Diagnostic accuracy of 18F-FDG PET/CT in restaging of well differentiated thyroid cancer

To evaluate the diagnostic accuracy of FDG PET/CT in detection of recurrent well differentiated thyroid cancer in patients with elevated Tg or anti-Tg levels. PET/CT images from February 2004 to February 2007 of well differentiated thyroid cancer patients with history of total thyroidectomy and radioiodine ablation were included. All patients had elevated Tg titer (>2.0 ng/mL with TSH stimulation, or >1.0 ng/mL without TSH stimulation) while diagnostic radioiodine scan was negative, or elevated anti-Tg titer (>50.0 IU/mL). The peak standard uptake value (pSUV) of the suspected lesions on PET/CT were recorded, and their sites were categorized as (1) thyroidectomy bed, (2) cervical lymph node (LN), (3) mediastinal LN, and (4) distant metastasis. The PET/CT findings were compared with histological or clinical follow-up results based on other imaging modality (CT, MRI, US, diagnostic I-123 scan, post therapeutic I-131 scan) findings and subsequent Tg and anti-Tg titers. Total of 48 PET/CT images from 43 patients were included, and histologic confirmation was available in 17 cases. Recurrence was positive in 36 cases, and negative in 12 cases. On patient basis, the overall diagnostic accuracy, sensitivity, and specificity of PET/CT for detecting recurrent thyroid cancer were 66.7%, 77.8%, and 33.3%, respectively. On lesion basis, from total of 50 sites with perceptible FDG uptake, true positive findings were noted in 11 cases out of 16 for thyroidectomy bed, 19 out of 26 for cervical LN, 4 out of 6 for mediastinal LN, and both of the 2 distant metastases cases. The pSUV of the recurrence positive group was statistically higher than the negative group (6.27.2 versus 2.81.8, p=0.013). FDG PET/CT aids in detecting recurrent thyroid cancer in patients with negative diagnostic radioiodine scan but elevated Tg or anti-Tg titers