Ethnic differences in the time trend of female breast cancer incidence: Singapore, 1968-2002

Abstract

Background: From 1968 to 2002, Singapore experienced an almost three-fold increase in breast cancer incidence. This increase appeared to be different across the three main ethnic groups: Chinese, Malays and Indians. This paper used age-period-cohort (APC) modelling, to determine the effects of age at diagnosis, calendar period, and birth cohort on breast cancer incidence for each ethnic group.

Methods: This study included all breast cancer cases (n = 15,269) in the three ethnic groups, reported to the Singapore Cancer Registry from 1968 to 2002 between the ages 25 to 79. Age-specific fertility rates from the Department of Statistics were used to explore the role of fertility.

Results: In the 1970s, Indian women had the highest age-standardized breast cancer but by the mid-1980s the highest rates were seen among the Chinese. Remarkable differences were seen in the age-specific incidence rates by ethnic groups. After age 49, the incidence rates for the Chinese and Malays leveled off whereas it continued to rise in the Indians. While our analyses provided some evidence that an age-drift model described the trend seen in the Indians, age-period-cohort model and age-cohort model had the best fit for the Chinese and Malays aged 25 to 79 respectively. Overall, Chinese and Malay women born in later cohorts were at increased risk of developing breast cancer relative to their counterparts in the earlier cohorts. The three ethnic groups experienced similar changes in their fertility in the 1970s, which likely explained much of the increase in their breast cancer incidence but not the ethnic differences. There was a stronger inverse association between total fertility rate and pre-menopausal breast cancer incidence in the Chinese and Malays than the Indians.

Conclusion: The observed dissimilarity among ethnic groups suggests ethnic differences in exposure or response to certain risk factors. It is likely that longer and subtler differences in childbearing trends and other risk factors may further explain these ethnic differences.

Figure 1

Age-standardized incidence rates of breast cancer stratified by ethnic group from 1968 to 2002 (as indicated by the mid-year of the 5-yearly intervals).

Figure 2

Overall age-specific breast cancer rates stratified by ethnicity and 5-yearly diagnosis period from 1968 – 2002 (as indicated by the first year of the 5-yearly intervals).

Figure 3

Curvature effects for a) age, b) period and c) cohort from full age-period-cohort model. (First cohort 1891–1895 and last cohort 1971–1975 are not shown as they are only based on one observation).

Figure 4

Total fertility rates for Singaporean women aged 15 – 44 from 1968 – 2002, stratified by ethnicity. Source: Reports on Registration of Births and Deaths (1968 – 2002). (Note: Only number of live births in each year is included and female population denominators used are based on interpolation of census data in 1970, 1980, 1990 and 2000).

Figure 5

Scatterplots of cumulative breast cancer incidence rates per 100,000 women (for ages 25 to 49) and total fertility per 1,000 women (for ages 15 to 44) for cohorts 1943 – 1944, 1945 – 1946, 1947 – 1948, 1949–1950 and 1951–1952. Over time, decreased total fertility seems to be associated with increased cumulative rate of breast cancer incidence across three ethnic groups.