Difference between observed and expected number of involved lymph nodes reflects the metastatic potential of breast cancer independent to intrinsic subtype

PURPOSE Poor prognosis associated with metastasis in breast cancer patients highlights the critical need to develop an effective evaluation model for metastatic potential (MP). We hypothesized that MP could be also indicated by primary tumor size and involved lymph nodes (LNs). METHODS The expected number of involved LNs is defined as tumor size (cm) divided by 1.5. The effect of the surrogate for MP (defined as difference between the number of observed and expected involved LNs) on breast cancer-specific survival (BCSS) was investigated in the first cohort from SEER (n = 108,814). Validation was performed in another SEER cohort (n = 50,414) and a third cohort (n = 3,755). RESULTS MP is an independent predictor for BCSS in the overall population [hazard ratio (HR) for high MP: 2.92; 95% confidence interval (CI): 2.80-3.03] and in subgroups. The effect of surrogate for MP on survival was independent to intrinsic subtype, with adjusted HRs of 3.46 (95%CI, 2.02-5.93), 2.30 (95%CI, 1.64-3.24), 4.05 (95%CI, 2.85-5.76), and 1.45 (95%CI, 1.04-2.03) in luminal-A, luminal-B, triple-negative, and HER2-positive subtypes, respectively. CONCLUSIONS Difference between the observed and expected number of involved LNs serves as an indicator for MP, which is independent to intrinsic subtype and could predict survival. Our findings need further validation.