The Number of Positive Lymph Nodes as Prognostic Factor in ypN1 Breast Cancer with Neoadjuvant Chemotherapy

This study aimed to assess the ability of the number of positive lymph nodes (LNP) after neoadjuvant chemotherapy (NAC) to stratify prognosis even within the same ypN stage in breast cancer patients.A 6,443 non-metastatic breast cancer patients with ypN1 disease after NAC following primary surgery were identified using the stage, stage evaluation field, treatment fields from Surveillance Epidemiology and End Results (SEER) database. Survival analysis was performed for overall survival (OS) using Kaplan-Meier plot and Cox proportional hazard models.Of patients, 1,132(17.6%) had 3 positive lymph nodes (LNP3), 4,363 (67.7%) patients were T1/2 stage and 538 (8.35%) patients were downstage after treatment. LNP3 group had a significantly worse OS than LNP1/2 group (5yr OS, 74.2% vs. 80.9%, p <0.001). Similar results were observed in cause specific survival analysis (5yr OS, 79.8% vs. 85.2%, p <0.001). In subgroup analysis, there was a significant difference in OS between LNP1/2 and LNP3 group for HER2 or triple negative subtype, (p <0.001), T1/2 stage (p <0.001) and stable/downstage of N stage (p <0.001 and 0.019, respectively), but not for luminal A/B subtype and T3/4 (p = 0.126, 0.629 and 0.154, respectively). Regarding the number of examined lymph node (LNE), OS was significantly decreased in patients with LNE ≤5 in LNP3 group (p <0.001) but not in LNP1/2 group. In multivariable analysis, the prognostic significance of LNP3 remained (p <0.001) and the following factors were significantly related to the prolonged OS; Luminal A/B (p<0.001 and 0.001, respectively), T1/2 stage (p <0.001), N1 stage (p <0.001), LNE (≤5 vs. >5, p = 0.001) and receipt of radiotherapy (p = 0.012). Even when PS matching was performed, the OS of LNP was worse than that of LNP1/2.LNP could more stratify the patients with ypN1 breast cancer after NAC. LNP3 is significantly associated with OS even after adjusting for other prognosticators. In particular, since there is a potential difference in the magnitude of the impact of LNP3 according to LNE, it is necessary to consider both of them together for decision-making.