Positron emission tomography-positive thyroid nodules: rate of malignancy and histological features

Thyroid nodules may be incidentally detected on (18) F-FDG-positron emission tomography (PET) scans. Previous reports suggest a high incidence of malignancy in FDG-avid nodules. The aims of this study were to examine the incidence of malignancy in a large cohort and to report on the histological features. The findings suggest that poor prognostic histologic features are often associated with FDG-avid thyroid nodules and this may have clinical implications.

A retrospective review of prospectively collected data was conducted. A database containing all patients who underwent PET scanning at a single tertiary referral centre from January 2006 to January 2013 was searched to identify those with incidental PET-positive thyroid nodules. Patients with known preexisting thyroid disease were excluded from analysis. The demographics, fine-needle aspiration (FNA) biopsy result and operative histopathology were analysed.

A total of 27 851 FDG-PET scans were performed of which 221 found incidental PET-positive thyroid nodules (incidence 0.8%). Fifty-three patients went on to have further investigation and 21 of these were found to have malignant disease (incidence 39.6%). Histopathological examination of 12 malignant nodules revealed an expected rate of poor prognostic features, including poorly differentiated subtype (8.3%), lymphovascular invasion (16.7%), perineural invasion (8.3%) and extrathyroid extension (33.3%).

Our data indicate that PET-positive thyroid nodules are associated with a high incidence of malignancy. This finding provides strong support for further investigation including FNA biopsy in all surgically suitable patients.