TUESDAY, July 7 (HealthDay News) -- The racial gap in breast cancer outcomes, with black women more likely to die from the disease, can't be explained completely by any one factor, new research shows.

For a quarter of a century, researchers have been aware of the so-called race gap in certain cancers. The racial disparity "first emerged about 25 years ago," said Idan Menashe, a postdoctoral fellow at the U.S. National Cancer Institute, who led one of two studies on the topic, both published in the July 15 issue of the Journal of the National Cancer Institute.

Since the mid-1980s, he said, the gap has been widening. "Today is the largest gap we have experienced," he noted.

The two new studies focus on why that may be so, and come to different conclusions. In the first, Menashe and his colleagues compared breast cancer rates for more than 244,000 black and white women, using data from the National Cancer Institute's Surveillance, Epidemiology and End Result (SEER) program.

They looked at the ratios between black and white women when it came to the incidence, mortality and hazard rate -- roughly defined as the probability of dying -- and looked at the tumor's estrogen-receptor status.

Estrogen receptor-negative tumors, or ER-negative, are known to be more aggressive than estrogen receptor-positive tumors, with some experts believing this explained the gap. "What we have shown is, this is not the case," Menashe said.

"We asked what would have happened if black women had the same ER-negative proportion as white women, and we found the mortality disparity would be reduced by only 10 to 20 percent if the proportion of ER-negative tumors were the same in each group," he said.

"We asked what would happen if we equalized the likelihood of dying from the disease. We ruled out that the proportion of ER-negative tumors explains the gap," he said.

They did find that early deaths are driving the disparity between outcomes for white and black women. And, they found that the deaths tend to occur soon after the diagnosis. "Most deaths occur in the two or three years after the diagnosis," he said.

So what drives the disparity?

"We think it's mostly access to care [with black women having less access], but we cannot rule out that the biological differences also contribute to the disparity," Menashe said.

In the second study, Dr. Kathy Albain of Loyola University's Cardinal Bernardin Cancer Center in Maywood, Ill., and her colleagues evaluated nearly 20,000 adult cancer patients with a variety of cancers who were in clinical trials from late 1974 through late 2001, all receiving identical treatments and access to care.

Black patients with breast cancer and other gender-specific cancers had worse survival than white patients, despite identical treatment and follow-up, they noted.

The patients were followed for at least 10 years after treatment. During that time, blacks were 21 percent to 61 percent more likely to die from gender-specific cancers than white patients.

Those findings, the researchers say, cast doubt on the theory that the lower survival rates for certain cancers are due solely to factors such as poverty and poor access to quality health care.

Dr. Mitchell Wong, an associate professor of medicine at the University of California Los Angeles David Geffen School of Medicine, published a study earlier this year on racial differences in cancer death rates.

"These two articles both suggest that something else is going on [in addition to cancer stage, tumor characteristics or treatment] that leads black patients to have a worse prognosis. We can only guess, but it may be due to differences in tumor biology that science does not yet understand. Then the question is: why do blacks have breast cancer with worse prognosis? It's still speculation, but the second article suggests it may be environmental effects. We cannot rule out genetics either."

In an editorial accompanying the studies, Dr. Otis W. Brawley, of the American Cancer Society, took this view: "Taken together, the two studies and others do not suggest that blacks have a different kind of breast cancer, but rather that there are multiple kinds of breast cancer and a higher proportion of black patients with breast cancer have the worst kind."

Both biological differences and access to care could account for the gap, in Menashe's view. The take-home point for breast cancer patients, he said, given the finding that deaths occur soon after diagnosis, is: "Once diagnosed, they should get treatment ASAP.''