Expert consensus on the clinical diagnosis and targeted therapy of HER2 breast cancer (2023 edition).

Human epidermal growth factor receptor 2 (HER2) is an important driver gene and prognostic indicator of breast cancer and also a key predictor of HER2-targeted therapies. The emerging anti-HER2 drugs have greatly changed the diagnosis and treatment modalities of breast cancer and dramatically improved the prognosis of HER2-positive breast cancer patients. To optimize the treatment of HER2 breast cancer an update of expert consensus on HER2 positive breast cancer was made to adjust the different recommendation levels from early stage to metastatic stage. Meanwhile, antibody-drug conjugate (ADC) like trastuzumab deruxtecan (T-Dxd) has been shown to have great efficacy in HER2-positive and HER2 low expression breast cancer patients. Clinically, on the basis of the original definition for HER2-negative breast cancer, patients with HER2 immunohistochemistry (IHC) 1+ or IHC 2+ and in-situ hybridization (ISH)-negative are defined as low HER2 expression (HER2-low). As both the low expression and the positive expression of the HER2 protein is clinically significant for disease treatment and prognosis, we added a new chapter of HER2 low to recommend a proper regimen for this kind of patients. In this consensus, we also talk about the importance of clinical research, real world evidence, biosimilars and so on. In addition, the whole-course management of HER2 breast cancer is even more critical during pandemic of coronavirus disease 2019 (COVID-19). An approach that gives preference to "low-toxicity regimens and oral preparations" are also recommended.