Challenges & recommendations for identification of human epidermal growth factor receptor -2 (HER2)-low metastatic breast cancer in India: Expert opinion statement.

Results from a recent Phase 3 clinical trial (DESTINY-Breast04) established the efficacy of the antibody-drug conjugate, trastuzumab deruxtecan (T-DXd) in patients with metastatic breast cancer with immunohistochemistry (IHC) score 1+ or 2+ and without in situ hybridization amplification, defining a new category of metastatic breast cancer known as human epidermal growth factor receptor 2 (HER2)-low. Across studies of patients with primary or metastatic breast cancer, approximately 50 per cent have HER2-low tumours, thereby emphasizing the importance of accurately identifying these tumours. Results from the DESTINY-Breast06 trial further corroborated the DESTINY-Breast04 results showing T-DXd as a new standard of care for patients with HER2-low and HER2-ultralow (defined as IHC score 0 with membrane staining) metastatic breast cancer following one or more lines of hormone therapy. Differentiating between HER2 IHC scores 0 and 1+ shows significant interobserver variability, possibly because the low scores did not have any clinical relevance until now. To establish a standardized approach in scoring of HER2-low tumours in India, a panel of experts comprising histopathologists, molecular pathologists, and clinicians met to discuss guidelines on HER2 testing. The identification of HER2-low expression depends on multiple methodological pre-analytical and analytical variables, including sample handling, fixation, processing, antigen clones, staining methodology, and substrates. The panel also focused on the challenges in the interpretation of HER2-low status. Challenges in the pre-analytical and analytical phases could be addressed by rigorous quality control procedures and training the pathologists. In the post-analytical phase, the subjective mode of HER2 assessment and HER2 intratumoural heterogeneity in HER2-low breast cancer are factors that influence HER2-low assessment. The panel recommended robust standard operating procedures to overcome these challenges. The central point of discussion was to implement clear guidelines, careful supervision of pre-analytical and analytical issues, and specialized training for accurate HER2 testing that would help select patients eligible for novel therapies.