Improved survival supports primary endocrine therapy in patients with hormone receptor positive/ HER-2 negative metastatic breast cancer

e13034 Background: Current ASCO guidelines recommend endocrine therapy as preferred primary treatment for hormone receptor positive (HR+) metastatic breast cancer (MBC). We assessed survival outcomes of HR+/HER2- MBC patients undergoing endocrine therapy with and without chemotherapy. Methods: The National Cancer Database was queried 2004-2016 for patients with de novo HR+/HER2- MBC. Exclusion criteria were treatment with surgery or radiation at the primary site and missing oncologic and follow up data. Overall survival was compared between systemic treatment groups using multivariable cox proportional hazards regression modes. Results: 19,317 patients met inclusion criteria, among whom 2,360 (12%) received no systemic therapy, 2,617 (14%) received chemotherapy only, 10,078 (52%) received endocrine therapy only and 4,262 (22%) received both chemotherapy and endocrine therapy. Patients treated with chemotherapy only more frequently had lung (38%, p