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"The breast cancer mortality rate along a section of the East Coast stretching from
New York City to Philadelphia is 7.4 percent higher than the rest of the Northeast," said
Martin Kulldorff, Ph.D., who led the study. "This is a statistically significant finding, indicating that elevated risk of breast cancer mortality in that area is not a random occurrence, but may be due to some underlying reason."
Kulldorff adds that "great caution should be exercised in interpreting the study of geographic clusters in cancer mortality." In particular, ascribing the cause of a mortality cluster to some local environmental exposure may be difficult or impossible.
Previous studies have shown that the northeastern United States (including Conn., Del., Md., Maine, Mass., N.H., N.J., N.Y., Pa., R.I., Vt. and D.C.) has about a 16 percent higher breast cancer mortality rate than the rest of the country. However, even within this region, there is substantial variation. Delaware, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Washington D.C. have higher breast cancer mortality rates than the national average. Breast cancer mortality rates in four other northeastern states -- Maryland, Vermont, Connecticut and Maine -- are very close to the national average of 26.4 per 100,000 women.
During the five years (1988-1992) covered by the analysis, 24,044 women in the
New York/Philadelphia region died of breast cancer. This was about 200 more deaths per year than researchers would have expected to find if this area had the same breast cancer mortality rate as the rest of the 11-state northeastern region of the U.S. The study also showed the existence of four smaller clusters of increased mortality within the larger New York/Philadelphia area: northeast New Jersey, central New Jersey, Philadelphia, and Long Island, N.Y.
Long Island has been a major focus of research on breast cancer, following a June 1993 Congressional mandate to initiate a comprehensive study to ascertain whether environmental factors might be related to elevated breast cancer rates there. In light of this new study, Long Island's elevated breast cancer mortality rate should be seen as part of a larger geographical trend in the New York/Philadelphia area.
In the new study, published in the July 15, 1997 American Journal of Epidemiology, researchers used a specially designed computer program that places a circular "window" on a map of the 11 most northeastern states and the District of Columbia.* They then calculated breast cancer mortality rates for the area inside the window's circumference and for the remainder of the northeastern United States.
A newly developed statistical method, called the spatial scan statistic, was used to compare these two rates. By resizing and moving the window across the entire map, the researchers were able to systematically scan for possible breast cancer mortality clusters and identify approximate boundaries for areas where mortality was elevated. The moveable window freed researchers from having to preselect which states or parts of states they would examine. Instead, they were able to let the data define the area for them.
Researchers also found a higher breast cancer mortality rate among women under 50 in the District of Columbia, but this cluster was explained by its proportionately larger African-American population. On average, African-American women have a higher mortality rate from breast cancer than do white women. In a paper published in the Feb. 5, 1997 Journal of the National Cancer Institute, NCI epidemiologist Robert E. Tarone, Ph.D., reported that breast cancer mortality rates for black women in the northeastern U.S. are no higher than those for black women in other regions. It is thus unlikely that widespread environmental exposures explain the elevated breast cancer mortality rates among white women in the Northeast.
The new study found that age, race, urbanization and average number of children born could not explain the elevated mortality rates for the larger New York/Philadelphia region, or for the four smaller clusters. However, the researchers could not include in their analysis other important risk factors for breast cancer such as age at first birth, menarche, or menopause, or factors that are known to affect mortality, including access to health care and regular mammography screening.
A study published in the July 2, 1997 Journal of the National Cancer Institute demonstrated that elevated breast cancer incidence in the San Francisco Bay Area could be entirely explained in terms of such known risk factors. Results of another study led by NCI epidemiologist Susan Sturgeon, Dr.P.H., and published in the Dec. 20, 1995 Journal of the National Cancer Institute showed that a large part of the regional differences in breast cancer death rates among white women in the United States would be explained by known factors such as age at first birth and mammography screening.
In 1997, there will be an estimated 180,200 new cases of invasive breast cancer among women in the United States, and an estimated 44,190 women will die from the disease. Breast cancer mortality rates vary widely from state to state. Hawaii's rate is 32 percent lower than the national age-adjusted average of 26.4 per 100,000 women, and Washington D.C.'s rate is
28 percent higher.
The new technique used in the study will help researchers overcome a major problem in "cancer cluster" investigations: the fact that the areas studied as clusters have generally been selected based on public concern rather than systematic comparison of rates in different areas.
NCI, in collaboration with the National Institute of Environmental Health Sciences, has undertaken two large initiatives to investigate reasons for elevated breast cancer death rates in the Northeast/Mid-Atlantic regions. These studies are being conducted by investigators at major medical institutions in the East. Six studies, which comprise the Northeast/Mid-Atlantic Study, are focusing on organochlorine compounds, which are commonly used as pesticides. The Long Island Breast Cancer Study Project is investigating whether environmental factors are responsible for breast cancer risk in Nassau and Suffolk counties. This research includes assessments of exposure to organochlorines and other chemicals, magnetic fields, contaminated drinking water, air pollution, and hazardous and municipal waste.
*The study is titled "Breast Cancer Clusters in the Northeast United States: A Geographic Analysis." The authors are M. Kulldorff, E.J. Feuer, B.A. Miller, L.S. Freedman. American Journal of Epidemiology, Vol. 146, No. 2, 1997.
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