Over the 17-year study period from 1988 to 2004, incidence rates of invasive breast cancer were 38% higher for US-born Hispanic than foreign-born Hispanic women, with elevations more pronounced for localized than regional/distant stage disease. The greater incidence differential between US-born and foreign-born Hispanics for localized disease may reflect higher mammography screening rates (33
) or higher levels of having a usual source of medical care among US-born Hispanics compared to foreign-born Hispanics (34
), consistent with CHIS population data.
This is the first study, to our knowledge, to describe trends in breast cancer incidence over time in US- and foreign-born Hispanic women, as prior reports (7
) examined incidence rates by nativity over a single time period. The trend analyses showed that, while differences in incidence between non-Hispanic whites and US-born and foreign-born Hispanics were consistent over the 17-year period, the differences were considerably more pronounced in women over 50 years of age, primarily post-menopausal women. In fact, among younger women, rates in US-born Hispanics were similar to those in non-Hispanic whites. As we have observed in US-born Asian women (25
), age-specific incidence rates did not increase as rapidly in Hispanic women, regardless of nativity, as they did in non-Hispanic whites. In women over 50 years of age, the rates of invasive breast cancer decreased significantly after 1998 in foreign-born Hispanics and non-significantly after 2002 in US-born Hispanics. However, we did not observe decreases in the incidence of estrogen receptor positive breast cancer in US- or foreign-born Hispanics after 2002, as seen in non-Hispanic whites (32
), even though the use of combined estrogen and progestin HT decreased by 48% in US-born Hispanics and 40% in foreign-born Hispanics from 2001 to 2003, possibly due to the lower use of HT in Hispanics.
Incidence differences by nativity may be due to differences in population distributions of breast cancer risk factors, as we found that US-born Hispanic women have a higher prevalence of certain risk factors, including advanced education, lower body mass index among women <50 years, nulliparity, late age at first birth, early menarche, alcohol consumption and use of estrogen/ progestin-containing HT for menopausal symptoms, as compared with foreign-born Hispanics. Other breast cancer risk factors, including physical inactivity and obesity in women ≥50 years, were less common in US-born Hispanics and are therefore unlikely to have contributed to the higher incidence rates of breast cancer in this group.
We also found that both living in a higher SES neighborhood and Hispanic enclave were important predictors of invasive breast cancer incidence rates in Hispanic women in California. Even though the two measures were highly correlated, neighborhood SES and living in a Hispanic enclave were independently associated with breast cancer incidence, with Hispanic enclave having a somewhat stronger association with breast cancer incidence than neighborhood SES. Our findings are consistent with a study using SEER cancer registry data that found higher breast cancer incidence (1988–1992) in Hispanic women living in US Census tracts with fewer Hispanics and higher incomes (35
). Our neighborhood measures likely capture both individual and neighborhood components of SES and acculturation; however, as cancer registries do not collect individual-level data on education or other measures of SES, we could not distinguish between the individual and neighborhood effects. Case-control studies have shown that women living in high SES neighborhoods had an increased risk of developing breast cancer above and beyond their individual SES (36
). Neighborhood SES may influence health through characteristics of the social (e.g., crime, social support, attitudes towards health), physical (e.g., pollution), and built (e.g., availability of health services, healthy food and recreation) environments of the neighborhood (37
Living in a Hispanic enclave may be an indicator of low acculturation to the U.S., as Hispanic immigrants have been found to initially reside in segregated enclaves, and over time, intermingle with people of other race/ethnicities in the host country (38
). Residence in enclaves may also be an indicator of resource availability and/or social support. Immigrants living in enclaves may be more likely to maintain the advantageous health behaviors, such as a healthier diet (14
), if ethnic food sources or other resources, such as services in their native language, are more readily available. One study found that Mexican-Americans living in Census tracts with higher percentages of Mexican-Americans consumed more traditional foods (e.g., corn, tomatoes, and legumes) and less of certain foods (e.g., some fruits, broccoli) than their counterparts living in less concentrated tracts (40
). There also may be fewer barriers in accessing medical care in areas with a high percentage of Hispanics (41
). Indeed, one study found that Mexican American immigrants living in areas with more Spanish speakers or Hispanic immigrants had better access to health care; however, this association was not seen for US-born Mexican Americans living in these areas (42
). On the other hand, Hispanic enclaves may be more likely to lack access to quality food and grocery stores (18
), quality medical care (43
) or a safe and walkable environment that promotes physical activity (14
). Our study lacked the population estimates for computing rates that account for both nativity and ethnic enclave, limiting our ability to differentiate these effects. Future research should differentiate the individual- and neighborhood-level effects, particularly the aspects of ethnic enclaves that positively or negatively affect breast cancer risk.
High acculturation to the U.S., as typically measured by duration of residence in the US and English language usage, has been found to be associated with a higher breast cancer risk in Hispanic women (13
). Similarly, higher neighborhood SES has been associated with a higher incidence of breast cancer among Hispanics (19
). Both high acculturation and high SES are correlated with certain lifestyle and reproductive risk factors that are related to higher breast cancer risk, including earlier age at menarche, later age at first birth, nulliparity or fewer children, and later age at menopause (12
). Additionally, higher physical activity has been associated with higher SES (45
), while shorter duration of breast feeding, increased HT use, higher caloric intake, increased alcohol consumption, sedentary lifestyle, and larger body mass have been associated with higher acculturation (12
The present study may be subject to some limitations. The imputation of immigrant status based on SSN, although an improvement over random assignment, is subject to some error. Given the similar sensitivity and specificity of the method, however, it is likely that misclassification as foreign-born was balanced with misclassification as US-born to produce accurate case counts overall for incidence rate calculations. The impact on overall case counts within groups defined by nativity is likely small, given that nativity was only imputed for 30% of cancer cases, and prior research showed high accuracy for cancer registry birthplace data (21
). Due to the sensitive nature of immigration status, it is also possible that undocumented immigrants may report being US-born rather than foreign-born; however, this would likely affect both our numerator and denominator, thus minimizing the effect of misclassification of nativity. Although only 2.4% of Hispanics in our study had missing or invalid SSNs, it is possible that some undocumented immigrants used false SSNs, as 2.8 million undocumented/illegal immigrants were estimated in California in 2006 (90% of these undocumented immigrants were from Latin America, with 65% from Mexico alone) (46
). Due to the increasing population of undocumented immigrants, the Social Security Administration has been identifying mismatched SSNs; in 2008, they found 4% of reports to be mismatched (47
). If false SSNs systematically classified foreign-born Hispanics as US-born, the incidence differences between foreign-born and US-born women observed here would be underestimated.
Our population estimates for Hispanics by nativity may also be subject to error, particularly for specific age-groups, which may have biased overall or age-adjusted incidence rates. While we relied on US Census data, the most definitive source of data for estimating populations, all population counts stratified by nativity are based on a sample of the population. To determine whether the assumptions underlying the methods we used for estimating annual populations were accurate, we compared our 2004 population estimates to those from the 2005 American Community Survey, a 2.3% stratified sample of the California population (48
), and found a 1.0% and 2.3% difference between the population estimates for US-born and foreign-born Hispanics, respectively. Lastly, the incidence in foreign-born Hispanics could be underestimated if immigrants returned home for medical care and their cancer were diagnosed in another country.
Our cancer registry data on Hispanic ethnicity data may also be subject to misclassification. However, several studies have shown registry classification of Hispanic ethnicity to be good (~80% sensitivity and positive predictive values) (22
). Our use of the North American Association of Central Cancer Registries Hispanic Identification Algorithm allows us improve the identification of Hispanics (24
), a group typically undercounted. In a recent study of breast cancer patients in the Los Angeles region, Hispanic classification using the NHIA algorithm, compared to self-report, had 97.7% sensitivity and 90.7% specificity (50
). Although Hispanics are heterogeneous with regard to country of origin and cancer incidence rates have been found to vary for national subpopulations (11
), we were not able to consider incidence trends in subpopulations because 56.8% Hispanics in our study did not have information on ancestry of origin. Our findings largely reflect the experience of Hispanics of Mexican descent, who comprise 77% of California’s Hispanic population (1
). Indeed, our incidence rates (1999–2001) in foreign-born Hispanics (70.2/100,000 (95% CI: 67.9–72.6) were similar to rates from Pinheiro et al. in primarily first generation/foreign-born Mexicans (71.9/100,000 (95% CI: 53.1–95.2)) (11
Our findings of consistently lower incidence rates of invasive breast cancer among foreign-born than US-born Hispanics suggest that nativity is a measure of acculturation that captures population distributions of breast cancer risk factors, and is important for assessing population-based patterns and trends in breast cancer incidence. In addition, our finding that foreign-born Hispanics were more likely to be diagnosed with later stage disease than US-born Hispanics suggests that measures of acculturation and SES also likely impact health care access and utilization of medical care. Future studies should not only examine individual-level measures of breast cancer risk factors and how they change with acculturation, but also examine individual- and neighborhood-level socioeconomic and cultural influences on health behaviors, health care access and utilization of medical care. A better understanding of breast cancer incidence rate variations in US-born and foreign-born Hispanic women will help identify modifiable risk factors relevant to breast cancer prevention in all women.