Ultrasound stratification of the FDG-avid thyroid nodule

To determine whether the malignancy risk in an 2-[(18)F]-fluoro-2-deoxy-d-glucose (FDG)-avid thyroid nodule can be stratified according to the presence or absence of suspicious ultrasound features and thereby identify which nodules require further cytological assessment.

A retrospective review of FDG-positron-emission tomography (PET) combined with computed tomography (CT) studies with FDG-avid thyroid nodules (defined as FDG uptake greater than blood pool) that were further assessed with ultrasound and fine-needle aspiration cytology or surgery was performed. FDG-avid thyroid nodules were classified as having either suspicious ultrasound features (marked hypo-echogenicity, irregular margins, microcalcifications, marked hypervascularity, or nodules that were taller than they were wide) or no suspicious ultrasound features and these findings were correlated with the subsequent cytological results.

Forty-eight FDG-avid thyroid nodules were assessed. On cytological assessment five nodules were malignant (10.4%), nine were indeterminate (18.75%), and 34 were benign (70.8%). On ultrasound, 24 (50%) had no suspicious features and 24 (50%) had one or more suspicious features. Of the nodules with no suspicious features, 22 (91.6%) were benign, two (8.3%) were indeterminate, and none were malignant. Of the nodules with suspicious features, five (20.8%) were malignant, seven (29.1%) were indeterminate, and 12 (50%) were benign. The absence of suspicious ultrasound features demonstrated a strong association with benign cytology (p=0.009). Out of the suspicious sonographic features, marked hypoechoic appearance (p=0.02), irregular margins (p=0.009), and taller than wide morphology (p=0.04) were statistically most significantly associated with malignancy.

The rate of malignancy in FDG-avid thyroid nodules is low in the absence of specific suspicious ultrasound features. The SUV values are non-discriminatory to differentiate between benign and malignant cytology. This suggests that ultrasound can be used to further stratify an FDG-avid thyroid nodule and invasive procedures to investigate the FDG-avid thyroid nodule may not be necessary in the absence of suspicious ultrasound features.