Evaluating the Intermediate Suspicion Category in Thyroid Nodules with AI Assistance: A Comparative Analysis of C-TIRADS, ACR-TIRADS, and ATA Guidelines.

OBJECTIVE: To compare diagnostic performance and fine-needle aspiration (FNA) decision-making for thyroid nodules classified as "Intermediate Suspicion" across three major ultrasound-based risk stratification systems (2017 ACR-TIRADS, 2015 ATA guidelines, and 2020 C-TIRADS), with and without AI assistance. METHODS: This retrospective study analyzed 1,911 ultrasound images of thyroid nodules from 1,040 patients (mean age, 46.1 ± 13.1 years) collected between 2021 and 2022. The Intermediate Suspicion category was defined as ACR-TR4, ATA-4, and C-TR 4B. Seven radiologists independently categorized nodules according to each guideline, and an artificial intelligence (AI) model independently evaluated all nodules. AI-assisted diagnostic and FNA strategies were implemented and compared across the three guidelines. Diagnostic performance and 95% confidence intervals (CIs) were assessed using patient-level clustered logistic generalized estimating equations. RESULTS: In the Intermediate Suspicion category, diagnostic accuracy was lower for both radiologists and the AI model than in the overall cohort. Radiologists' overall accuracy increased from 66.4%-71.4% without AI to 74.1%-79.4% with AI across all three guidelines (P < 0.001). With AI, C-TIRADS achieved the highest accuracy (79.4%), specificity (66.7%), and PPV (77.4%) (all P < 0.05), with no significant differences between ACR-TIRADS and ATA guidelines. Without AI, FNA rates were lowest with ACR TI-RADS (37.7%) and highest with C-TIRADS (52.2%). AI assistance reduced FNA rates by 20.3%-34.7%, increased malignant detection rate (MDR) by 15.0%-23.4%, and decreased missed malignancy rate (MMR) by 23.7%-36.8% (all P < 0.001). With AI assistance, ATA-4 had the lowest FNA rate (14.7%), whereas C-TR 4B had the lowest MMR (29.8%) and the numerically highest MDR (70.0%). DISCUSSION: AI-assisted interpretation improves diagnostic accuracy and FNA decision-making for Intermediate Suspicion nodules. C-TIRADS combined with AI shows superior performance among the three systems.