Correlation of 18F-FDG PET/CT findings with histopathological results in differentiated thyroid cancer patients who have increased thyroglobulin or antithyroglobulin antibody levels and negative 131I whole-body scan results.

This study aimed to investigate the correlation of 18F-FDG PET/CT findings with histopathological results in defining the recurrence of the disease in patients with differentiated thyroid cancer (DTC) who have increased thyroglobuline (Tg) or anti-Tg antibody (TgAb) levels and negative 131I whole-body scan (WBS) result. A total of 59 patients with DTC (44 women, 15 men; mean [SD] age, 48.2 [22.6] years) were included in the study. All of the patients had previous papillary thyroid cancer, and all of them had undergone radioiodine ablation after a total or near-total thyroidectomy. After radioiodine ablation, patients were followed up for approximately 2.5 years. In the follow-up, the patients with negative 131I-WBS results and increased Tg or TgAb levels under thyroid-stimulating hormone-stimulated conditions underwent an 18F-FDG PET/CT scan to determine any recurrence of disease. There were negative or uncertain findings in the neck ultrasonography and/or thorax CT in most of the patients. The 18F-FDG PET/CT findings were compared with the histopathological results in all patients. Although 49 patients had increased Tg levels, the remaining 10 patients had increased TgAb levels. In patients with high Tg levels, 18F-FDG PET/CT scan results were negative in 10 and positive in 39 patients. In this patient group, 18F-FDG PET/CT findings were true positive, true negative, false positive, and false negative in 32, 3, 7, and 7 patients, respectively. The sensitivity, specificity, positive predictive value, negative predictive value and accuracy of 18F-FDG PET/CT in this group were calculated as 82%, 30%, 80%, 30%, and 71%, separately. In the receiver operating characteristic analysis, a 4.5 cutoff SUV(max) was calculated with 75% sensitivity and 70% specificity for predicting disease recurrence. Cutoff Tg level was calculated as 20.7 ng/mL with 75% sensitivity and 55% specificity. In 10 patients with high TgAb levels, 18F-FDG PET/CT was true positive, true negative and false positive in 6, 2, and 2 patients, respectively. The sensitivity, specificity, positive predictive value, negative predictive value and accuracy of 18F-FDG PET/CT in this patient group were calculated as 100%, 50%, 75%, 100%, and 80%, respectively. 18F-FDG PET/CT may be a useful imaging modality in defining recurrence of the disease in patients with DTC who have increased Tg or TgAb levels, negative 131I-WBS results, and negative or suspicious neck ultrasonography and/or thorax CT results. Although 18F-FDG PET/CT seems to be a more sensitive method in patients with increased TgAb levels, the number of patients is not enough to make a substantiated comment.