• Accurate assessment of lower ranges of HER2 immunohistochemical (IHC) expression has become important since trastuzumab deruxtecan (T-DXd) was approved for treatment of patients with HER2-low metastatic breast cancer (BC).
• Findings from DESTINY-Breast06 Phase 3 trial showed that T-DXd offers a favourable outcome over standard chemotherapy in patients with HER2-low and HER2-ultra-low metastatic BC.
• Several studies showed low concordance among pathologists in distinguishing low levels of HER2 IHC expression.
• There is an emerging need for a standardized and reproducible HER2-low testing methodology.
• A dedicated EQA programme for the assessment of IHC stain quality of HER2-low testing was established by UK NEQAS ICC & ISH in 2023 providing support to clinical laboratories and gathering evidence about the reproducibility of HER2-low testing.
• We report results from the application of digital image analysis (DIA) for the HER2-low IHC assessment with a view of implementing DIA in the HER2-low EQA programme.

Lila Zabaglo¹, Suzanne Parry¹, Dawn Wilkinson¹, Andrew Dodson¹

1. UK National External Quality Assessment Scheme for Immunocytochemistry and In-Situ Hybridisation, London UK.