Identifying Patients With Low Relapse Rate Despite High-Risk Estrogen Receptor–Positive/Human Epidermal Growth Factor Receptor 2–Negative Early Breast Cancer: Development and Validation of a Clinicopathologic Assay

Escalation of adjuvant systemic therapies (eg, with cyclin-dependent kinase 4 and 6 inhibitors) is now indicated for patients with clinically defined high-risk estrogen receptor–positive (ER+)/human epidermal growth factor receptor 2–negative (HER2–) early breast cancer, although it is unclear which will benefit from additional therapies. We developed and validated a prognostic clinicopathologic assay identifying a subpopulation of high-risk patients with good prognosis after standard adjuvant therapies, who may safely forgo treatment escalation.