Magee Equation 3 demonstrates predictive and prognostic value for neoadjuvant chemotherapy outcomes in estrogen receptor-positive, human epidermal growth factor receptor 2-negative breast cancer.

[OBJECTIVE] This study assesses the effectiveness of Magee Equation 3 (ME3) in predicting response to neoadjuvant chemotherapy (NACT) and its prognostic value.

[METHODS] In total, 263 clinical stage I to III estrogen receptor (ER)-positive, human epidermal growth factor receptor 2 (HER2)-negative breast cancers diagnosed between 2014 and 2020 were assessed for response to NACT. Pathologic complete response (pCR) was defined as no invasive carcinoma in the breast or regional lymph nodes.

[RESULTS] The pCR rate for the 4 different categories of ME3 scores (<18, 18 to 25, >25 to <31, and ≥31) was 0% (0 of 44), 4% (4 of 96), 10% (7 of 68), and 31% (17 of 55), respectively. The ME3 score inversely correlated with residual cancer burden (RCB) score (Pearson correlation coefficient, -0.374; P < .001; 95% CI, -0.473 to -0.265). The pCR group also had a lower rate of multifocality (P = .007) and smaller tumor size (P = .008). Patients who achieved pCR (RCB-0) experienced favorable outcomes. In patients with residual disease after NACT, worse recurrence-free survival (RFS), distant recurrence-free survival (DRFS), overall survival (OS), or breast cancer-specific survival (BCSS) was noted for patients with a pretherapy ME3 score greater than 25. In the group with substantial residual disease (RCB-3), an increasing ME3 score was associated with significantly worse RFS (hazard ratio [HR], 1.103; 95% CI, 1.038-1.171), DRFS (HR, 1.086; 95% CI, 1.020-1.156), OS (HR, 1.069; 95% CI, 1.002-1.140), and BCSS (HR, 1.088; 95% CI, 1.002-1.182) in the multivariable analysis. The RCB index and posttherapy grade were also prognostic in the RCB-3 group.

[CONCLUSIONS] The ME3 score is predictive of response to neoadjuvant chemotherapy and is prognostically informative, particularly for RCB-3 cases.