Mammography service screening and breast cancer mortality in New Zealand: a National Cohort Study 1999-2011

Abstract

Background: This breast cancer mortality evaluation of service screening mammography in New Zealand, the first since commencement of screening in 1999, applies to the 1999-2011 diagnostic period. Individual-level linked information on mammography screening, breast cancer diagnosis and breast cancer mortality is used to analyse differences in breast cancer mortality according to participation in organised screening mammography, as provided by BreastScreen Aotearoa (BSA).

Methods: Women were followed from the time they became eligible for screening, from age 50 years (1999-2004) and 45 years (⩾2004). Breast cancer mortality from cancers diagnosed during the screening period from 1999 to 2011 (n=4384) is examined in relation to individual screening participation or non-participation during preceding person-years of follow-up from the time of screening eligibility. To account for changes from never- to ever-screened status, breast cancer mortality is calculated for each year in relation to prior accumulated time of participation and non-participation in screening. Breast cancer mortality is also examined in regularly screened women (screened ⩾3 times and mean screening interval ⩽30 months), and irregularly screened women compared with never-screened women. Statistical analyses are by negative binomial and Poisson regression with adjustment for age and ethnic group (Māori, Pacific women) in a repeated-measures analysis. Relative risks for breast cancer mortality compared with never-screened women, are adjusted also for screening selection bias, to indicate the extent of breast cancer mortality reduction in a population offered and not offered mammography screening. Prognostic indicators at diagnosis of breast cancer are also compared between different screening participation groups, including by grade of tumour, extent of disease (spread), multiple tumour status and maximum tumour size using χ² statistics, t-tests and two-sample median tests.

Results: For 1999-2011, after adjusting for age and ethnicity, breast cancer mortality in ever-screened women is estimated to be 62% (95% CI: 51-70) lower than in never-screened women. After further adjustment for screening selection bias, the mortality reduction in NZ is estimated to be 29% (95% CI: 20-38) at an average screening coverage of 64% for 2001-2011, and 34% (95% CI: 25-43) for recent screening coverage (2012-13, 71%). For irregularly screened women, the mortality reduction is estimated to be 31% (95% CI: 21-40), and 39% (95% CI: 22-52) in regularly screened women compared with never-screened women, after adjusting for age, ethnicity and screening selection bias (using recent 2012-2013 screening coverage of 71%). Ever-screened women diagnosed with breast cancer have more favourable prognostic indicators than never-screened women, with a higher proportion of localised cancer (63 compared with 46%), a higher proportion with a well-differentiated tumour (30 compared with 18%), lower risk of multiple tumours (RR=0.48) and smaller median tumour size (15 mm compared with 20 mm)-all differences are statistically significant (P<0.0001).

Conclusions: This is the first total population cohort study of an established nation-wide screening mammography programme using individual-level information on screening participation and mortality outcomes from breast cancer. The findings are in accord with other mammography screening service evaluations and with randomised trials of mammography screening.

Figure 1

Individual examples of person-year contributions to screening exposure and non-exposure under a variety of screening participation scenarios. The total cumulated person-years of participation and non-participation in screening up to the end of 2003 (vertical line) in a hypothetical cohort of 20 women aged 40–65 years in 1999. Cumulated person-years contributing to participation is 18, and the total cumulated person-years from first eligibility contributing to screening non-participation is 30.

Figure 2

Differences (%)† in breast cancer mortality by mammography screening group, New Zealand women, 1999–2011. ^†Adjusted for age and ethnicity by regression; and adjusted for screening selection bias (Duffy et al, 2002a) assuming relative risk in non-screeners to women not offered screening=1.17 and recorded screening participation rate of 71% for 2012–2013. ^§Trend test of regression estimates.