Using new, rigorously derived cancer and population data estimates for women by immigrant status, we identified subpopulations of US Asian women who appear to have higher burdens of breast cancer than were previously described. Specifically, we found that US-born Asian women have incidence rates of invasive breast cancer nearly 2-fold higher than do foreign-born Asian women in all groups examined except Japanese women. These findings suggested that breast cancer rates reported for specific US Asian populations are sensitive to the proportion of foreign-born women. Our results also suggest that rates for US-born Chinese, Japanese, and Filipina women approached and, among Filipina women exceeded, those for non-Hispanic White women; this pattern was consistent with a strong environmental influence on breast cancer risk. Our findings also suggested that US-born Asian women younger than 55 years in particular have higher risks of breast cancer than previously estimated. To our knowledge, our study is the first to suggest that rates among pre- and peri-menopausal US-born Chinese and Filipina women were higher than were those for non-Hispanic White women.
These relatively higher rates in Asian populations together with their younger age distributions lead to a considerably greater proportion of breast cancer cases diagnosed among young women than occurs in other racial/ethnic groups. For example, from 1988–2004, the percentage of breast cancers diagnosed among women younger than 55 years was 67.5% among US-born Filipina women but only 28.9% among non-Hispanic White women. Furthermore, age-specific incidence patterns among US- and foreign-born Asian women differed from patterns seen in non-Hispanic White women, with rates differing very little between women 55 years and older and those aged 45–54 years. By contrast, among non-Hispanic White women, incidence rates were nearly 2-fold higher in the 55 years and older age group. In this regard, the age-specific incidence patterns we estimated for US-born Chinese and Filipinas in particular were noteworthy in light of the age-specific incidence for African American women, which have been shown to be higher than that for non-Hispanic Whites among women diagnosed who were younger 40 years, but “crossing over” to be relatively lower for women diagnosed at older ages.36,37
Although our1988–2004 data show that the rates for women younger than 40 years were indeed higher for African American (15.1 per 100000) than they were for non-Hispanic White women (12.8 per 100000), the rates for US-born Chinese and Filipina women were even higher (20.3 and19.8 per 100000, respectively). Thus, the observed ethnic and immigrant differences in the age-specific patterns of breast cancer incidence relative to menopause may lend insights into breast cancer etiology among Asian women, particularly as these age-specific patterns relate to changes in acculturation that may be occurring during this critical time period.
Previous studies have reported comparable breast cancer incidence rates for US-born Japanese and non-Hispanic White women,38,39
despite the lower prevalence among Japanese women of particular breast cancer risk factors (e.g., lower weight or body mass index and less alcohol consumption).40
We identified increasing rates of breast cancer for US Japanese women, regardless of immigrant status, during the period 1988–2000, mirroring the rising rates of breast cancer in Japan,41
nearly doubled between 1978 and 1998.41,43,44
Women in Japan have become increasingly similar to US White women with respect to the population prevalence of lifestyle-related breast cancer risk factors. After World War II, the number of women in the Japanese workforce has risen,45
which has produced a declining birthrate.46,47
This societal shift may have resulted in relevant changes in the population distribution of breast cancer risk factors, such as higher socioeconomic status,48
higher body mass index, fewer births, and later age at first birth.42,49
Consequently, breast cancer risk profiles may be more similar for foreign- and US-born Japanese women than for other Asian populations, causing a narrowing in the incidence rate gap between Japanese immigrants to the United States and those who are US-born. In addition, it is likely that the foreign-born Japanese immigrated earlier in life, and thus may be more acculturated, than other foreign-born Asians, based on the younger age of SSN issuance among Japanese observed in our data.
Some of our results differ from those reported in the only prior examination of breast cancer incidence rates by immigrant status.17
In an analysis based on 1973 to 1986 SEER data from Seattle, Washington, and San Francisco, Oakland, and Los Angeles, California, US-born Chinese and Japanese women had 50% higher incidence rates than did their foreign-born counterparts, whereas rates among US-born Filipinas were slightly lower than those among foreign-born Filipinas.17
The difference in main findings may be because of the choice in this prior study to randomly distribute immigrant status among the 13% to 22% of cancer cases with unknown registry birthplace data, which resulted in underestimated rates among US-born and overestimated rates among foreign-born women.22
Second, whereas the 1973–1986 rates were not directly comparable to ours given different populations used for age standardization, the rates among US-born Filipinas were substantially lower in the earlier analysis17
than they were in our study. The higher rates observed in our study may suggest a large increase in incidence rates in US-born Filipinas, as indicated by the 4% increase per year during the period 1988–2004, as well as immigrant cohort differences in risk factor profiles.
Our study had several possible limitations. Our imputation of immigrant status based on SSN, although an improvement over prior methods,50
had an 84% sensitivity and 80% specificity of classification. The remaining misclassification had a greater relative impact on the smaller immigrant group, usually US-born. The assignment of immigrant status in some groups is fairly sensitive to the SSN age cutoff for imputing immigrant status; for example, using an age cutoff of 20 years instead of 25 years would have yielded 21%, 8%, and 34% fewer US-born Chinese, Japanese, and Filipina breast cancer cases but would have been subject to higher misclassification rates than the cutoff we used. However, the primary function of immigrant status in our study was to indicate acculturation, with those characterized as US-born considered to be more acculturated than those characterized as foreign-born. Even if imputed immigrant status was not completely accurate, it was likely to be correlated with acculturation level.
Our population estimates for Asian populations by immigrant status may also have been subject to error, particularly for specific age groups, which may have biased overall or age-specific rates. Although the Census data we used were the most definitive for estimating populations, counts stratified by immigrant status were available only for a population sample, which reduced their robustness. To evaluate the accuracy of the assumptions underlying our methods for estimating annual populations, we compared our 2004 population estimates to those from the 2005 American Community Survey (ACS), a 2.3% stratified sample of the population, 51
for California. We found less than a 2% difference in the estimates for US- and foreign-born Filipinas, suggesting that our findings for Filipinas were unlikely to be biased by inaccuracies in the denominator data. For Chinese women, our population estimates were higher than the ACS estimates by about 8% for US-born Chinese and about 1.5% for foreign-born Chinese; however, if the ACS estimates were more accurate, the bias was conservative, as we would have underestimated the relative difference in rates between US- and foreign-born Chinese women. Our estimates for US- and foreign-born Japanese women were both 15% higher than the ACS estimates; however, even if the ACS data were more accurate, this difference would be unlikely to explain our finding of a lack of difference in breast cancer rates between US- and foreign-born Japanese women, given that the direction and degree of difference were similar.