Development of circulating tumor cell-endocrine therapy index in patients with hormone receptor-positive breast cancer.

Background: Endocrine therapy (ET) fails to induce a response in one half of patients with hormone receptor (HR)-positive metastatic breast cancer (MBC), and almost all will eventually become refractory to ET. Circulating tumor cells (CTC) are associated with worse prognosis in patients with MBC, but enumeration alone is insufficient to predict the absolute odds of benefit from any therapy, including ET. We developed a multiparameter CTC-Endocrine Therapy Index (CTC-ETI), which we hypothesize may predict resistance to ET in patients with HR-positive MBC.