Breast Cancer: Explaining Ethnic Differences

New insights have emerged about why breast cancer (search) seems to be deadlier in women of different racial and ethnic groups.

Breast cancer is the most common type of cancer among women, apart from skin cancer (search). It’s also the second leading cause of cancer death in women, after lung cancer, says the American Cancer Society.

But the numbers fall a bit differently among women of various ethnic groups. White women have more cases of breast cancer than women of any other ethnic group, yet more black women than whites die of the disease.

Why is that? It’s mainly a matter of risk factors — except for black women, according to a new study.

“Differences in breast cancer incidence rates between most racial/ethnic groups were largely explained by risk factor distribution, except in African-Americans,” the study notes.

“However, breast cancers in African-American women more commonly had characteristics of poor prognosis, which may contribute to their increased mortality,” the study continues.

Gauging Breast Cancer Risk Factors

When a woman gets breast cancer, doctors might have a hard time telling her exactly why it happened. Her genes, medical history, diet, lifestyle habits (such as smoking, alcohol, or activity level) could be important. So could her age, environment, hormone use, and family history, among other possible influences.

Looking at the broader picture, several general risk factors have been identified. For instance, the risk of breast cancer increases with age. Early age of first menstruation and later age of menopause are also risks. Alcohol, hormone use, smoking, diet, and activity may also be important. Using health care services, such as screening mammograms, could help. Social and economic status could matter, too.

Those risk factors seem to explain a lot of the racial and ethnic patterns in breast cancer, researchers say.

Breast Cancer Deadlier for Black Women

Data came from more than 156,000 post-menopausal (search) women enrolled in the Women’s Health Initiative, a long-term study of women’s health.

Participants provided an abundance of personal information about their breast cancer risk and mammograms.

Over six years, nearly 4,000 of the women were diagnosed with breast cancer. Rates of breast cancer for all minority groups were lower than for white women. Taking into account breast cancer risks explained the racial differences in the rates of breast cancer, except for African-American women.

Here’s how the numbers broke down by the women’s racial/ethnic group:

—White: 3,455 cases

—Black: 242 cases

—Hispanic: 103 cases

—Asian/Pacific Islander: 88

—American Indian/Native American: 11

—Unknown race/ethnicity: 39

Black-White Differences

The numbers showed that white women have more breast cancer than other groups. In most cases, differences in risk factors between the groups account for that, write researchers.

While black women had a lower rate of breast cancer then whites, their tumors had more aggressive characteristics such as not being responsive to the hormone estrogen (search) (estrogen-receptor negative). Women with estrogen-receptor negative tumors (search) — which are not affected by estrogen — were more likely than those with estrogen receptor-positive tumors to die from breast cancer.

These aggressive characteristics may contribute to the increased death rates seen in black women with breast cancer compared with whites, say the researchers.

The tumor type could worsen black women’s chances of surviving breast cancer. Research has repeatedly shown that although fewer black women than whites get breast cancer, black patients are more likely to die of it.

Obesity might also play a role, write the researchers, who included Rowan Chlebowski, MD, PhD, of the Los Angeles Biomedical Research Institute. About half (51 percent) of the black participants were obese, compared with 28 percent of white women, they note.

Still, the reasons for the black-white gap aren’t clear. Access to health care or mammography doesn’t completely explain it, write the researchers. “It remains to be determined whether differences in unidentified environmental exposures, genetic makeup, or other factors” are at work, they write.

The study appears in the March 16 issue of the Journal of the National Cancer Institute.

By Miranda Hitti, reviewed by Brunilda Nazario, MD

SOURCES: Chlebowski, R. Journal of the National Cancer Institute, March 16, 2005; vol 97: pp 439-447. American Cancer Society, “How Many Women Get Breast Cancer?” News release, Journal of the National Cancer Institute.