Epidemiology and outcomes associated with brain metastases among patients with metastatic breast cancer - a cohort study in US electronic health record data.

BACKGROUND: There are limited real-world data on the prevalence of brain metastases (BM) in metastatic breast cancer (mBC) across the treatment pathway, especially when stratified by human epidermal growth factor receptor 2-positive (HER2+) or HER2-negative (HER2-) status. The goals of this study were to estimate the prevalence of BM at metastatic diagnosis and at the start of each line of systemic therapy (LOT), and to describe treatment patterns and overall survival (OS) in patients with and without BM. METHODS: This retrospective cohort study included adult patients in the US with mBC diagnosed between January 2013 - May 2020, with known HER2 status from an electronic health record-derived, deidentified database. Patients were followed from mBC diagnosis to last activity date or death. Descriptive statistics were used for BM prevalence, patient characteristics, and treatment patterns. OS was estimated using the Kaplan-Meier method. RESULTS: Of 12,644 patients with mBC in the database, 1923 (HER2+) and 9693 (HER2-) were included. The prevalence of BM at mBC diagnosis was 12.5% (HER2+) and 1.7% (HER2-). An NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines®) recommended systemic treatment for BM was received by 25.0% of patients with BM (HER2+) versus 12.8% (HER2-) during first-line treatment. The prevalence of BM (documented before or within the same month of LOT start) was 11.2%, 22.8%, and 33.0% in those with HER2+ diseases who had at least one, two, and three prior LOTs, respectively. The prevalence of BM among patients with HER2- disease was 1.6%, 2.0%, and 2.8% in those who had at least one, two, and three prior LOTs, respectively. Median OS from mBC diagnosis among patients with versus without BM was 24 versus 37 months (HER2+) and 12 versus 27 months (HER2-). CONCLUSIONS: In this real-world study of patients receiving care in US oncology clinics, the prevalence of BM in patients with mBC increased by LOT, and most were not receiving NCCN Guideline®-recommended systemic therapies. OS was poorer in patients with BM versus without BM, especially in the HER2- population. These results highlight a need for more effective treatments for patients with mBC and BM.