High incidence of thyroid cancer in focal thyroid incidentaloma detected by 18F-fluorodeoxyglucose [corrected] positron emission tomography in relatively young healthy subjects: results of 3-year follow-up.

As 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) is becoming a common imaging modality, the number of thyroid incidentalomas identified by FDG-PET (PET incidentaloma) is increasing. The purpose of this study was to elucidate the risk of cancer in focal thyroid PET incidentaloma in healthy subjects of relatively younger age as well as the usefulness of repeated FDG-PET. The study was conducted with an observation period of three years. A total of 1,501 healthy volunteers (mean age, 43.5+/-9.7 years) underwent the first FDG-PET from August 2003 to July 2004. When focal thyroid PET incidentaloma was found, further diagnostic examination was conducted. When thyroid cancer was suspected, surgical resection was performed with the patient' s agreement. Patients with PET incidentaloma without surgery were offered annual US and FDG-PET and finally FNAB was performed in the fourth year. Focal thyroid PET incidentaloma was observed in 20 subjects. The final diagnoses in 20 subjects were malignant in 11 (ten papillary thyroid carcinoma (PTC) and one thyroid carcinoma showing thymus-like differentiation), indeterminate in one, and benign in eight subjects. Seven patients not treated surgically at the first examination had annual FDG-PET. One patient with PTC showed increasing SUVmax, but another with a benign nodule exhibited a similar increase. Others (one with PTC, one with an indeterminate nodule, and three with benign nodules) exhibited negligible SUVmax changes. When closely examined, focal thyroid PET incidentaloma in relatively young healthy adults has a high probability of malignancy. Repeated FDG-PET to follow up patients with thyroid nodules is ineffective.