The Impact of Adjustment for Socioeconomic Status on Comparisons of Cancer Incidence between Two European Countries

Background. Cancer incidence rates vary considerably between countries and by socioeconomic status (SES). We investigate the impact of SES upon the relative cancer risk in two neighbouring countries. Methods. Data on 229,824 cases for 16 cancers diagnosed in 1995–2007 were extracted from the cancer registries in Northern Ireland (NI) and Republic of Ireland (RoI). Cancers in the two countries were compared using incidence rate ratios (IRRs) adjusted for age and age plus area-based SES. Results. Adjusting for SES in addition to age had a considerable impact on NI/RoI comparisons for cancers strongly related to SES. Before SES adjustment, lung cancer incidence rates were 11% higher for males and 7% higher for females in NI, while after adjustment, the IRR was not statistically significant. Cervical cancer rates were lower in NI than in RoI after adjustment for age (IRR: 0.90 (0.84–0.97)), with this difference increasing after adjustment for SES (IRR: 0.85 (0.79–0.92)). For cancers with a weak or nonexistent relationship to SES, adjustment for SES made little difference to the IRR. Conclusion. Socioeconomic factors explain some international variations but also obscure other crucial differences; thus, adjustment for these factors should not become part of international comparisons. 1. Introduction International comparisons of cancer incidence highlight considerable differences in incidence rates between various countries [1]. The incidence rates in these studies are routinely age-adjusted due to the relationship between cancer and age and the variation between countries in their demographic makeup. It has been well established that socioeconomic status (SES) also influences the incidence rate of many types of cancer [2]. However, it is not usual to take into account the relationship between cancer and SES in international comparisons. The Republic of Ireland (RoI) and Northern Ireland (NI) are the only two countries on the island of Ireland, although NI is one of the constituent countries making up the United Kingdom. Recent studies have shown differences between the two countries in incidence rates for lung, bladder, brain, prostate, cervical, uterine and male colorectal cancer, leukaemia, and female melanoma [3]. This is despite the proximity of the two countries and their similar demographics and proportion of different ethnic groups [4, 5]. Additionally in the 2001 NI [4] and 2002 RoI censuses [5] 40% of the NI 16–74-year-old population was economically inactive compared to 34% in RoI, while of the economically active population 7% in NI was unemployed