From 1968 to 2002, Singapore experienced an almost three-fold increase in breast cancer incidence. This increase appears to be different across the three main ethnic groups: Chinese, Malays and Indians. The aim of this study was to explore possible explanations behind any observed differences through the use of age-period-cohort (APC) modelling. We applied Poisson regression to determine the effects of age at diagnosis, calendar period, and birth cohort on breast cancer incidence for each ethnic group. Akaike’s information criteria (AIC) and deviance tests, based on the distance of predicted rates from observed breast cancer rates, were used to assess goodness-of-fit of the models. This study included all breast cancer cases (14,522) in the three ethnic groups, reported to the Singapore Cancer Registry from 1968 to 2002. In the 1970s, Indian women had the highest age-standardized incidence rates followed by Chinese and Malays but by the mid-1980s the highest rates were seen among the Chinese. As the aetiology of breast cancer is believed to be different in pre-menopausal and post-menopausal women, separate models were fitted for women younger than 45 years and women at least 45 years of age at the time of diagnosis. While our analyses provided some evidence that an age-period model described the trend seen in post-menopausal Indians, age-cohort model has the best fit for the Chinese and Malays, both pre- and post-menopausal. Overall, Chinese and Malay women in later cohorts were at increased risk of developing breast cancer relative to their counterparts born in the earlier cohorts. All three ethnic groups experienced similar changes in their fertility in the 1970s, including a drop in the total fertility rates and delayed age at first birth, but resulted in ethnic differences in breast cancer incidence. More subtle differences in childbearing trends or other risk factors may explain these ethnic differences.
[Proc Amer Assoc Cancer Res, Volume 47, 2006]