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One hindrance to any discussion of global breast-cancer incidence is the limited data available for many countries. Incidence figures are based on data from small geographic areas that are often pooled and extrapolated to large regions. Reported rates may reflect only the women who are easiest to reach or who have the highest standard of living. Thus, current global figures cannot truly reflect the underlying economic and cultural diversity driving increased incidence and related mortality.
Nevertheless, the trends are clear enough to warrant concern and action. Registry data show that breast-cancer incidence has been increasing in virtually all regions since 1973. Although East Asian women still have the lowest rates (about 21 per 100,000, as compared with 101 per 100,000 in the United States and 85 per 100,000 in Western Europe), countries with the most developed registries have documented increases: rates in Japan, Singapore, and Korea have doubled or tripled in the past 40 years, and China's urban registries document 20 to 30% increases in the past decade. India reports similar trends, with increases concentrated in urban areas.
In Africa, the trends are difficult to evaluate, given the general lack of large registries and accurate population data. However, it is certain that the incidence of breast cancer in African countries (reported to be 23 per 100,000) is lower than overall rates in North America or Western Europe, as well as lower than rates among black women residing in these Western countries. Local registries in Africa report a doubling of rates over the past 40 years, but the degree to which these figures represent real increases, as opposed to changes in disease tracking and reporting, is unclear.
The most widely cited reason for the global increase in breast cancer is the “Westernization” of the developing world. The term encompasses generally desirable changes (socioeconomic improvements that increase life expectancy and allow women reproductive control) as well as the adoption of less desirable habits (dietary changes, decreased exercise), all of which could increase breast-cancer risk. In affluent Western countries, women have relatively high social status, tend to delay childbearing, have relatively few children and, until recently, commonly used hormone-replacement therapy (HRT). Some such behavioral factors — delayed childbearing, lower parity, and reduced breast-feeding — are becoming more prevalent in lower-income countries. The challenge is understanding how these reproductive risk factors, which are primarily associated with an increased risk of postmenopausal breast cancer, relate to incidence and mortality in lower-income countries, where rates of postmenopausal breast cancer are much lower than in Western countries. The cancers apparently causing the most deaths in lower-income countries may not be those influenced by such factors; more data are needed to clarify their true effect in poorer countries.