Some of the differences in breast cancer outcomes between white and black women in the U.S. can be attributed to biology, suggests a new study.

Compared to black women, non-Hispanic white women were more likely to be diagnosed when their breast cancer was at an early stage, the research showed. And black women had a higher risk of dying from their breast cancer, compared to white women.

The study team thinks these disparities are due to "a combination of biology and other factors that warrant further investigation to improve survival of all women,” said Dr. Ophira Ginsburg, one of the study’s authors from the Women’s College Hospital in Toronto.

The researchers write in JAMA that women diagnosed with early invasive breast cancer – known as stage I - have generally good outlooks, but there are racial differences in a number of factors that affect how well they do.

For the new study, the researchers used a U.S. database to analyze nearly 450,000 invasive breast cancers diagnosed between 2004 and 2011.

Overall, about 51 percent of white non-Hispanic women in the sample were diagnosed with early-stage cancers, which are thought to have the best prognosis if properly treated. Only about 37 percent of black women were diagnosed with early cancer, making them less likely to have that good prognosis.

The researchers also found that about 6 percent of black women were predicted to die within seven years after their breast cancer diagnosis, compared to about 3 percent of white women.

Black women were also more likely to die from small tumors, compared to white women.

Differences still remained after the researchers excluded women with a more aggressive form of breast cancer known as triple-negative, which is known to be more common among black women.

After accounting for various factors that are known to affect breast cancer outcomes - including income - the differences in outcomes persisted, which suggests to the researchers that other biological reasons - such as the aggressiveness of the tumors, for example - were behind the disparities.

They don’t believe a lack of screening among black women leads to a later diagnosis of cancer, because the relationship is even seen in age groups that don’t normally receive mammograms.

Ginsburg said that other characteristics that could not be completely accounted for within the study may influence the results, such as access to care.

“We do know that breast cancers in African American women do tend to be more aggressive than breast cancer in white women,” said Dr. Nancy Keating, a cancer researcher at Brigham and Women’s Hospital in Boston who was not involved with the new study. “This - as well as other studies - supports this.”

However, it’s still a question of how much of the differences between diagnosis and care can be attributed to biology, said Keating, who is also a professor of healthcare policy at Harvard Medical School.

Regardless of where the blame for more aggressive cancers lies, Dr. Funmi Olopade told Reuters Health that it’s time women with breast cancer be treated as individuals.

“I think we’re in an era of precision medicine now,” said Olopade, who co-authored an editorial accompanying the new study. “What’s really been rewarding about the advances in breast cancer research is that we’re beginning to have the granular data to treat each patient as a unique individual and to look at their tumors as a unique tumor that we need to develop a strategy to treat.”

Historically the U.S. focused on getting everyone screened,” said Olopade, a breast cancer expert at the University of Chicago.

“The one size doesn’t fit all,” she said. “The conversation we should be having is ‘we have breast cancer, but what type do I have?’”