Analysis of HER2-low breast cancer in Aotearoa New Zealand: a nationwide retrospective cohort study

Aim To perform the first national analysis of demographic and clinicopathological features associated with the HER2 positive, HER2-low and HER2-zero invasive breast cancers in New Zealand. The study will inform the proportion of women who benefit from new HER2-targeted antibody drug conjugate (ADC) therapies. Methods Utilising data from Te Rēhita Mate Ūtaetae (Breast Cancer Foundation NZ National Register), the study analysed data from women diagnosed with invasive breast cancer over a 21-year period. The HER2 status of tumours was classified into three categories – HER2-zero, -low, - positive. Results From 2009-2021, 94% of women underwent HER2 testing, with 14% diagnosed with HER2-positive breast cancer. For advanced-stage disease, 38% formerly classified as HER2-negative were reclassified as HER2-low. Including HER2-positive breast cancers, this indicates 60% of women with advanced breast cancer would be eligible for the new HER2-directed ADCs (approximately 120 women per year). In future, these therapies may provide a targeted option for 40% of women with early-stage triple negative breast cancer now classified as HER2-low. Conclusion The findings suggest a significant proportion of women with invasive breast cancer in New Zealand could benefit from new HER2-targeted treatments. There is a need to standardise HER2 testing to enhance personalised treatment and improve outcomes.