Insufficient and delayed breast cancer screening may be the primary reason black women tend to be diagnosed at significantly more advanced stages of the disease, according to a study released Tuesday.

In the study of more than one million women, black women were found to be significantly less likely than white women to have ever had a mammogram before a positive diagnosis, or to have had one before age 55. While the breast cancer mortality rate in the United States has decreased significantly over the past 15 years, the rate for black women has changed little.

“Physicians need to know that African American patients are not getting mammograms,” the study’s authors wrote.

The study, published in the Annals of Internal Medicine, found black women were more likely than any of their female counterparts to have large and advanced-stage tumors upon diagnosis.

Furthermore, Sarah Gehlert, director of the Center for Interdisciplinary Health Disparities Research at the University of Chicago, said the problem is compounded because women of African descent may actually need screening earlier than other ethnic groups. She said this is because of genetic differences that play a role in the age of onset and progression of breast cancer, an assertion also maintained in the study.

Gehlert said there is not substantial data for black women, but research funded by the National Institutes of Health has found that among West African women, 74 percent of breast cancer cases occur before menopause; 12 percent before the age of 30.

By contrast, only 1 percent of white women have breast cancer before age 30, and incidence of the disease among white women increases each year after menopause, Gehlert said.

“If we set policy and procedure to one group it’s a disservice to other groups,” Gehlert said. ” We’re seeing too many African American women die from this. There has been a failure to appreciate the possibility that there might be a different, more lethal form of breast cancer among women of African ancestry.”

For women who were equally screened, black women were no more likely to be in advanced-stages or have lymph node involvement than white women with the same screening history. However, black women had higher rates of high-grade tumors regardless of screening history, supporting the likelihood that biological differences are a factor, researchers said.

In the past, reasons for differences in breast cancer mortality rates among various racial and ethnic groups have been difficult to determine. Despite a lower incidence of breast cancer among black women, the mortality rate is significantly higher.

Disparities in access to health care have been shown to be a significant challenge to health outcomes for minorities in the United States. Studies show that black populations bear a disproportionate burden of deaths in cases of breast, colorectal, and cervical cancer.

“Because African American women are over-represented in poverty and have lower rates of insurance, it’s much more difficult for them to obtain screening,” Gehlert said.

Gehlert and colleagues at the University of Chicago are conducting research to help understand why the gap between black and white women is growing, and to examine the interplay between genes and social environment with regard to occurrences of breast cancer. They are studying women in 15 neighborhoods on the South Side through research currently funded until 2008 by the National Institutes of Health.

University of Chicago researchers said they have found that women, in particular those with a family history of breast cancer, had a hard time obtaining screenings because physicians would tell them to wait to have a mammogram until after they reached age 40.

Gehlert said most if not all health insurance companies base their coverage on breast cancer statistics of white women. They do not cover mammograms until a woman is age 40 or older, which for many black women may be too late.

Although the rate of incidence for white women is higher than for black women, 141.1 compared to 119.4 per 100,000, the mortality rate for black women is higher, 34.7 compared to 25.9, according the American Cancer Society.

Martha Haley, 46, who is black and was first diagnosed with breast cancer in 1996, said she is not surprised by the study findings.

“This is something that I’ve known for some time,” she said. “A lot of us don’t have access to health care to allow us to go in and get mammograms.”