Impact of mammographic screening on ethnic and socioeconomic inequities in breast cancer stage at diagnosis and survival in New Zealand: a cohort study

From this study we found that breast cancer survival inequities between Māori and NZ European women in the screening age group were significant for women diagnosed through the non-screen pathway but absent for women diagnosed through screening. Five and 10-year survival for women diagnosed through screening were 2.8% and 5.2% higher respectively for Māori compared with NZ European women, while Māori women diagnosed through the non-screen pathway had a 9.9% lower 5-year and a 17.7% lower 10-year survival than NZ European women. We found that patterns of inequities by socioeconomic deprivation were similar to those found between Māori and NZ European women. For non-screen detected cancer, 10-year breast cancer specific survival was 16.3% lower for most deprived compared with women from most affluent socioeconomic quintile.

Benefits of population based mammographic breast cancer screening have been proven by several randomized trials [3,11]. It has been shown that provision of biennial screening with a 70% coverage confers an approximately 30% reduction in breast cancer mortality for the screening population [12]. The breast cancer incidence for NZ European women is similar to the rates seen in breast cancer screening trials [12,13], but incidence in Māori is much higher [14]. Hence, at a 70% biennial screening rate, more Māori women may benefit than non-Māori, and given that Māori women with symptomatic cancer tend to present at a later stage than non-Māori, Māori women may also have a greater reduction in breast cancer mortality with screening. Our data lend support this hypothesis.

Compared with NZ Europeans, a higher proportion of breast cancer is observed among younger Māori women due to the younger age structure of Māori population [15]. Further, the screen detection rate for Māori women between 45 to 49 years in initial mammographic screens is more than twice that for non-Māori, and is 50% greater even for subsequent screening mammograms [5]. Breast cancer in younger women tends to be more aggressive and is more likely to be associated with poorer outcomes [16]. Thus, it may be worthwhile exploring the usefulness of providing breast cancer screening for Māori women below the current screening age limit, for instance for women between 40 and 44 years. However, a thorough evaluation of potential benefits versus harms of mammographic screening should be an essential part of such an initiative.

Obese women are more likely to present with more advanced breast cancer compared with non-obese women and are more likely to be diagnosed through screening mammography than non-obese [17]. Approximately 48% of adult Māori women are obese, which is almost twice the rate observed in NZ European women [18]. Evidence from the USA suggests that up to 30% of later stage breast cancer diagnosed in African American women compared with White American women are attributable to higher rates of obesity in African American women [19]. At present, no New Zealand data are available on the impact of obesity on ethnic differences in breast cancer stage at diagnosis or outcomes. Nonetheless, higher obesity prevalence is likely to be another factor that would confer a greater benefit from increased mammographic screening coverage for Māori compared with NZ European women.

Survival differences for screen detected breast cancer were not seen, either by ethnicity or socioeconomic deprivation, while differences in survival were marked and significant for non-screen detected cancer by both. Similar observations have been reported from the UK, the USA and the Netherlands [14,20,21]. Inequities in breast cancer survival by ethnicity and socioeconomic status for women diagnosed through non-screen pathway are likely due to a range of factors. While later stage at diagnosis for Māori and women of socioeconomically deprived groups is the likely major factor for these differences, higher rates of obesity, comorbidities and differences in treatment compared with NZ European and more affluent women, respectively are also likely to be important. In addition, women who participate in mammographic screening may be more likely to have better access to health care, education, health literacy and health seeking behaviours compared with women from similar ethnic or socioeconomic backgrounds, who do not participate in screening [22].

Furthermore, there is a stringent quality framework and an audit process for the treatment of breast cancer detected through BSA, but not for cancers diagnosed outside BSA [4]. For instance, all women with BSA detected breast cancers are reviewed through a multidisciplinary meeting, and according to BSA quality standards, at least 90% of BSA detected women are expected to receive their first surgical treatment within 20 working days of receiving their final diagnostic result. Results for each regional screening provider are regularly audited and a feedback process ensures that adequate measures are undertaken by providers who fail to achieve these targets. Similar audit processes or quality frameworks are not in place for non-screen detected cancer treatment. Lack of a framework is a likely key reason for the disparities observed amongst women with non-screen detected cancers by ethnic and socioeconomic grouping. This is further supported by previous work which was based on the Waikato Breast Cancer Register on delay for primary surgical treatment of breast cancer [23]. This study showed significantly longer delays for Māori compared to NZ European women for non-screen detected breast cancer, but not for cancers diagnosed through BSA. To overcome these disparities in cancer care, the Ministry of Health is in the process of introducing quality measures for the management of all common cancers through the National Cancer Control Strategy. These include the Faster Cancer Treatment Indicators [24] and the Standards of Service Provision for Breast Cancer Patients in New Zealand [25]. These measures will provide benchmarks for quality cancer care and are expected to improve care for all women with breast cancer as well as reducing and hopefully eliminating inequities in access, timeliness and quality of care along the symptomatic breast cancer care pathway.

The main strengths of this study include the completeness and comprehensive nature of the study sample which include more than 98% of all breast cancers in eligible women over the study period. Data linkage with the national screening database enabled us to clearly define and validate screen detected, interval and symptomatic non-interval cancers. Furthermore, relatively longer follow up has provided fairly robust survival data with five and 10-year survival rates for groups of women of interest. Limitations of this study include the absence of data on previous breast cancer screening behaviours of these women, which is a factor known to be associated with cancer stage at diagnosis and long term outcomes [21]. Also, we were unable to ascertain the reasons for non-screening participation in women with symptomatic non-interval cancer, which however is beyond the scope of the present study. This study has used NZDep2006 as a measure of socioeconomic status, which is based on area level deprivation [8]. Although area level deprivation is not as accurate as individually measured socioeconomic status, NZDep2006 has been validated as an accurate proxy measure for assessment of individual deprivation [26].