Thyroid carcinoma detected by ¹⁸F-fluorodeoxyglucose positron emission tomography among individuals without prior evidence of thyroid disease: Relevance and clinicopathologic features

Objective: The expanding use of ¹⁸F-fluorodeoxyglucose positron emission tomography (¹⁸F-FDG-PET) has contributed to an increasing number of thyroid incidentalomas. The present study aimed to estimate the prevalence of ¹⁸F-FDG-PET thyroid incidentalomas and evaluate the clinicopathologic features of thyroid malignancies detected by ¹⁸F-FDG-PET.

Methods: We reviewed all ¹⁸F-FDG-PET exams performed at the Portuguese Institute of Oncology, Lisbon, between 2007 and 2012 (n = 9,374). The inclusion criteria were focal thyroid uptake and absence of known thyroid disease.

Results: Focal thyroid uptake was observed in 60 out of 9,374 ¹⁸F-FDG-PET exams (prevalence of 0.64%). Fine-needle aspiration cytology (FNAC) was performed in 23 patients and reported as malignant in 14 cases (56.5% primary thyroid carcinoma; 4.3% secondary malignancy), as benign in 7 cases (30.5%) and as follicular lesion of undetermined significance in 2 cases (8.7%). Fourteen patients had surgery. A final histologic diagnosis of papillary thyroid carcinoma was established in 12 cases (52.2%). Three were multifocal (25.0%); 8 had extrathyroidal extension (66.7%); 5 had angioinvasion (41.7%); 3 had lymph nodes metastases (25.0%) and 2 showed lung metastases (16.7%). Overall, 91.7% were classified as intermediate or high risk. All patients had radioiodine therapy. At the last observation (mean follow-up was 29.9 months), persistent or recurrent disease was identified in 4 patients (33.3%) and none died from thyroid malignancy.

Conclusions: Thyroid carcinomas disclosed by ¹⁸F-FDG-PET are associated with aggressive histological criteria likely to carry a worse prognosis.