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Geographic Differences in Breast Cancer Mortality:
Known Factors Explain Some of the Disparity
Why are breast cancer death rates higher in some regions of the United States than in others? Recognized risk factors and factors predictive of mortality could account for a sizable part of the difference, according to a detailed analysis of geographic disparities in breast cancer mortality risk by researchers at the National Cancer Institute (NCI). The analysis, published in the Dec. 20 Journal of the National Cancer Institute,* provides a new perspective on this longstanding question.
Mortality data have shown for several decades that breast cancer death rates are higher in the Northeast than in other regions of the country, particularly the South. But the new analysis demonstrates that the Northeast, Midwest, and West have a similar mortality risk after taking into account factors such as age at first live birth. Moreover, the differences between these regions and the South narrowed after accounting for such factors.
"After adjustment for a number of known factors, the three regions look much more alike in relation to the South," said the NCI's Susan Sturgeon, Dr.P.H., who led the analysis. "The Northeast is no longer singled out."
Sturgeon and her colleagues emphasize that the study does not explain all the regional differences in breast cancer mortality. "This is not so much a study that breaks ground as a study that lays groundwork," Sturgeon said. "It helps put into perspective what the problems really are." For instance, research questions may now be more likely to revolve around why rates are low in the South than why they are high in the Northeast.
Contributing the most to geographic variations in mortality were differences related to childbearing. Women in the Northeast were more likely to delay childbearing until a later age or to have no children than women in other regions of the country, particularly the South. Both a late age at first birth and having no children are well established breast cancer risk factors.
The investigators examined a range of additional factors as well. Other factors known to increase the risk of developing or dying from breast cancer include a family history of the disease, earlier age at the beginning of menstruation, and later age at menopause. Other known factors are a history of biopsy proven benign breast disease, higher alcohol intake, greater body mass (weight in relation to height), measures of socioeconomic status, and lower use of mammography. Data on the prevalence of these factors in different regions of the country came from questionnaires on cancer risk that were part of the 1987 National Health Interview Survey.
After adjusting for age only, breast cancer mortality risk for women age 50 and older was 30 percent higher in the Northeast compared to the South, 18 percent higher in the Midwest, and 15 percent higher in the West. But after adjusting for all factors, mortality risks in these three regions were more similar to each other.
Moreover, the gap between each of these regions and the South grew smaller. After adjustment, mortality risk compared to the South was 13 percent higher in the Northeast rather than 30 percent, 8 percent in the Midwest rather than 18 percent, and 13 percent in the West rather than 15 percent.
Among women under age 50, geographic differences in breast cancer mortality risk were more modest to begin with and, after adjustment for risk and predictive factors, almost disappeared. In the Northeast and West, mortality risk for younger women dropped slightly below that of the South after adjustment and was 5 percent higher in the Midwest.
The authors point out that after adjustment for known factors, 13 percent of the 30 percent excess mortality in the Northeast (relative to the South) remains unexplained. They suggest that regional differences in factors not considered in this study need to be examined as well. These include possible environmental and dietary factors, quality of death certificate information, and factors that influence survival, such as extent of disease at time of diagnosis.
Within regions, differences in breast cancer mortality also remain a puzzle. Studies are under way to examine whether and how much known risk factors contribute to these differences.
* The study is titled " Geographic variation in mortality from breast cancer among white women in the United States." The authors are S.R. Sturgeon, C. Schairer, M. Gail, M. McAdams, L.A. Brinton, R.N. Hoover. Journal of the National Cancer Institute, Dec. 20, 1995.
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