Oncocytic adenomas of thyroid-mimicking benign or metastatic disease on 18F-FDG-PET scan

Background: The literature is sparse concerning 18F-fluorodeoxyglucose (18F-FDG) accumulation in the Hürthle cell neoplasm (HCN) of the thyroid. Given the difficulty of accurately diagnosing HCN, even with ultrasound (US) and fine needle aspiration biopsy (FNAB), the ability to accurately characterize these lesions by 18F-FDG positron emission tomography (PET) would be of value.

Purpose: To describe six cases of oncocytic proliferation in the thyroid gland that mimics the presence of metastatic disease and was detected incidentally by an 18F-FDG PET scan.

Material and methods: We conducted whole-body 18F-FDG PET examinations for cancer staging in 1862 oncological patients from 2012 to 2013. Among them, six subjects (4 women, 2 men; age range, 45-85 years) with focal-enhanced 18F-FDG accumulation in the thyroid gland were selected from the study population. This study group was further investigated using 99 m-Tc-pertechnetate scintigraphy, US, and FNAB. Two experienced nuclear physicians reviewed the images. Gray-scale US and color Doppler (CD) sonographic examinations of the thyroid were undertaken for all subjects using a sonographic device Logiq 5 Expert (GE Medical Systems, Osaka, Japan) equipped with a 7-12 MHz linear array transducer.

Results: In all six cases, abnormal 18F-FDG uptake was found locally in the thyroid. The average SUVmax of the HCN was 5.8 (range, 2.6-16). In all six cases, 99 m-Tc-pertechnetate scintigraphy showed a cold spot. Compared with normal parenchymal vascularity, five of the six masses were shown to be hypervascular by CD ultrasonography.

Conclusion: On PET scans, oncocytic proliferations of the thyroid may mimic metastases of other malignancies. The focal-enhanced uptake of 18F-FDG PET may be associated with a focal increase in the metabolic activity of the thyroid parenchyma due to the presence of oncocytes. Our study emphasizes the importance of obtaining cytological evidence before making a diagnosis of metastatic disease.