Social inequalities or inequities in cancer incidence? Repeated census-cancer cohort studies, New Zealand 1981-1986 to 2001-2004

We examine incidence trends for 18 adult cancers, by ethnicity and socioeconomic position in New Zealand. The 1981 to 2001 censuses were linked to subsequent cancer registrations, giving 47.5 million person-years of follow-up. Ethnicity: Pooled over time, differences were marked: Pacific and MAori rates of cervical, endometrial, stomach and pancreatic cancers were 1.5-2.5 times European/Other rates; MAori, Pacific and Asian rates of liver cancer were 5 times European/Other; European/Other rates of colorectal, bladder and brain cancers were 1.5-2 times the rates of other groups and melanoma rates 5-10 times higher; Pacific and Asian kidney cancer rates were half those of MAori and European/Other. Over time, MAori and Pacific rates of cervical cancer fell faster and MAori rates of colorectal and breast cancer increased faster, than European/Other rates. Male lung cancer rates decreased for European/Other, were stable for MAori and increased for Pacific. Female lung cancer rates increased for all ethnic groups. Income: Other than lung (rate ratio 1.35 men, 1.56 women), cervical (1.35) and stomach cancer (1.23), differences in incidence by income were modest or absent. Tobacco explains many of the social group trends and differences and constitutes an inequity. Cervical cancer trends are plausibly explained by screening and sexual practices. Faster increases of colorectal and breast cancer among MAori are presumably due to changes in dietary and reproductive behaviour, but the higher MAori breast cancer rate is unexplained. Ethnic differences in bladder, brain, endometrial and kidney cancer cannot be fully explained.