Contrast-Enhanced Ultrasound Combined with Methylene Blue for Sentinel Lymph Node Biopsy After Neoadjuvant Chemotherapy in Patients with Initially cN1 Breast Cancer: A Multicenter Prospective Cohort Study

Background: Reliable axillary restaging after neoadjuvant chemotherapy (NAC) is essential for patients with initially cN1 breast cancer considered for surgical de-escalation. Although radioisotope plus blue dye dual-tracer mapping can reduce the false-negative rate (FNR) of sentinel lymph node biopsy (SLNB), its routine use is limited by radioisotope access, infrastructure requirements, and procedural complexity. We evaluated contrast-enhanced ultrasound (CEUS) combined with methylene blue as a non-radioactive dual-mapping strategy. Methods: In this two-center prospective cohort study, patients undergoing surgery after NAC were enrolled if they received preoperative CEUS, intraoperative methylene blue mapping, SLNB, and axillary lymph node dissection. The primary analysis focused on patients with initially cN1 disease, whereas cN2–3 patients were included only for exploratory analyses. Detection rate (DR), FNR, predictive values, CEUS–methylene blue concordance, and CEUS performance for predicting SLN metastasis were assessed. Results: A total of 269 patients were enrolled, including 208 with initial cN1 disease, 45 with cN2 disease, and 16 with cN3 disease. In the cN1 cohort, CEUS plus methylene blue achieved the highest SLN DR (99.04%; 95% CI, 96.56–99.74), followed by CEUS alone (98.08%) and methylene blue alone (71.63%). The FNR was 9.38% (95% CI, 4.37–18.98) for both CEUS plus methylene blue and CEUS alone, versus 12.50% for methylene blue alone. CEUS–methylene blue concordance was 91.16%. For predicting SLN metastasis, CEUS showed 65.52% sensitivity, 86.30% specificity, and 80.39% accuracy. No immediate procedure-related adverse events occurred. Conclusions: CEUS combined with methylene blue showed high SLN detection and a promising false-negative profile in patients with initially cN1 breast cancer after NAC. These findings support further evaluation of this feasible non-radioactive strategy with larger comparative studies, particularly in settings with limited access to radioisotope-guided mapping.