To evaluate the diagnostic value of American College of Radiology thyroid imaging report and data system (ACR TI-RADS) and 2011 Kwak TI-RADS in thyroid isthmic nodules.

This retrospective study included 308 patients with 308 thyroid isthmic nodules diagnosed pathologically by fine needle aspiration biopsy or surgery at department of ultrasound, the Second Affiliated Hospital of Zhejiang University School of Medicine from January 2015 to January 2020. All nodules were classified according to the ACR TI-RADS and Kwak TI-RADS classifications. Multivariate logistic regression analysis was performed to analyze the independent risk factors formalignant thyroid isthmic nodules.The ACR TI-RADS and Kwak TI-RADS classifications results for the nodules were compared with the pathological results, and the receiver operating characteristic (ROC) curves of the two methods were plotted to assess their diagnostic efficiency.

Of the 308 thyroid isthmic nodules, 197 were malignant and 111 were benign. Compared with benign thyroid nodules, malignant nodules located in the isthmus were more likely to have the following ultrasonic features: solid type (99.5% vs 77.5%), hypo-/very hypo-echogenicity (94.9% vs 46.8%), taller-than-wide shape (21.3% vs 5.4%), lobulated or irregular or extra-thyroidal extension (41.1% vs 7.2%), and punctate echogenic foci (43.2% vs 11.7%); the differences of these features were statistically significant between the two groups (P＜0.05). Solid type (P=0.047), hypo-/very hypo-echogenicity (P＜0.001), lobulated or irregular or extra-thyroidal extension (P=0.002), and punctate echogenic foci (P=0.007) were independent risk factors for malignancy, while taller-than-wide shape was not an independent risk factor for malignancy (OR=2.683, P=0.050). All the ACR TR 1-2 nodules were benign. The malignancy rates of ACR TR 4 and 5 were 70.1%and 87.0%, respectively. All the Kwak TR 2-3 nodules were benign. The malignancy rates of Kwak TR 4b, 4c, and 5 were 70.0%, 83.2%, and 100%, respectively. The areas under the ROC curves of the ACR TI-RADS and Kwak TI-RADS classifications were 0.823 and 0.820, respectively. The optimal cut-off values for diagnosing malignancy by ACR TI-RADS and Kwak TI-RADS were ≥TR4 (total score ≥4 points) and ≥TR4b, and at these points, the sensitivity, specificity, accuracy, positive predictive value, negative predictive value, and Youden index were 99.5% vs 99.0%, 53.1% vs 53.1%, 82.8% vs 82.5%, 79.0% vs 78.9%, 98.3% vs 96.7%, and 0.526 vs 0.523, respectively.

Both of the ACR TI-RADS and Kwak TI-RADS have high diagnostic valuein diagnosing thyroid isthmic nodules. Further studies of TI-RADS are needed to assess the threshold of malignant risk and management in isthmic nodules.