Optimizing the ER Cutoff in HER2-Positive Breast Cancer: ER ≥ 50% Predicts Low pCR Rates and Resistance to Antibody-Drug Conjugates in the Neoadjuvant Setting

Background

Estrogen receptor (ER)-positive/HER2-positive breast cancer demonstrates significantly lower rates of pathological complete response (pCR) to neoadjuvant HER2-targeted therapies compared to ER-negative/HER2-positive disease. However, the optimal ER positivity cutoff for clinically meaningful patient stratification remains undefined.