Correlation of MammaPrint to Histopathologic Variables and Comparison to Magee Equations in Adjuvant and Neoadjuvant Cohorts of Estrogen-Receptor Positive HER2-Negative Breast Carcinoma.

MammaPrint (MP) is a multigene assay utilized for prognostic and predictive use in early stage ER+/HER2-negative breast cancer (BC). There is limited data on MP correlation to histopathologic variables, including multivariable model Magee Equations (MEs). We compared pathologic variables to MP on 365 ER+/HER2-negative BCs in the adjuvant setting. Further, we analyzed MP and ME results in 26 core biopsy-proven ER+/HER2-negative BCs subjected to neoadjuvant chemotherapy (NACT). Post-NACT response was assessed by residual cancer burden (RCB) score. In the adjuvant cohort (n=365), high-risk (HR) MP correlated with high Nottingham score, low progesterone receptor H-score, ductal morphology, high Ki-67 index, and high ME score. In the neoadjuvant cohort (n=26), 24 were MP-HR and 2 were MP low-risk (MP-LR). The 2 MP-LR cases correlated with average ME scores of ≤25 and showed RCB scores of 2 or 3. Of the 24 MP-HR, 2 showed RCB-0 or RCB-1. Average ME>25 was seen in both of these cases and 7 others corresponding to an RCB-0 and RCB-1 rate of 22% (2 of 9) with ME>25 compared with 8% (2 of 24) with MP-HR. Lack of significant benefit from NACT (RCB-2 or RCB-3) was accurately predicted by average ME≤25 in 17 of 26 cases (65%) compared with only 2 of 26 cases (8%) with MP-LR. MP correlates with aggressive histopathologic features. While both MP and ME identified cases with robust chemotherapeutic response, MP overpredicted cases that would benefit. Therefore, MEs may more accurately predict response to NACT.