Abstract
Breast cancer incidence differs by ethnicity in New Zealand (NZ) with Māori (the indigenous people) women having the highest rates followed by Pakeha (people primarily of British/European descent), Pacific and Asian women, who experience the lowest rates. The reasons for these differences are unclear. Breast density, an important risk factor for breast cancer, has not previously been studied here. We used an automated system, Volpara™, to measure breast density volume from the medio-lateral oblique view of digital mammograms, by age (≤50 years and >50 years) and ethnicity (Pakeha/Māori/Pacific/Asian) using routine data from the national screening programme: age; x-ray system and mammography details for 3,091 Pakeha, 716 Māori, 170 Pacific and 662 Asian (total n = 4,239) women. Linear regression of the natural logarithm of absolute and percent density values was used, back-transformed and expressed as the ratio of the geometric means. Covariates were age, x-ray system and, for absolute density, the natural log of the volume of non-dense tissue (a proxy for body mass index). Median age for Pakeha women was 55 years; Māori 53 years; and Pacific and Asian women, 52 years. Compared to Pakeha women (reference), Māori had higher absolute volumetric density (1.09; 95% confidence interval [95% CI] 1.03–1.15) which remained following adjustment (1.06; 95% CI 1.01–1.12) and was stronger for older compared to younger Māori women. Asian women had the greatest risk of high percentage breast density (1.35; 95% CI 1.27–1.43) while Pacific women in both the ≤50 and >50 year age groups (0.78; 95% CI 0.66–0.92 and 0.81; 95% CI 0.71–0.93 respectively) had the lowest percentage breast density compared to Pakeha. As well as expected age differences, we found differential patterns of breast density by ethnicity consistent with ethnic differences seen in breast cancer risk. Breast density may be a contributing factor to NZ’s well-known, but poorly explained, inequalities in breast cancer incidence.
Highlights
Breast cancer remains the most important cancer for New Zealand women, who experience high breast cancer incidence and mortality rates compared to many other developed countries [1] with breast cancer accounting for fully 28% of all cancer registrations and 16% of all cancer deaths among women in 2009 [2]
In the United Kingdom (UK), density was found to be lowest in British South Asian women, intermediate in African-Caribbean women, and highest in white women [12]
This paper reports the results of a pilot study to obtain novel data on the distribution of breast density in New Zealand women, by ethnicity and age, using VolparaTM on digital mammographic images already collected by one of the Breast Screen Aotearoa (BSA) Lead Providers, Breast Screen Waitemata Northland (BSWN)
Summary
Breast cancer remains the most important cancer for New Zealand women, who experience high breast cancer incidence and mortality rates compared to many other developed countries [1] with breast cancer accounting for fully 28% of all cancer registrations and 16% of all cancer deaths among women in 2009 [2]. The burden of breast cancer is not shared across ethnic groups [2,3,4] and while earlier data reported rates to be reasonably similar between Maori and non-Maori women [5,6,7], more recent work highlights an increasing incidence of breast cancer among Maori [2,3]. United States (US) data show that, compared to white American women, African-American [13] and native Hawaiian [14] women have lower percentage breast density, which is partially accounted for by higher body mass index (BMI), while results for Asian women are inconsistent [13,14,15]
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