Our analyses demonstrated the disparities and changes in breast cancer incidence trends by race/ethnicity in Los Angeles County. While the NH white women continued to have the highest breast cancer risk, the risk among Filipinas and Japanese are fast-approaching. After a persistent rise in AAIR throughout the 1980s and 1990s in all racial/ethnic groups examined, breast cancer incidence rates began to decline during 2000–2007 significantly for NH whites, slightly for blacks, and remained stable for Hispanics. Statistical analyses showed continued growing trends with statistical significance for all Asian subgroups.
Breast cancer incidence rates increased gradually over time in Asian countries, including Japan, Philippines, China (including mainland China, Taiwan, and Hong Kong), and the Republic of Korea.21–22
Although the breast cancer incidence rates in Asia are currently not as high as those of the Asian-American women, they are expected to rise rapidly for the coming decades. Much of the variations in breast cancer risk in Asia is thought to be attributable to the country-specific differences in the prevalence of risk factors and westernization of lifestyles. Nationwide data in the U.S. also reported similar declines in invasive breast cancer incidence rates in NH white, black, and Hispanic women, respectively since 1999.7
The recent downward trends in invasive breast cancer incidence rates in the U.S. were reportedly limited to postmenopausal women with estrogen/progestin receptor positive tumors. Reduction in the prescription and use of menopausal hormone therapy (HT)2–6
and saturation in mammogram screening7–9
were thought to be largely responsible for the recent decline in invasive breast cancer incidence rates.
Our ability to examine breast cancer incidence trends by individual Asian ethnic groups not only confirmed the previous reports of rapidly rising breast cancer risk among Japanese and Filipinas as compared to the Chinese and Korean women,11–15
but also revealed the persistence of such alarming trends among these groups in recent years. In sharp contrast to the consistently and rapidly rising breast cancer risk among Asians, Hispanic women successfully maintained a stable risk level for the past 20 years. Since both the Hispanics and Asians are major immigrant populations in Los Angeles, their markedly different breast cancer risks warrant further investigations into the acculturation experiences and exposure to breast cancer risk factors in both groups.
A clear distinction in the association between age and breast cancer risk was documented in our findings between Asian and non-Asian women. As breast cancer risk consistently increases with age among non-Asians, after menopause the older Asian women did not necessarily have higher risk as compare to the younger women; by 2000–2007 all Asian ethnicities showed declining risk after age 65. The risk of developing breast cancer is known to increase rapidly with age until menopause then slows, indicating the involvement of reproductive hormones in breast cancer etiology.23
Among Asian women, breast cancer risk plateaus after menopause.24–25
The differences in age-specific patterns in incidence rates between Asians and non-Asians have been attributed to a combination of multiple factors that favored the Asians in terms of breast cancer risk, including late age at menarche, low body weight, low premenopausal ovarian estrogen and progesterone serum levels, and low postmenopausal estrogen levels.23,26–27
Our analyses of ASIR documented that over time the Filipinas and Japanese have moved away from the traditional Asian patterns of ASIR approaching that of the NH whites, as the Chinese and Koreans appear to be following the same trend. Since our ASIR data are cross-sectional, the age-specific patterns may also reflect a cohort effect. Previous studies have shown that breast cancer risk increases steadily with more recent birth cohorts.22
Studies of immigrants have shown that women who migrate to the U.S. from Asia experience substantial increases in breast cancer risk after living in the U.S. a decade or more and the risk among their offspring approaching that of U.S. born women.28–30
These observations underline the environmental and behavioral influences through acculturation on the development of breast cancer. The process of acculturation involves changes in lifestyle, including adoption of a western diet and sedentary living are believed to lead to a variety of chronic diseases, including breast cancer.31–32
The positive association between breast cancer risk and western diet that is high in meat and dairy products and sweets has been well established by many studies.33–37
Asian women who adopted the Western diet pattern had higher breast cancer risk as compared to those with a healthy traditional diet pattern that is high in vegetable, fruit, soy, and fish consumption.38–39
To date, fewer studies, particularly prospective cohort studies on dietary factors have included premenopausal women. Thus the associations between dietary components and breast cancer risk among premenopausal women have yet to be established. The effects of diet on the development of breast cancer are likely achieved through body weight/size and circulating blood hormone concentrations in postmenopausal women.33,40
Body size as a risk factor is inversely associated with breast cancer risk in premenopausal women, but positively associated in postmenopausal women.40
Studies of dietary acculturation among Japanese Americans showed clear generational differences with the younger generation more accustomed to western dietary patterns.41
Longer U.S. residency has been linked to overweight or obese among foreign-born Asian Americans.42
Physical activity has been found to be a means of reducing breast cancer risk.43
Physical activity that is sustained over lifetime has the greatest benefit. However activities done later in life or specifically after menopause have a larger impact than activities before menopause.44
While vigorous activities have the greatest benefit on breast cancer reduction, activities that are of moderate intensity were also found to confer a sizeable decrease in breast cancer risk.44
Although physical activity helps to reduce breast cancer risk, Asian Americans were found to have significantly lower levels of physical activities.45
It is important to note that each of the four Asian ethnic groups included in the study has its own unique cultural practices, dietary habits, immigration history, socioeconomic status, genetic makeup, and so on that may influence breast cancer risk. For example, Filipino- and Japanese-Americans generally have higher educational levels, lower poverty rates, and higher proportions of people with social security income and ability to speak English “very well”, as compared to their Chinese and Korean counterparts.46–47
Consequently, Filipina and Japanese women had higher rates of participation in Pap screening, mammography, and health insurance coverage than the Chinese and Korean women in California.14
Filipina-Americans tend to have higher body mass index (BMI) than other Asian-American women,14,48
which likely increases their risk for developing breast cancer.40
The possible limitations of our study relate to the fact that the analyses depended on the accurate and reliable racial/ethnic classification of cancer cases and annual population estimates. Over-counting cancer cases by racial/ethnic misclassification or under-estimating the underlying population will produce artifactually increased incidence rates. As in all population-based cancer registries in the U.S., the CSP collects the racial/ethnic information based on medical charts and administrative records. There have been reports evaluating the quality of race and Hispanic origin classifications in population-based cancer registry data.49–50
The overall quality was found to be excellent on race and moderate to substantial on Hispanic ethnicity among the SEER registries.49
Misclassification was very minimal for NH Whites, Blacks, Hispanics, Japanese, and Filipinos, while underestimates were found for Chinese by 16%, according to one study conducted in northern California.50
Being one of the SEER registries, in addition to following all the SEER reporting and coding rules, the CSP has two distinct advantages in this regard: 1) It has historically actively collected pathology reports on all microscopically confirmed cases diminishing the risk of underreporting, and 2) it performs visual editing for quality control on demographics (including race/ethnicity) for 100% of cases.
We would like to note the methodological difference in the racial/ethnic identification between the cancer patients and the population at risk. As described above, patient demographic information in the registry database was abstracted from the medical records that were not always self-identified. The estimates of the population at risk by race/ethnicity were based on self-reported census data. This less than ideal difference in the sources of racial/ethnic data is part of the nature of registry-based studies.
It is difficult to assess the quality of the annual population estimates by age, sex, and race/ethnicity without a large-scaled actual enumeration. Different sources of data and different estimation methodology may result in discrepancies in numbers. Our postcensal estimates of 2001–2007 compares well overall with the official population estimates for Los Angeles County by either the American Community Survey or the National Center for Health Statistics in collaboration with the Census Bureau. Furthermore, as our findings confirmed the known trends and patterns among the major racial/ethnic categories, we believe that any possible misclassifications of race/ethnicity or errors in population estimates are minimal and should not bias the overall observations.