Alteration of HER2 Status Following Neoadjuvant Chemotherapy in Breast Cancer: A Clinicopathological Analysis Focusing on HER2-Low Status

Purpose This study aimed to investigate alteration of HER2 status after neoadjuvant chemotherapy (NAC) in breast cancer and its impact on clinical outcomes of patients, focusing on HER2-low status. Materials and Methods We retrospectively reviewed clinicopathological data of 1,063 breast cancer patients who underwent NAC between 2013 and 2020. Using paired samples of 670 patients with residual disease after NAC, we analyzed HER2 discordance rates between pre- and post-NAC samples, relationships between HER2 discordance and clinicopathological characteristics of tumors, and clinical outcomes of the patients. Results Pre-NAC HER2-low status was associated with a lower pathological complete response rate and higher Residual Cancer Burden class compared with HER2-zero and HER2-positive status. However, in subgroup analysis by hormone receptor (HR) status, no statistical differences were found in chemo-responsiveness between them. Following NAC, the overall HER2 discordance rate was 21.2% (κ = 0.676), and the most common type of alteration was zero-to-low (11.5%) conversion, followed by low-to-positive (3.6%) conversion. HER2 discordance was significantly associated with lower HER2 levels and HR positivity before NAC, as well as lymphovascular invasion, higher ypT stage, and axillary node metastasis in residual disease after NAC. In survival analyses, HER2 discordance was found to be an independent prognostic factor for poor disease-free survival of the patients, particularly within the HR-positive subgroup. Conclusion Given the prognostic implications of HER2 discordance which primarily involves zero-to-low conversion and the therapeutic benefits of newly developed antibody-drug conjugates in HER2-low breast cancers, HER2 status should be re-evaluated in surgical resection specimens following NAC.