Migrant studies aid the search for factors linked to breast cancer risk.

Breast cancer is by far the most common nonskin cancer in U.S. women. However, breast cancer rates vary widely around the world. Most cases occur in industrialized countries in Europe and North America, whereas the disease is less common among developing countries in Africa and Asia. Rates in the Netherlands, France, Denmark, and the United States, for example, cluster around 90 – 100 cases per 100,000 women, whereas those in China, India, Turkey, and Rwanda range from 10 – 20 cases per 100,000 women. Over the past four decades, many studies have shown that breast cancer rates change when women move to a new country, providing evidence for the importance of lifestyle and environment in breast cancer risk. However, most of the studies have not been designed to answer questions about which factors are driving the increased risk. Now, data from several recent studies of people who have migrated from their home countries are beginning to suggest some answers. “ Migration provides a kind of natural experiment allowing the comparison of populations of similar genetic background living in different environments, ” said Max Parkin, M.D., an epidemiologist at the University of Oxford in the United Kingdom. Migrants typically move to countries with higher breast cancer rates than those of their country of origin. Two studies published in the l990s showed that the Italian and Polish women had a higher risk of breast cancer after they migrated to Australia, where incidence rates are higher. Another study compared incidence rates of Japanese women who migrated to Los Angeles, San Francisco, and Hawaii to the rates of Japanese women still living in Japan. The incidence rates — 63 per 100,000 in Los Angeles, 68 per 100,000 in San Francisco, and 73 per 100,000 in Hawaii — were more than twice as high as the rates for Japanese women living in Japan (24 – 31 per 100,000). A separate study showed that the third generation of Asian-American women living in the United States has rates similar to or greater than white women in the United States. The factors frequently associated with these changes in breast cancer risk include “ acculturation, Westernization of lifestyle, later marriage, fewer children later in life, changing infant feeding patterns, women getting taller, increasing rates of obesity, dietary changes, and exposures to carcinogenic agents, ” according to a recent literature search of studies of breast cancer in migrant populations by Valentina Andreeva-Cook at the University of Southern California in Los Angeles.