Original articleAdherence to adjuvant endocrine therapy: Is it a factor for ethnic differences in breast cancer outcomes in New Zealand?
Introduction
Breast cancer is the most common cancer and the second most common cause of cancer deaths for New Zealand women [1]. Māori, the Indigenous population in New Zealand are known to have one of the highest incidences of breast cancer among all populations in the world [2]. Age standardized rates of incidence and mortality from breast cancer are 28% and 60% higher for Māori compared to non-Māori women, who are predominantly of European origin [3]. Breast cancer mortality inequity between Māori and non-Māori women is greater than breast cancer mortality inequities between Indigenous and non-Indigenous populations in Australia, Canada and the USA [4]. While there has been an improvement in breast cancer survival both for Māori and non-Māori women over last two decades, a significant gap in survival persists [5]. An advanced cancer stage at diagnosis in Māori women has the greatest impact on breast cancer mortality inequity [3], while differences in treatment are also believed to make a substantial contribution [6].
Adjuvant endocrine therapy forms an integral part in breast cancer treatment and has shown to reduce mortality from hormone receptor positive breast cancer by about 30% [7], [8]. Traditionally, endocrine therapy [tamoxifen or an aromatase inhibitor (AI) as single agent or in sequence] was prescribed for 5 years, although recent studies have shown additional improvement of breast cancer specific survival by continuing tamoxifen beyond 5 years [9]. Despite proven benefits, many women either do not take their medication daily as prescribed (i.e. low adherence) or do not complete the full duration of treatment (i.e. discontinuation) for the minimum of 5 years [10], [11]. Based on previous studies, up to 22% of women discontinue endocrine therapy before the end of first year of therapy and only about 50% complete the full 5-years, while maintaining an optimum level of adherence [12], [13], [14]. These studies have also shown higher risks of breast cancer recurrence and mortality in women who are sub-optimally adherent or who discontinue their treatment [11], [15].
We conducted this study to estimate the degree of adherence to adjuvant endocrine therapy and to investigate ethnic, socio-demographic, tumour and treatment related factors associated with poor adherence among women with hormone receptor positive breast cancer in New Zealand. We also investigated the association between sub-optimal adherence and breast cancer outcomes to determine the impact of adherence on ethnic inequities in breast cancer outcomes.
Section snippets
Study population
Eligible women for this study were identified from the Waikato Breast Cancer Registry (WBCR). The WBCR is a prospective database that records breast cancers of women who were residents of the Waikato District Health Board area at the time of diagnosis since 1999. The WBCR includes more than 98% of the cancers diagnosed over the study period and validity of its data has been reported previously [16]. The Waikato District Health Board covers a population of approximately 380,000 out of a total
Results
Median age of the cohort (n = 1149) was 60 years (range 24–99). Median ages of NZ European and Māori were 62 (range 24–99) and 57 (range 28–89) years, respectively. Median follow-up duration was 51 months (inter-quartile range 32.1–73.0) months. There were a total of 131 (11.4%) cancer recurrences and 164 (14.3%) deaths, out of which 77 (47%) were due to breast cancer. Overall, 51% of women were followed up for at least 5 years or until death.
A total of 509 (42.2%) women were started on
Discussion
From this population-based cohort study we report that Indigenous Māori and Pacific women do have significantly higher rates of sub-optimal adherence to adjuvant endocrine therapy compared with NZ European women. This is important especially in light of our finding that risk of death and recurrence from breast cancer were significantly higher among women with sub-optimal adherence. This suggests that sub-optimal adherence to endocrine therapy may be a contributing factor to breast cancer
Conflict of interest statement
The authors have declared that they have no financial conflicts of interest.
Acknowledgements
We acknowledge funding support received from the Waikato Breast Cancer Trust, the Cancer Society of New Zealand, the New Zealand Breast Cancer Foundation, the Lion Foundation, the Grassroots Trust and the WEL Energy Trust towards the WBCR and for additional data collection included for this study.
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