Background and purpose: We investigated the prognosis of breast cancer (BC) with low expression of human epidermal growth factor receptor 2 (HER2), as previous studies have found varying impacts on survival of HER2-low BC compared with HER2 0 BC (HER2 IHC score of 0). HER2-low is defined as a score of 1+ or 2+ in an immunohistochemical (IHC) assay without HER2 gene amplification.
Materials and methods: Patients with HER2 0 or HER2-low BC from the national Danish Breast Cancer Group database were examined by multivariable survival analysis in a retrospective noninterventional investigation. Patients were grouped as either HER2 0 or HER2-low. The primary endpoint was time to recurrence (TR), and the secondary endpoints were overall survival (OS) and distant recurrence-free interval (DRFI).
Results: 41,610 patients were included (12,981 with HER2 0 BC and 28,629 with HER2-low BC). HER2-low BC was associated with a lower risk of recurrence (hazard ratio [HR]: 0.92, p = 0.03). Regarding secondary endpoints, HER2-low disease was linked to improved overall OS (HR: 0.94, p = 0.02). No statistically significant effect of HER2-low was found for DRFI, along with no differential effect of HER2-low according to estrogen receptor (ER) status.
Interpretation: HER2-low BC was found to show an improved HR for OS and DRFI compared with HER2 0 BC; however, further studies are need to establish whether it represents a separate biological entity.