Aim: 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.
Patients and methods: 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.
Results: 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.
Conclusions: 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.