Abstract
The standard of care for HER2-positive and hormone receptor-positive breast cancer patients who receive neoadjuvant chemotherapy (NACT) combined with trastuzumab, with or without pertuzumab, is to continue with adjuvant T-DM1 in cases of an incomplete response according to KATHERINE trial results. However, the optimal management for patients with residual disease with loss of HER2 expression is not widely studied. Loss of HER2 expression after NACT with anti HER2 is a rarer event with questionable value both as a predictive prognostic marker.