Preoperative MRI versus Ultrasound in Axillary Nodal Staging in Breast Cancer: Additive Role of MRI.

OBJECTIVE: To evaluate the additive role of breast MRI in axillary nodal staging following a benign axillary US and compare diagnostic performance of US-first versus MRI-first imaging sequences. METHODS: This retrospective study included 524 patients with stage I-III breast cancer who underwent preoperative breast MRI and axillary surgery. Imaging assessments were categorized as US-first (n = 367) or MRI-first (n = 157). Diagnostic performance metrics were calculated based on the first modality used to evaluate the axilla. Incremental detection by MRI after benign US was assessed. Associations between false-negative results and clinicopathologic factors were analyzed. RESULTS: US-first patients were younger (51.8 vs 55.2 years, P = 0.002), had larger tumors (37.8 mm vs 26.0 mm, P < 0.001), and higher Ki-67 indices (42.6% vs 33.0%, P < 0.001) compared to MRI-first. Sensitivity for nodal metastasis was higher for US-first (71.7%) than MRI-first (50%, P = 0.01), while MRI-first demonstrated greater specificity (85.6% vs 74.6%, P = 0.02) and negative predictive value (87.0% vs 69.2%, P < 0.001). Among 182 axillae assessed as benign on initial US, MRI identified 24 additional abnormal cases (13.2%), of which 15 were true positives, yielding an incremental detection rate of 8.2%. False-negative results were associated with younger age in both groups and with larger tumor size and multifocality in US-first cases. CONCLUSION: MRI provided clinically relevant incremental detection of axillary metastases after benign US and demonstrated higher negative predictive value when performed first. False-negative results were more common in patients with younger age, larger tumors, and multifocal disease, highlighting the need to consider these factors when interpreting axillary imaging.