Women in the U.S., New Zealand, the European Union, Israel, Canada, Australia, and Uruguay have breast cancer incidence rates that rank as the highest in the world.5 Among women living in the U.S., breast cancer is the most commonly diagnosed cancer and the second leading cause of cancer-related mortality. Projections for 2004 show an expected 215,990 new cases of invasive breast cancer, representing 32% of all new cancer diagnoses, and 40,110 breast cancer-related deaths, accounting for 15% of cancer-related mortality.6
Breast cancer incidence has increased over the past 30 years in the U.S.6 This trend is thought to reflect increased diagnosis due to mammographic screening,7 and perhaps also to secular trends in the prevalence of obesity and hormone replacement therapy (HRT) use by postmenopausal women.8 Over the same period, breast cancer mortality rates have declined, reflecting earlier breast cancer detection and treatment, and improvements in breast cancer therapies.9
The risk of breast cancer increases with age. There is a rapid rise in breast cancer incidence with age up to about age 50; then the rate of increase slows dramatically.10
In the U.S., breast cancer incidence and mortality rates vary by race and ethnicity. White women have the highest incidence rates of breast cancer, while Asian Americans and American Indians/Alaskan natives have the lowest incidence rates.6 African American women have the highest breast cancer death rates, with mortality rates 30% higher than those of white women, in spite of significantly lower disease incidence.11 These differences have been attributed to disparate use of screening services and diagnoses at later stages, higher rates of early and more aggressive cancers, and undertreatment of disease.12
Breast cancer rates vary widely among different countries. The countries cited above have incidence rates six times higher than countries in Asia and Africa. Migrant studies suggest the importance of early life experience in affecting breast cancer risk. Japanese women who were born in Japan and migrated to the U.S. have higher breast cancer rates in comparison to their counterparts in Japan. While Japanese women who migrated to the U.S. as young adults experience a modest increase in their breast cancer rates; those born in the U.S. have rates approaching their white counterparts.13 International differences and migrant studies have pointed to the importance of environmental and lifestyle factors in determining breast cancer risk. Among the potential risk factors, we discuss those of particular interest.
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