Risk of Subsequent Breast Cancer in Women with Early Stage HER2-Positive Breast Cancer in a Large Community Health Plan.

Purpose: Clinical outcomes have improved for women with early stage, HER2-positive breast cancer following the FDA approval of adjuvant trastuzumab use in 2006. However, only limited information exists on such patients' outcomes in real-world settings outside of clinical trials. We examined the risk of subsequent breast cancer in women with HER-2 positive disease, and the impact of trastuzumab use, in a large California community-based health plan. Patients and Methods: A cohort of 3550 women with HER2-positive breast cancer (stages I-III) from 2009-2017 were followed through December 2018. We calculated subsequent breast cancer (SBC) rates overall and by trastuzumab use. Multivariable Cox proportional hazards modeling was used to compute hazard ratios (HR) and 95% confidence intervals (CI) for SBC by trastuzumab use. Results: Within the cohort diagnosed with HER2-positive disease, 81% received adjuvant trastuzumab. After 4.1 mean years follow-up (maximum 10 years), the risk of SBC was 22% lower with adjuvant trastuzumab use (hazard ratio [HR] = 0.78, 95% confidence interval [CI]: 0.66-0.92) compared with non-use. The cumulative incidence of SBC precipitously rose two years after diagnosis and by the 10th year, the cumulative incidence was 31% among those who had trastuzumab therapy versus 34% without this therapy. Conclusion: In community practice settings, the cumulative incidence of SBC in patients with early stage HER2-positive BC was 31% at 10 years in a cohort treated with adjuvant trastuzumab. Trastuzumab use was associated with a 22% reduced risk of developing SBC. This residual disease burden suggests breast cancer outcomes may be improved with further treatment given the advent of next-generation HER2-targeted therapies.