Purpose: This study aimed to evaluate the diagnostic value of 18F-FDG PET/CT and identify the best parameter to detect subcentimeter cervical nodal metastasis in patients with a well-differentiated thyroid carcinoma (WDTC), elevated serum thyroglobulin (Tg) levels, but negative findings in the 131I whole-body scan (WBS).
Materials and methods: We prospectively studied 30 consecutive patients with WDTC after standard surgery and radioiodine treatment. All patients had serum Tg greater than 10 ng/mL during thyroid hormone withdrawal but negative findings in the therapeutic 131I WBS. One whole-body CT scan and serial whole-body and neck PET scans were performed between 10 and 170 minutes after 18F-FDG injection. Parameters studied were SUVmax, percent change in SUVmax, SUV ratios of lesions to reference organs, and their percent change. Result in the PET/CT scan was correlated with histopathology and follow-up information. Patient-based and lesion-based (subcentimeter cervical lymph nodes) analyses were performed. Outcome of Tg level after lymph node resection was also analyzed.
Results: The overall sensitivity, specificity, accuracy, and positive and negative predictive values were 100%, 78%, 93%, 91%, and 100%, respectively. In lesion-based analysis, the differential SUVmax between 2 time points did not provide higher sensitivity than the individual SUVmax at the 60th or 90th minute. A combined SUVmax at the 90th minute greater than 2.75 and a percent change of SUVmax between the 60th and 90th minute greater than -1.1% provides the best diagnostic value with sensitivity, specificity, accuracy, and positive and negative predictive values of 81%, 90%, 83%, 97%, and 56%, respectively. After surgery, patients with completely resected PET-positive nodes without distant metastasis showed reduction of suppressed Tg to less than 2 ng/mL.
Conclusions: Combined SUVmax at the 90th minute and the percent change of SUVmax between the 60th and 90th minute provides the best diagnostic value to differentiate benign from malignant conditions in subcentimeter lymph nodes.