21.2.2 Breast cancer

The major aetiological factor for breast cancer is lifetime average oestrogen exposure, from both endogenous and exogenous sources (Veronesi et al., 2005 Key et al., 2001, International Agency for Research on Cancer, 2011a). Protection is afforded by breastfeeding and full-term pregnancies at an early age. Increased risk has been shown to be positively related to affluence. This relationship is likely to be due to a number of reasons, including patterns of childbearing, post-menopausal use of hormone replacement therapy and obesity. The average age at first birth in RoI fell from 27.5 in 1975 to 24.8 in 1995, but rose thereafter to its 2009 level of 29.1. In the 1990s, age at first birth tended to be higher in the west of RoI and relatively low in NI, with the exception of Castlereagh, North Down and Ards (Map 21.3).

Map 21.3 Average age of mother at first birth, RoI 1989-1998, NI 1997-1998

The relationship between employment levels and breast cancer risk found here was modest; that with educational levels was stronger. However, ascertainment bias due to screening may have had the effect of masking the relatively modest differentials in risk caused by socio-economic factors. The overall variation in risk across Ireland was relatively small.

Map 4.2 shows a clear gradient in 1995-2001 between higher risk in NI, where breast screening began in 1989 (HSC Public Health Agency, 2011), and lower risk in RoI, where screening began only in 2000 (National Cancer Screening Service, 2010) and then only in the eastern half of the country. Observed incidence rates were higher in NI up to 2000, but became higher in RoI from 2001 on (National Cancer Registry, 2011, Donnelly et al, 2010) (Figure 21.3). The introduction of population-based breast screening is the most likely explanation for the increase in incidence in RoI (Møller et al., 2005).

In both countries, socio-economic and geographical differences in incidence may also result from variations in screening coverage and compliance. Given the long history of breast screening in NI, the higher risk seen north and east of Derry, to the east of Belfast and in Newry and Mourne is likely to reflect a true difference in risk rather than an artefact due to screening. In RoI, in the period before organised screening began, the risk was higher in urban areas and, in Dublin, in the more affluent areas to the east and south of the city. Once population-based screening was established in RoI the areas of higher risk became more extensive. The area of higher risk in south Dublin seen during 1995-2001 was much less defined in 2002-2007, and extended to cover most of Dublin, Kildare, Louth and Meath. An area of above-average risk can also be seen in a triangle bounded by Cork, Waterford and Athlone. However, many areas in the east which would have been covered by screening in the east did not show as great an increase in relative risk.

Figure 21.3 Trends in female breast cancer incidence for RoI and NI 1994-2009 (European age-standardised incidence rates ± 95% confidence limits)

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