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(USATODAY.com) - The two-decade-long divide in breast cancer survival rates among black and white women has not budged, and the difference is driven more by how sick women are when they come in for a diagnosis rather than the differences in how they're treated, a new study says.

Despite declines in breast cancer deaths over the past 20 years and strides in diagnosis and treatment, black women are 40% more likely to die of breast cancer than white women, according to the Centers for Disease Control and Prevention.

For years, researchers have scratched heads over why the racial disparity exists, suggesting differences in screening, existing health problems, socioeconomic status and treatment.

But the low survival rate can't be explained solely by the types and frequency of treatment, says a new study, published Tuesday in theJournal of the American Medical Association,

The study suggests that characteristics at diagnosis explain the differences in survival, not differences in the type, duration and frequency of treatment. Black patients had poorer health at diagnosis, including more advanced disease, worse biological features of the disease, and larger tumor size.

"Can you blame the oncologists? Probably not. But what we are saying is that in order to improve diagnoses, we have to turn to primary care," says Jeffrey Silber, study author and director of the Center for Outcomes Research at the Children's Hospital of Philadelphia. "This is an issue of prevention."

Silber is also a professor of pediatrics, anesthesiology, critical care and health care management at the University of Pennsylvania.

Breast cancer is the second-leading cause of cancer deaths among women (after lung cancer), killing about one in 36 women in the USA annually. Death rates have been declining since the 1980s as a result of earlier detection through screening, increased awareness and improved treatment, but they have leveled off in recent years.

In the study, researchers compared five-year survival rates of more than 7,000 black women over 65 with three sets of white women who were matched on factors such as year of diagnosis, state of their health upon diagnosis (including any existing health problems) and the state of their tumors. They were also compared by treatment such as surgery, radiation therapy and chemotherapy. The women were all insured by Medicare to eliminate any effect from socioeconomic background.

Overall five-year survival rates were 55.9% for black women and 68.8% for whites, a 12.9 percentage point difference. But when the researchers also accounted for how sick the patients were when first diagnosed, the black-white difference in survival dropped to 4.4%,meaning two-thirds of the difference was because of how sick patients were at diagnosis. Looking further at treatment, as well as their condition at time of the diagnosis, dropped the difference to 3.6%.

"Therefore, the overall survival disparity is greatly influenced by how sick patients are at diagnosis, but only trivially influenced by differences in treatment," Silber says.

This comprehensive matching system allowed researchers to eliminate major biases that affect analysis, wrote Jeanne Mandelblatt and her colleagues in an editorial accompanying the story. Mandelblatt is the associate director of population sciences at Georgetown University.

Though the study tracked patients diagnosed between 1991 and 2005 and through 2009, the results likely hold true today, Silber says, adding that advances in treatment - however startling - can't offset the disparities in diagnosis.

Still, 12.6% of black patients did not receive treatment, compared with 5.9% of whites. Black patients also had longer average times between diagnosis and treatment. Prior to diagnosis, many black patients also received less adequate primary care than white patients in the same demographic, the study says.

It's possible that the role of treatment could be understated, especially since the database that collects records of all Medicare users diagnosed with cancer through 2009 - SEER-Medicare (Surveillance, Epidemiology, and End Results) - doesn't include details on the doses, intensity and completion of dose cycles.

Reducing the disparity in survival rates means reducing the differences in how sick patients are when first diagnosed, Silber says. "Reducing treatment disparities, while important, will not solve the survival disparity problem. We must find ways to have black patients present at diagnosis ... with less advanced disease, smaller tumors and with less chronic conditions like diabetes and heart failure, all of which lead to worse survival."

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