Purpose: To assess the accuracy and reproducibility of ultrasonography classification systems in characterizing cytologically indeterminate thyroid nodules.
Methods: We retrospectively identified 49 nodules that had been surgically resected owing to features classified as indeterminate according to 2010 Italian Consensus on Thyroid Cytology criteria. Three experienced sonographers independently reviewed original sonographic images of each nodule and classified it using the 2015 American Thyroid Association (ATA) guidelines and the Thyroid Imaging Reporting and Data System (TI-RADS) classification proposed by Korean radiologists; later, images were reviewed jointly to obtain consensus classifications of each nodule. Original cytology slides were similarly reviewed by three experienced cytopathologists, who reclassified the nodule (independently, then jointly) according to revised Italian Consensus on Thyroid Cytology (ICTC-2014) criteria. Consensus ICTC-2014, ATA, and TI-RADS classifications were analyzed against surgical histology reports to estimate each system's sensitivity, specificity, positive and negative predictive values.
Results: Of the 49 indeterminate nodules examined, 30 (61.2 %) were histologically benign. Consensus ICTC-2014 classification correctly classified malignant nodules with positive predictive value of 50 % and negative predictive value of 90 %. Sonographic classification of nodules as intermediate to high suspicion by ATA or TI-RADS category 4c displayed positive predictive value of 63 and 71 %, respectively; positive predictive values dropped to 44 and 42 % when lower positivity thresholds were used (ATA low suspicion, TI-RADS category 4a). Negative predictive values for ATA and TI-RADS were 91 and 74 %, respectively, with higher positivity thresholds and 100 % for both with lower thresholds. All systems displayed appreciable inter-observer variability (Krippendorff alphas: ATA 0.36, TIRADS 0.42, ICTC-2014 0.74).
Conclusions: With stringent negativity cut-offs, American Thyroid Association and Thyroid Imaging Reporting and Data System assessment of cytologically indeterminate thyroid nodules allows high-confidence exclusion of malignancy.
Keywords: Indeterminate; TIRADS; Thyroid nodule; Ultrasonography.