The dynamics of HER2-low expression during breast cancer progression.

Purpose: Low HER2 expression is emerging as an actionable target for the treatment of breast cancer (BC) with the antibody drug conjugate Trastuzumab deruxtecan. The aim of the study was to characterize the dynamics of HER2 expression during BC progression.

Methods: We evaluated the evolution of HER2 expression in 171 paired primary and metastatic BCs (pBCs/mBCs) by including the HER2-low category.