Prospective Optimization of Malignancy Risk Prediction in Indeterminate Thyroid Nodules: Diagnostic Synergy of ACR TI-RADS and the 2023 Bethesda System

Background: Risk stratification of indeterminate thyroid nodules (Bethesda III–IV) remains difficult and often triggers unnecessary procedures. Ultrasound-based ACR TI-RADS and the 2023 Bethesda System are widely used, but the incremental value of combining them and the role of size thresholds needs prospective validation. Objective: The objective of this study was to prospectively compare the diagnostic performance of ACR TI-RADS and the 2023 Bethesda System, alone and in combination, for predicting malignancy in thyroid nodules, with dedicated analyses of indeterminate lesions (Bethesda categories III–IV), including subtypes of Bethesda III (nuclear atypia vs. other atypia), and the impact of nodule size. Methods: Histopathology was available for 131 nodules. Diagnostic metrics (sensitivity, specificity, PPV, NPV), ROC curves (DeLong comparison), and Youden indices were calculated for individual and combined thresholds; a 16 mm size cut-off was explored. Results: Malignancy was confirmed in 105/131 nodules (80.2%). Bethesda outperformed TI-RADS (AUC 0.87 vs. 0.69; DeLong p = 0.041). Malignancy rates rose with higher categories (e.g., TI-RADS 5: 93.6%; Bethesda category V: 100%; Bethesda category VI: 100%) and were markedly elevated in the histologically confirmed subset for Bethesda category III (32/41; 78.0%) and IV (6/8; 75.0%). The combined requirement of TI-RADS ≥ 4 and Bethesda ≥ 4 maximized specificity (96.2%) and PPV (98.4%) with a high Youden J (0.552), supporting a rule-in strategy in category IV of Bethesda. Size alone was a weak discriminator (AUC 0.66); within Bethesda III–IV nodules, malignancy did not differ significantly by the 16 mm threshold (p = 1.00). ROC using continuous tumor size yielded AUC = 0.66; the ROC-derived optimal cut-off was 16 mm. Applying this split produced sensitivity 0.80 and specificity 0.50. Conclusions: Integrating ACR TI-RADS with Bethesda cytology significantly improves specificity and PPV for indeterminate thyroid nodules, supporting a morphology-driven approach over traditional size-based thresholds. Incorporation of combined sonographic–cytologic criteria into management algorithms may reduce unnecessary interventions and optimize patient care.