1044 Background: HER2-Low (or HER2-equivocal, FISH negative) breast cancer has historically been treated as HER2-negative; however, recent evidence suggests that there may be prognostic and/or predictive differences between the two. We explore demographic characteristics and clinical outcomes of HER2-negative and HER2-low metastatic breast cancer (MBC) patients using real world data. Methods: We queried the National Cancer Database to identify MBC patients that were HER2 0, HER2 1+, or HER2 2+ per immunohistochemical staining, with the latter two defined as HER2-low and the former HER2-negative. A multivariable binomial regression analysis identified demographic and clinical correlates of each subtype. A Cox multivariable regression analysis (MVA), propensity matched to HER2 status, was performed to identify correlates of survival. Results: After excluding missing data, 24,636 MBC patients diagnosed between 2008-2015 were identified, 6,865 (27.9%) of whom were HER2-negative and 17,771(72.1%) of whom were HER2-low. There were no differences between the two groups with respect to age, race, year treated, location, income, insurance status, Charles Deyo comorbidity score, laterality, T stage, N stage, or use of systemic therapy. HER2-low tumors were half as likely to have concomitant hormone receptor-negative status (OR = 0.49, 95% CI 0.46-0.53). The 3-year survival rate among hormone receptor-negative patients was 33.8% for HER2-low and 32.2% for HER2-negative, and 60.9% and 55.6% in HER2-low and HER2-negative cases among hormone receptor-positive patients, respectively. HER2-low cases were associated with better survival on MVA (HR = 0.91, 95% CI 0.87-0.95), and remained superior with propensity-matching (HR = 0.92, 95% CI 0.89-0.96). In a subset analysis isolated to hormone receptor-positive cases, HER2-low remained correlated with improved survival (HR = 0.93, 95% CI 0.89-0.98) with propensity-matched MVA. Correlates of worse survival include older age as a continuous variable (HR = 1.02), Black race (HR = 1.13), uninsured (HR = 1.18), comorbidity score > 0 (HR = 1.28), higher T stage (HR = 1.17 to 2.34), node positivity (HR = 1.17) and, as the most influential, hormone receptor-negative status (HR = 1.94) [all P < 0.01]. Conclusions: Consistent with recent data in non-MBC, our study demonstrates a small but statistically significant association with improved survival for HER2-low tumors compared to HER2-negative tumors in MBC. Randomized data are necessary to further validate this discrepancy and determine if different management is warranted for each subtype.

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