15IN - Which Luminal BC need Chemotherapy?

Abstract

ABSTRACT Estrogen Receptor positive (ER+) HER2- breast cancer is the largest subset of breast cancers and the most chemotherapy resistant. For small ER+ node negative breast cancers with favorable genomic or clinical risk profiles this is an irrelevant concern as management with endocrine therapy alone is the standard of care. However multiple studies have shown that once HER2+ ER+ are removed from a node positive ER+ population, adjuvant chemotherapy, whether anthracycline or anthracycline/taxane-based has limited efficacy. To investigate new approaches to the treatment of ER+ HER- higher stage tumors we have therefore focused on the neoadjuvant setting. This is because the most critical question in the management of ER+ HER2- tumors is whether the patent has an endocrine therapy sensitive tumor. Endocrine therapy resistance in the neoadjuvant setting can be identified in a straightforward manner using the Ki67 proliferation index. Tumors that have an elevated Ki67 (above 10%) at 2 to 4 weeks after starting an aromatase inhibitor (AI) are defined in our prospective studies as endocrine therapy resistant. This cut-point was established because if Ki67 is >10% the tumor is very unlikely to fall into the lowest relapse risk category at post neoadjuvant endocrine therapy surgery (preoperative endocrine prognostic index zero or PEPI-0 (defined as ER+ Stage 1 or 2A, Ki67 Disclosure The author has declared no conflicts of interest.