Health statistics aggregated across broad ethnic categories, such as Asians, may mask disparities between specific ethnic populations. Indeed, studies that present statistics on cancer risk factors, screening, incidence, or outcomes disaggregated by Asian nationality have shown striking variation.1–3,5–7,18,37–47
However, population-based studies have not been able to address the roles in this variation played by differences in immigration and acculturation histories across those groups, in part because of the incompleteness of data on birthplace in registry data.
In our examination of the association between immigrant status and breast cancer survival across 6 Asian ethnic populations, made possible through enhancement of the cancer registry data, we found that, with the exception of Japanese women, foreign-born Asian women had consistently more advanced breast cancer stage at diagnosis and, consequently, lower survival rates than their US-born counterparts. Moreover, these differences in survival by nativity were not explained by the many sociodemographic and clinical factors we evaluated. We further found that immigrant status had a more substantial effect on survival in some ethnic groups (Vietnamese, Chinese, and Filipinas) than in others and that the magnitude of the effects of certain demographic and clinical factors also varied across groups. By contrast, neighborhood SES was associated with survival only in Japanese women.
To our knowledge, only 1 previous population-based study assessed the effect of immigrant status on breast cancer survival. In Surveillance, Epidemiology, and End Results Program data from San Francisco–Oakland, Hawaii, and Seattle–Puget Sound that excluded cases with missing birthplace information, Pineda et al. found no differences in survival after breast cancer by place of birth for Japanese, Chinese, and Filipina women.12
However, because patients in the cancer registry without available birthplace data were more likely than those for whom this information was available to be US born and alive,14
these authors’ exclusion of patients with unknown birthplace from the survival analyses would lead to underestimation of mortality differences, a concern supported by our finding of a significantly detrimental influence of immigrant status.
Although disease stage at diagnosis is the single strongest contributor to differential rates in mortality among cancer patients, we found little contribution of stage or other tumor characteristics to the observed differences in mortality by immigrant status. Within ethnic groups, a significant effect of immigrant status remained for Chinese, Filipina, and Vietnamese women. Clearly, more research is warranted to understand the complex factors that encompass immigrant status.
Studies of healthy populations have found that, compared with their US-born counter-parts, foreign-born Asians present an apparently contradictory profile for breast cancer risk and outcomes. Compared with US-born descendents of immigrants from their countries, foreign-born Asian women generally are thinner and smaller; consume more soy, less fat, and more vegetables; engage in more physical activity; are older at menarche; have children earlier in life and are more likely to breastfeed them; and are less likely to receive postmenopausal hormone replacement therapies,18,48–55
all of which are factors associated with a lower risk of breast cancer. However, they also have lower SES; more linguistic barriers, reported health conditions, and Asian-specific beliefs and health behaviors; and are less likely to have health insurance and to be screened for breast cancer,18,42,56
all of which potentially raise their risk of poor outcomes after diagnosis. Asian cultures tend to value self-sufficiency, fatalism, and familial obligation over individual physical health, and these cultural values have been found to significantly affect breast cancer treatment choices.57–61
Several studies have shown that women with certain Asian ethnicities are less likely than the general population to receive guideline-based or preferred treatment, including adjuvant radiation after breast-conserving surgery6,62
and breast-conserving treatment instead of mastectomy,17,62–65
and that these patterns are in part associated with immigration and acculturation factors.17,65
Factors associated with the process and consequences of immigration may also be relevant. Not only is immigration related directly to stress, particularly when associated with personal or political turmoil, it often requires separation from friends and family, major lifestyle changes, and acceptance of lower-status occupations than immigrants had in their home countries. Future research on the impact of the specific immigrant experience, process, and context may better help to explain the disparities we observed in breast cancer outcomes in foreign-born Asian women.
We found few differences between foreign- and US-born Japanese women in prognostic factors or survival patterns after breast cancer. We previously reported similar rates of breast cancer incidence between foreign- and US-born Japanese.2
Women in Japan have become increasingly Westernized and the population prevalence of lifestyle-related breast cancer risk factors has risen significantly. Since World War II, the number of women in the Japanese workforce has risen,66
leading to a declining birth rate.67,68
This societal shift may have resulted in changes in the population distribution of breast cancer risk factors such as higher SES,69
higher body mass index, lower parity, and later age at first birth,68,70
as well as in factors that may affect survival. This may explain why foreign- and US-born Japanese women were more similar than were immigrant and nonimmigrant members of the other Asian populations we studied. Indeed, in our data, neighborhood SES and ethnic enclave did not differ for foreign-and US-born Japanese. In addition, because Japanese women in our study generally were younger when they received an SSN (83% before age 40) than were women in other ethnic groups (65% before age 40), it is likely that foreign-born Japanese immigrated earlier in life and were thus more acculturated than other foreign-born Asians.
Our study had the distinct strength of obtaining data from a database with sufficient statistical power to examine breast cancer outcomes in 6 nationalities of Asian women subclassified by immigrant status and with a significant (19-year) follow-up period. Nonetheless, our data had some limitations. Abstracted from medical records, cancer registry data on race and ethnicity are likely not recorded consistently across reporting facilities and may also be subject to some misclassification.29
We applied an algorithm to minimize this misclassification.
Our measurement of immigrant status also relied on imputation of nativity for 26% of the patients. Although imputation was an improvement over the assumptions about missing nativity data used in previous studies, the resulting classification had 84% sensitivity and 80% specificity. Even if imputed immigrant status was not completely accurate, it was likely to be correlated with acculturation, for which immigrant status served as a proxy.
Our findings of survival differences may also have been biased by the salmon effect,71–76
the phenomenon of migrants with serious illnesses returning home to their birth countries to die. We noted slightly higher loss to follow-up among the foreign- than US-born Asians, suggesting that the HRs comparing foreign-born women to US-born women may have been slightly underestimated. Other limitations of our study included a lack of detailed or complete registry data on clinical factors, treatment, and behavioral, environmental, and genetic factors that may influence survival after breast cancer diagnosis. Some groups stratified by ethnic population and immigrant status yielded small sample sizes.
Asians are the fastest-growing racial group in the United States,77
composing 4.2% of the national population and 12% of the California population.78
The process of immigration and acculturation, as well as intrinsic cultural differences between groups, may have important and lasting effects on the health of these populations, yet very little is known about patterns of health and disease across Asian populations stratified by immigration and acculturation characteristics. US Asian women are the only ethnic group for which cancer far outweighs heart disease as the leading cause of mortality,79–81
with breast cancer as the leading cause of cancer incidence and second leading cause of cancer mortality.1,3
Our study takes the first step toward providing the evidence base for a better understanding of how population-based cancer patterns vary with immigration and acculturation. A paucity of detailed health and other data specific to distinct Asian ethnic groups in the United States has caused health disparities between them to be largely overlooked. By enhancing cancer registry data, we uncovered important breast cancer survival disparities among Asian immigrant groups, which require that we shape public health policies for specific Asian subpopulations in the United States.