How do you approach ER, PR weakly positive, HER2 negative breast cancer in the metastatic setting?

Adjuvant studies have suggested benefit to hormone therapy in low ER positive patients, although the absolute benefit may be less. In the metastatic setting, the decision to approach low ER positive, HER2 negative patients with endocrine therapy versus chemotherapy depends on the individual patient and tumor biology characteristics - degree of ER positivity, extent of disease, comorbidities, prior exposure to endocrine therapy, etc. While a tumor with ER <10% is likely biologically triple negative, in a patient with minimal, bone only, and asymptomatic disease, I would likely still attempt endocrine therapy in some patients as tumor heterogeneity is possible. However, for example, a patient who develops metastatic disease while on an adjuva...