Abstract PS1-10-14: Defining the prognosis of estrogen receptor low-positive de novo metastatic breast cancer

Abstract Background: Recent studies have reported the prognosis and heterogeneity of estrogen receptor (ER) low-positive early-stage breast cancer. Survival and treatment outcomes have not been reported for patients with ER low-positive, metastatic breast cancer. The primary objective of this study was to compare overall survival (OS) among patients with ER low-positive de novo metastatic breast cancer (dnMBC), ER-negative dnMBC, and ER-positive dnMBC. The secondary objective was to determine if progesterone receptor (PR) status can define two prognostic groups among ER low-positive dnMBC patients. Methods: A retrospective cohort of patients with dnMBC was created using data from the National Cancer Database (NCDB). Adults diagnosed with human epidermal receptor 2 (HER2) negative dnMBC between 2018-2021 were included. The primary exposures were ER status, based on immunohistochemistry and defined as negative,