Breast cancer in New Zealand: trends, patterns, and data quality.

Aims: To determine if incidence of cancer of the female breast in New Zealand is changing by age, ethnic group, and geographic region, and if there are differences in rates for stage of tumour by urban and rural residence.

Methods: Age-adjusted incidence rates for Maori and nonMaori were computed from all primary cancers of the breast registered in the National Cancer Registry, 1978-92. Analyses of time trends and geographic variations were conducted using standard statistical methods.

Results: There were steady, but nonsignificant, increases in the Maori and nonMaori incidence rates between 1978-92 which represents important increases in case numbers. The age-specific rates increased sharply from age 20, levelled out at age 45 and slowly increased through 85 years and older. There was a significant trend over time for the rate of "not staged" cases which was due to an artifact. No statistically significant variations in the age-adjusted rates by area (14 area health board districts, 4 regional health authorities), or by stage of tumour in three urban-rural groups were found. Maori women showed less shift in incidence rates from regional to local stage of tumour than nonMaori. The proportion of tumours reported without stage increased in the period 1988-92. A large proportion of cases had nonspecific morphology codes.

Conclusions: There are small differences between rates of breast cancer in rural and urban residences in New Zealand, as compared to some other countries. The quality of the Cancer Registry as a basis for evaluating the planned breast cancer screening programme is adversely affected by the high proportion of cases found with no reported stage of tumour. The proportion of nonspecific morphologies in these data is also of concern. The recent passing of the Cancer Registry Act should ensure better reporting of morphologies.