Adherence to adjuvant endocrine therapy: Is it a factor for ethnic differences in breast cancer outcomes in New Zealand?

PurposeDespite the benefits of adjuvant endocrine therapy for hormone receptor positive breast cancer, many women are non-adherent or discontinue endocrine treatment early. We studied differences in adherence to adjuvant endocrine therapy by ethnicity in a cohort of New Zealand women with breast cancer and its impact on breast cancer outcomes.MethodsWe analysed data on women (n = 1149) with newly diagnosed hormone receptor positive, non-metastatic, invasive breast cancer who were treated with adjuvant endocrine therapy in the Waikato during 2005–2011. Linked data from the Waikato Breast Cancer Registry and National Pharmaceutical Database were examined to identify differences by ethnicity in adherence to adjuvant endocrine therapy and the effect of sub-optimal adherence on cancer recurrence and mortality.ResultsOverall, a high level of adherence of ≥80% was observed among 70.4% of women, which declined from 76.8% to 59.3% from the first to fifth year of treatment. Māori women were significantly more likely to be sub-optimally adherent (<80%) compared with European women (crude rate 37% vs. 28%, p = 0.005, adjusted OR = 1.51, 95% CI 1.04–2.17). Sub-optimal adherence was associated with a significantly higher risk of breast cancer mortality (HR = 1.77, 95% CI 1.05–2.99) and recurrence (HR = 2.14, 95% CI 1.46–3.14).ConclusionsSub-optimal adherence to adjuvant endocrine therapy was a likely contributor for breast cancer mortality inequity between Māori and European women, and highlights the need for future research to identify effective ways to increase adherence in Māori women.