Prevalence and risk of cancer of focal thyroid incidentaloma identified by 18F-fluorodeoxyglucose positron emission tomography for cancer screening in healthy subjects.

The aim of this study was to define the clinical prevalence and rate of malignancy of focal thyroid incidentaloma in a relatively large number of individuals with application of 18F-fluorodeoxyglucose (FDG) Positron Emission Tomography (PET) in asymptomatic individuals. The subjects consisted of 4803 physical check-up examinees (2638 men, 2165 women; mean age+/-SD, 52.8+/-9 years old) with non-specific medical history. Whole-body FDG PET was performed on all patients. Focal hypermetabolic areas of thyroid, with an intensity equal to or exceeding the level of FDG uptake in the liver, were considered abnormal and interpreted as thyroid incidentaloma. Among the 4803 FDG PET examinations, thyroid incidentaloma was present in 60 examinations, among which, 50 had further examination with ultrasound-guided fine-needle aspiration (FNA), revealing 43 benign lesions and 7 papillary carcinomas. Of 7 examinees diagnosed with cancer, 2 had lymph nodes metastasis. The mean and standard deviation of standard uptake value (SUV) in benign and carcinoma was 2.6+/-1.01 and 6.7+/-3.66, respectively. Thyroid incidentaloma identified by FDG-PET occurred with a frequency of 1.2% (60/4803). Of the thyroid incidentalomas that underwent FNA and surgery, 14% (7/50) were found to be malignant. It is possible to differentiate benign from carcinoma of thyroid incidentalomas by the increased rate of glycolysis (SUV) in the carcinoma. The small size and moderate FDG uptake of thyroid incidentalomas per se cannot guarantee low risk in incidentally found thyroid cancers.