Incidence and Significance of Incidental Focal Thyroid Uptake on 18F-FDG PET Study in a Large Patient Cohort: Retrospective Single-Centre Experience in the United Kingdom

Abstract

Objective: To assess the prevalence and pathological nature of incidental focal thyroid uptake on ¹⁸F-FDG (2-[¹⁸F]-fluoro-2-deoxy-<smlcap>D</smlcap>-glucose) PET (positron emission tomography) and examine the role of the maximum standardised uptake value (SUV_max) to differentiate benign from malignant thyroid pathology. Material and Methods:¹⁸F-FDG PET reports were retrospectively reviewed. Incidental focal tracer uptake in the thyroid was noted in 147 patients (0.5%). Patients with known primary thyroid malignancy were excluded. The final diagnosis was made following ultrasonography of the neck, fine-needle aspiration cytology (FNAC) or histopathology of the surgically resected specimen where surgery was indicated. A Mann-Whitney U test was used to compare the SUV_max of benign and malignant thyroid pathology. Receiver operating characteristic (ROC) analysis was performed to identify an SUV_max cutoff in differentiating benign from malignant pathology. Results: A final diagnosis was achieved in 47/147 (32%) of the patients. The diagnoses included benign lesions in 36 patients and malignancy in 9 patients. In 2 patients, FNAC demonstrated indeterminate follicular lesions; however, surgical excision was not performed. There was a highly significant difference in the mean SUV_max of malignant focal thyroid uptake (15.7 ± 5.9) compared to that of benign lesions (7.1 ± 6.8) with a p value of 0.000123. An SUV_max of 9.1 achieved a sensitivity of 81.6%, specificity of 100% and area under the curve of 0.915 in the ROC analysis differentiating benign from malignant disease. Conclusion: The malignancy potential of incidental focal thyroid uptake remains high and warrants prompt and appropriate follow-up by the clinician. The SUV_max may aid in further characterisation of the lesion and its management.