New findings from the Women's Health Initiative (WHI) suggest that variances in breast-cancer rates between racial and ethnic groups can be largely explained not by race or ethnicity, but by differences in the distribution of previously discovered risk factors, except in African-American women. Public Health Science Division researchers contributed to the study, published in the March 16 issue of the Journal of the National Cancer Institute.
The study of 156,570 postmenopausal women, led by Dr. Rowan Chlebowski at Harbor-UCLA Medical Center, found that white women had a higher incidence of breast cancer than women in minority groups when only age differences were taken into account. The lower incidence of breast cancer in Hispanic, Asian/Pacific-Islander and American-Indian/Alaskan-Native women mostly disappeared when differences in known risk factors such as family history, reproductive history, education level, alcohol consumption and mammography use were considered. Risk factors are characteristics or exposures that increase the likelihood of developing a disease.
"The fact that minority women of similar ages have lower rates of breast cancer than whites has been established for some time. We wanted to know whether these differences could be attributed to other factors or, alternatively, if there was something more fundamental about these minority groups," said Dr. Garnet Anderson, co-principal investigator of the WHI coordinating center and one of the study's co-authors. "In these data, only the lower rate among African-American women persists."
Differences in breast-cancer risk factors also explained some, but not all, of the difference in the death rate from breast cancer between African-American and white women. Despite the lower incidence of breast cancer among African-American women, a larger fraction died from the disease. Reduced access to health care and mammography may be contributing factors to more breast-cancer deaths in the general population. The current analyses, however, point to a different pattern of disease as a contributing factor.
The African-American women were more likely to have poor-prognosis tumors than whites. Their tumors were more often estrogen receptor-negative (a marker associated with poor clinical outcome), larger, and at a more advanced stage. African-American women with breast cancer were nearly five times likelier than white women to have these three unfavorable characteristics. The authors suggested that higher rates of obesity and high-grade cancer in the African-American women could explain some of the mortality difference.
Other studies point to genetic factors as a potential influence on the breast-cancer characteristics in African Americans. In study results published last year, Dr. Peggy Porter of the Human Biology Division found breast-cancer tumors from African-American women almost always exhibited more aggressive characteristics than those from whites. The findings held true even for breast tumors of equally advanced stages in the two groups of women. Porter and colleagues also found that tumors from African Americans were more likely to contain abnormal amounts of several cell-cycle regulatory proteins compared to tumors from white women. Cancers that lose control of these proteins tend to be more aggressive and harder to cure.
"The next step is to determine whether this different pattern of breast cancer in African Americans — meaning lower incidence overall, but no reduction in the rate of poor-prognosis tumors — can be explained in this group of women by genetic factors," Anderson said.
Established in 1991, the WHI is the largest-ever women's health study focusing on strategies for heart disease, breast and colorectal cancer and osteoporosis prevention in postmenopausal women. Fred Hutchinson is home to both the WHI coordinating center and the Seattle clinical center.
Co-authors included Aaron Aragaki and Drs. Anne McTiernan and Ross Prentice, PHS; Dr. Zhao Chen, University of Arizona; Dr. Thomas Rohan, Albert Einstein College of Medicine; Dr. Dorothy Lane, State University of New York at Stony Brook; Dr. Nancy Dolan, Feinberg School of Medicine; Dr. Electra Paskett, Ohio State University; Dr. F. Alan Hubbell, University of California at Irvine; and Dr. Lucile Adams-Campbell, Howard University Cancer Center.
The study was funded by the National Heart, Lung and Blood Institute, National Institutes of Health, Department of Health and Human Services, National Centers for Research Resources and generous Fred Hutchinson donors.