Diagnostic performance of Kaiser score, ADC, and their combination for BI-RADS 4 breast lesions: subgroup and misclassification analysis.

BACKGROUND: The Kaiser score and apparent diffusion coefficient (ADC) have been proposed as diagnostic adjuncts to reduce unnecessary biopsies in Breast Imaging Reporting and Data System (BI-RADS) 4 breast lesions. However, which specific histopathologic entities are prone to misclassification by each diagnostic model remains poorly characterized, limiting clinical guidance for borderline cases. OBJECTIVE: To compare the diagnostic performance of the Kaiser score, ADC, and their combination (Kaiser+) stratified by lesion morphology, and to identify histopathologic subtypes most susceptible to misclassification. METHODS: This retrospective study included 222 patients with BI-RADS 4 breast lesions confirmed by histopathology. Two radiologists independently evaluated lesions using the Kaiser score and ADC values (threshold: 1.4 × 10-3 mm²/s). Diagnostic performance was assessed using receiver operating characteristic (ROC) analysis, and misclassification patterns were analyzed by pathologic subtype using McNemar's test with Bonferroni correction. RESULTS: The Kaiser score achieved the highest area under the curve (AUC) of 0.901 (95% CI: 0.856-0.941), followed by Kaiser+ (AUC 0.890; 95% CI: 0.845-0.930) and ADC alone (AUC 0.733; 95% CI: 0.666-0.800; p < 0.001 vs. Kaiser). Kaiser demonstrated superior performance in mass lesions (AUC 0.926; 95% CI: 0.878-0.965) compared with non-mass enhancement (NME) (AUC 0.801; 95% CI: 0.681-0.905). Pathology-stratified analysis revealed distinct misclassification patterns: fibroadenoma, breast hyperplasia, and intraductal papilloma accounted for most FP cases, with ADC showing significantly higher FP rates (66.1%; 95% CI: 57.3%-73.9%) than Kaiser (19.8%; 95% CI: 13.7%-27.8%) or Kaiser+ (17.4%; 95% CI: 11.6%-25.1%; p<0.001). Among malignant lesions, ductal carcinoma in situ (DCIS) and mucinous carcinoma were the majority of frequently misclassified as FN across all models. CONCLUSION: The Kaiser score outperformed ADC alone in BI-RADS 4 lesions. Pathology-stratified analysis identified subtype-specific diagnostic pitfalls: benign cellular and proliferative lesions drive FP cases (especially with ADC), while low-conspicuity malignancies (DCIS, mucinous carcinoma) drive FN cases. Awareness of these patterns may guide individualized biopsy decisions in clinical practice.