MIGRATION PATTERNS AND BREAST-CANCER RISK IN ASIAN-AMERICAN WOMEN

Background: Breast cancer incidence rates have historically been 4-7 t imes higher in the United States than in China or Japan, although the reasons remain elusive. When Chinese, Japanese, or Filipino women migr ate to the United States, breast cancer risk rises over several genera tions and approaches that among U.S. Whites. Purpose: Our objective wa s to quantify breast cancer risks associated with the various migratio n patterns of Asian-American women. Methods: A population-based, case- control study of breast cancer among women of Chinese, Japanese, and F ilipino ethnicities, aged 20-55 years, was conducted during 1983-1987 in San Francisco-Oakland, California, Los Angeles, California, and Oah u, Hawaii. We successfully interviewed 597 case subjects (70% of those eligible) and 966 control subjects (75%). Results: A sixfold gradient in breast cancer risk by migration patterns was observed. Asian-Ameri can women born in the West had a breast cancer risk 60% higher than As ian-American women born in the East. Among those born in the West, ris k was determined by whether their grandparents, especially grandmother s, were born in the East or the West. Asian-American women with three or four grandparents born in the West had a risk 50% higher than those with all grandparents born in the East. Among the Asian-American wome n born in the East, breast cancer risk was determined by whether their communities prior to migration were rural or urban and by the number of years subsequently lived in the West. Migrants from urban areas had a risk 30% higher than migrants from rural areas. Migrants who had li ved in the West for a decade or longer had a risk 80% higher than more recent migrants. Risk was unrelated to age at migration for women mig rating at ages less than 36 years. Ethnic-specific incidence rates of breast cancer in the migrating generation were clearly elevated above those in the countries of origin, while rates in Asian-Americans born in the West approximated the U.S. White rate. Conclusions: Exposure to Western lifestyles had a substantial impact on breast cancer risk in Asian migrants to the United States during their lifetime. There was n o direct evidence of an especially susceptible period, during either m enarche or early reproductive life. Implications: Because heterogeneit y in breast cancer in these ethnic populations is similar to that in i nternational comparisons and because analytic epidemiologic studies of fer the opportunity to disentangle correlated exposures, this study sh ould provide new insights into the etiology of breast cancer.