocrine tumors;11 metastases of unknown primary origin;12 and non-Hodgkin’s lymphoma. 13 The ability of FDG-PET to visualize tumors is based on the increased carbohydrate metabolism of malignant cells, with subsequent elevated glucose utilization and selective 18FDG uptake. Once phosphorylated by hexokinase, FDG accumulates intracellularly because it cannot be metabolized as glucose. Although well-differentiated thyroid malignancies with sufficient 131I uptake are associated with decreased glucose metabolism, and in contrast, dedifferentiated tumors lacking the possibility of 131I accumulation show elevated glucose utilization, FDG-PET has been also introduced in the follow-up of thyroid cancer. 14–17 Our study assesses the experience with FDG-PET in the detection of recurrent thyroid carcinoma in the presence of a negative 131I scan and pathologic serum thyroglobulin values.