Use and results of systemic treatments for de novo and recurrent metastatic breast cancer: a population-based cohort study.

AIMS To describe the systemic treatments in patients with de novo metastatic breast cancer (dnMBC, initial metastatic diagnosis) and recurrent metastatic breast cancer (rMBC). METHODS Women diagnosed with dnMBC and rMBC in 2010-2017 were identified. Adjusted odds ratios of receiving systemic treatments were estimated by logistic regression model. Cox proportional hazards regression was used to estimate adjusted hazard ratio of breast cancer-specific mortality by treatments. RESULTS The adjusted odds ratio of having chemotherapy and trastuzumab (for human epidermal growth factor receptor 2 positive (HER2+) disease) for Pacific women was 0.43 and 0.13 compared to European women. Patients receiving chemotherapy had improved survival for HER2+ non-luminal and triple negative metastatic breast cancer (MBC) (hazard ratios: 0.30, 0.66). Those with endocrine therapy was associated with better survival for luminal A and luminal B HER2+ MBC (hazard ratio: 0.25, 0.26). Trastuzumab was associated with superior survival in luminal B HER2+ and HER2+ non-luminal disease (hazard ratio: 0.34, 0.40). CONCLUSIONS Pacific women with MBC were less likely to receive chemotherapy and trastuzumab than non-Pacific women. Chemotherapy was associated with improved survival in HER2+ non-luminal and triple negative MBC. Endocrine therapy improved survival in luminal A and luminal B HER2+ disease. Trastuzumab was associated with improved survival in luminal B HER2+ and HER2+ non-luminal disease.