Trastuzumab as Adjuvant Therapy for Early Breast Cancer: The Importance of Accurate Human Epidermal Growth Factor Receptor 2 Testing

Context.—Overexpression of human epidermal growth factor receptor 2 (HER2) is an adverse prognostic parameter for patients with breast cancer. Patients with HER2-positive tumors are, however, likely to gain significant benefit from targeted therapy with trastuzumab. Four large-scale trials have assessed the efficacy and safety of adjuvant trastuzumab. In all 4 trials, disease-free survival and overall survival were significantly improved with adjuvant trastuzumab (36%–52% and 33%–41%, respectively), and improvements were observed in all patient subgroups. Objective.—To describe, in detail, the clinical benefits of using trastuzumab in the adjuvant setting, and to emphasize the importance this places on the pathologist to correctly identify all patients with HER2-positive tumors with a high degree of accuracy. Data Sources.—Published literature on both HER2 testing and the adjuvant use of trastuzumab. Conclusions.—Immunohistochemistry and/or fluorescence in situ hybridization are routinely used to determine HER2 status. Maintaining quality assurance throughout a standardized testing process is essential to achieve accurate and reproducible assay results. Adherence to the new American Society of Clinical Oncology/College of American Pathologists HER2 testing guidelines will help to ensure correct identification of all patients who may benefit from adjuvant trastuzumab and has significant implications for patient outcomes.