This report presents trends in ethnic and socioeconomic inequalities in the incidence of 28 cancers over a quarter of a century (1981–2004). It is the first report to document these inequalities in a comprehensive, consistent and reliable way.
Purpose
Cancer is a major public health issue in New Zealand, as in other established market economies. It ranks second as cause of death (after cardiovascular disease), currently accounting for almost one-third of all deaths. Furthermore, the burden of cancer – especially tobacco-related cancer – falls disproportionately on Māori and on socioeconomically disadvantaged individuals, families and communities, thus contributing to health inequality. Importantly, the causes of many cancers are now understood, meaning that a substantial proportion of the cancer burden is preventable, or amenable to early detection and cure through screening.These facts led to the development in 2003 of a cancer control strategy for New Zealand. This strategy has two objectives:
- to reduce the impact of cancer; and
- to reduce inequalities in the impact of cancer.
This report uses linked Census and New Zealand Cancer Registry datasets to produce estimates of cancer incidence, including inequalities and trends in inequalities in cancer incidence. It is an output of an ongoing collaboration between the Ministry of Health, the University of Otago and Statistics New Zealand that has used data linkage to generate information on health outcomes and inequalities in these outcomes – including cancer mortality – for some years.
Methodology
Record linkage- Five Censuses (1981–2001) were anonymously and probabilistically linked to cancer registrations, creating five separate cohort studies of the entire population. 73–82 percent of eligible cancer registrations were linked, of which at least 95 percent were estimated to be correct linkages. To avoid underestimation of rates using the linked datasets, linkage weights were calculated for strata of age, sex, ethnicity and deprivation.
- For each of the cohorts ethnicity was categorised as Māori, Pacific or Asian using a total response definition of ethnicity.
- The residual group (that is, those who did not identify as Māori, Pacific or Asian) were categorised as non-Māori non-Pacific non-Asian (described herein as European/Other).
- Household income, equivalised for the number of children and adults in the household and inflation-adjusted across cohorts, was categorised into tertiles.
- Cancer sites were categorised using International Statistical Classification of Diseases and Related Health Problems 10th Revision (ICD10) coding (having excluded in situ cases and second diagnoses of the same cancer within one cohort) for 24 adult cancer sites, ‘other’ adult cancers, ‘ill-defined’ adult cancers, child cancers and adolescent cancers.
- Age-standardised rates of cancer by ethnic group, and age-and ethnicity-standardised rates of cancer by income group, were calculated for each cohort.
- Standardised rate differences and ratios (SRDs and SRRs) were calculated within each cohort, and pooled over time, to quantify absolute and relative differences in cancer incidence between ethnic and income groups.
- For income differences, regression-based measures of inequality, the slope and relative indices of inequality (SII and RII), were also calculated. P values were calculated for statistical tests of linear trends over time in rates and measures of association.
Key Results
Information on selected cancer sites only is reported in this Executive Summary.- Bladder: Incidence increased by one-quarter overall. European/Other rates were about one-third greater than Māori or Pacific rates. Rates were 10–20 percent higher among lower income groups.
- Brain: Incidence increased modestly, and European/Other rates were usually highest. There were no consistent differences by income.
- Breast (female): Rates increased more among Māori, such that the Māori rate was one-quarter higher than the European/Other rate by 2001–2004. Incidence was consistently 10–20 percent higher among high-income women.
- Cervix: Incidence halved over the 25 years, and decreased dramatically among Māori and Pacific women. Nevertheless, the Māori rate was still at least twice the European/Other rate by 2001–2004. Gaps by income closed markedly.
- Colorectal: Incidence increased overall by 10–20 percent, and more so among Māori. Pacific rates tended to increase, but were usually less than Māori rates.
- Endometrial: There was no notable change in incidence over time. Pacific rates were consistently highest (up to three times that of European/ Other).
- Liver: Rates increased over time, and rates were five to ten times higher among Māori and Pacific than among European/Other.
- Lung: Rates halved over time among males, but were stable among females. Disparities between Māori and European/Other widened: Māori rates were three-to five-fold higher in 2001–2004. Rates among low-income people were up to twice those of high-income people.
- Melanoma: Incidence increased (although note that figures may have been influenced by the Cancer Registry Act 1993). Consistent relative inequalities were recorded over time: high-income people had rates one-quarter to one-third higher than low-income people; Pacific rates were 10 percent of European/Other rates; Māori rates were 20 percent of European/Other rates.
- Prostate: Rates increased profoundly over time (probably due to prostate-specific antigen testing). Māori, Pacific and European/Other rates were very similar at all points in time. Modest income differences were evident over time, rates among higher-income men being 10–20 percent higher in 2001–2004.
- Stomach: Rates fell over time; more so for males. Rates among Māori and Pacific people were at least twice those among European/Other. Rates tended to be higher in low-income people.
- Testicular: Rates increased by about one-third over time. Māori rates were usually greater than European/Other rates, and Pacific rates consistently lower than European/Other. There was a modest tendency towards higher rates among low-income men.
- Thyroid: Rates increased moderately over time. The only clear social patterning was evident in consistently elevated rates among Pacific women (often four times higher than those among European/Other).