Characteristics of primary papillary thyroid carcinoma with false-negative findings on initial (18)F-FDG PET/CT

We often observe that uptake of tracer is not detected in the primary cancer focus in patients with histologically proven papillary thyroid carcinoma (PTC) on preoperative (18)F-fluorodeoxyglucose positron emission tomography-computed tomography ((18)F-FDG PET/CT). Therefore, we analyzed the clinical and pathologic variables affecting false-negative findings in primary tumors on preoperative (18)F-FDG PET/CT.

We retrospectively reviewed the medical records of 115 consecutive patients who underwent (18)F-FDG PET/CT for initial evaluation and were diagnosed with PTC by postoperative permanent biopsy. The clinical and pathologic characteristics that influence the (18)F-FDG PET/CT findings in these patients were analyzed with respect to the following variables: age, gender, tumor size, multifocality of the primary tumor, perithyroidal invasion, lymphovascular or capsular invasion, and central lymph node metastasis-based final pathology.

Twenty-six (22.6%) patients had false-negative (18)F-FDG PET/CT findings. In patients with negative (18)F-FDG PET/CT findings, tumor size, and perithyroidal and lymphovascular invasion were significantly less than in patients with positive (18)F-FDG PET/CT findings. Tumors >1 cm in size were correlated with (18)F-FDG PET/CT positivity. On multivariate analysis, perithyroidal invasion (P = 0.026, odds ratio = 7.714) and lymphovascular invasion (P = 0.036, odds ratio = 3.500) were independent factors for (18)F-FDG PET/CT positivity. However, there were no significant differences between (18)F-FDG PET/CT positivity and age, gender, capsular invasion, and central lymph node metastasis based on final pathology.

Tumor size and perithyroidal and lymphovascular invasion of papillary carcinoma can influence (18)F-FDG PET/CT findings. Absence of perithyroidal and lymphovascular invasion were independent variables for false-negative findings on initial (18)F-FDG PET/CT in patients with PTC.