Summary: A new study by American Cancer Society researchers shows that the risk factors that affect whether women die of breast cancer are similar among white women and black women. The study is one of the largest to examine these risk factors and one of very few that compare risk factors by race.

Why it's important: Breast cancer is the most common cancer in both African-American and white women in the US, but the disease affects each race differently. African-American women are less likely than white women to get breast cancer, but are more likely to die from it if they do get it. Some of that gap is due to social factors like poverty and access to health care. Some studies have also found genetic differences in the type of breast cancer that develops in black and white women. But little is known about whether risk factors have a different impact in women of different races.

"Most of what we know about risk factors for breast cancer is from studies of white women," explained Marji McCullough, ScD, a senior epidemiologist at ACS and the study's lead author.

Studying risk factors by race may help explain some of the differences in death rates seen in US women, and may give researchers clues on how to close the gap between the races further.

What's already known: Many of the risk factors for dying of breast cancer overlap with the risk factors for developing breast cancer, McCullough explained. Risk factors women cannot change include their age, when they started getting their periods, when they reached menopause, and whether breast cancer runs in their family.

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Risk factors women can control include their weight, how much exercise they get, their diet, and how much alcohol they drink. Some studies have suggested that certain risk factors may vary by race, but data on this are limited.

How this study was done: McCullough and her colleagues studied more than 21,000 black women and more than 400,000 white women who were already taking part in a larger ACS study of lifestyle and cancer. All of the women were past menopause. In 1982 the women answered questions about their medical history, reproductive history, height and weight, diet, and exercise. The researchers followed them for 20 years. The findings were published in the American Journal of Epidemiology.

What was found: Over the course of the study, 257 African-American women died of breast cancer, as did 4,265 white women. Most of the risk factors for dying of breast cancer seemed to have an equal impact in women of both races. Factors that were bad for white women (like being overweight) were also bad for black women, and those that had a positive effect on white women (like getting regular exercise) also had a positive effect on black women, although there were some minor differences in the degree of risk or protection.

"The good news is that it appears that the modifiable risk factors [those a woman can control] like physical activity and controlling obesity are important for black women, too," McCullough said.

Study limitations: The study only looked at risk factors associated with dying from breast cancer. It couldn't measure how important these factors were to developing breast cancer. And it couldn't account for other things that have an impact on whether women survive breast cancer, like the size and stage of the tumor, what kind of treatment the women received, or how quickly they got treated. The study also couldn't measure whether some of these risk factors changed over time -- whether women gained or lost weight, for instance -- and what effect that might have had.

"This is one piece of the puzzle," McCullough said.

The bottom line: This new study shows that the factors that influence the risk of breast cancer death are similar in black women and white women. Still, more research will be needed to really tease out any differences in how risk factors affect women of different races -- not just African-American and white women, but Hispanics, Asians, and other women, too.

"It's important to get minority groups involved in these types of studies so that risk factors for disease can be better understood," McCullough said.

ACS is planning a new, large study in hopes of doing just that, she added. The Cancer Prevention Study 3, which will begin a pilot phase next year, will aim to recruit at least 25% of its participants from racial and ethnic minority populations.

Citation: "Risk Factors for Fatal Breast Cancer in African-American Women and White Women in a Large US Prospective Cohort." Published in the American Journal of Epidemiology (Vol. 162, No. 8:1-9). First author: Marjorie L. McCullough, ScD, American Cancer Society.