A new study gives a possible explanation for why breast cancer is more deadly in black women: they are more likely to have tumors that do not respond to the hormone-based treatments that help many others with the disease.

The study is the largest yet to link a biological factor to the racial disparity, which also has been blamed on black women getting fewer mammograms and less aggressive treatment.

"This puts biology more to the forefront," said Dr. Julie Gralow, a cancer specialist at the University of Washington School of Medicine familiar with the work. "It's not just access to care, access to treatment and other factors that have been implicated in the past."

The study was led by Dr. M. Catherine Lee of the University of Michigan Comprehensive Cancer Center and is to be presented at a conference starting Friday in San Francisco, organized by the American Society of Clinical Oncology and other cancer groups.

Breast cancer is the most common cancer in American women. An estimated 178,480 new cases and 40,460 deaths from it are expected in the United States this year.

Blacks are less likely than whites to develop breast cancer but are more likely to die from it, doctors have long known. Blacks also are diagnosed at younger ages and at later stages of disease.

Researchers for the first time used the National Cancer Data Base, a tumor registry maintained by the American College of Surgeons, to explore these issues, using more than 170,000 cases diagnosed in 1998. Ten percent were in black women.

The study focused on the 95,500 women whose cancers were invasive rather than still confined to a milk duct. About 39 percent of such tumors in black women were estrogen receptor-negative, or ER-negative, compared with 22 percent of those in white women.

Estrogen helps tumors grow. Drugs that block this hormone, like tamoxifen and a newer class of medications called aromatase inhibitors, work against these cancers.

ER-negative tumors are resistant to such therapies and harder to treat. Other tools like chemotherapy, radiation and targeted biological drugs then become more important for such women, and doctors should consider this when they evaluate black women with the disease, Lee said.

In the study, ER-negative tumors were more common in black women at every stage of disease and at all ages.

For example, only 17 percent of early stage tumors in white women were ER-negative, but 31 percent in black women were. Of the most advanced cancers, 31 percent in whites and 46 percent in blacks were ER-negative.

Echoing previous research, the new study found that black women were diagnosed at younger ages — an average of 57 years old versus 62 for white women — and with more advanced disease: only 29 percent had early stage tumors versus 42 percent of white women. They also had larger tumors and more cell traits that are signs of a poor prognosis.

Smaller studies have suggested biological differences between breast cancer in blacks and whites. Earlier this year, the Carolina Breast Cancer Study found that young black women were more likely to have an aggressive form called the basal-like subtype.

Last fall, two studies by researchers from the University of Texas M. D. Anderson Cancer Center found that black women were more likely to have larger, later-stage tumors and lower survival rates than Hispanic and white women given similar treatments.

But these findings do not mean that differences in screening and health care are not contributing to the trend, especially in certain parts of the country, said Dr. Wendy Woodward, a breast cancer specialist at M.D. Anderson.

"You really have to kind of go at the problem from all angles. If you solve the access problem and women come in and you don't have an adequate therapy for them, you haven't taken a step forward," she said.

Dr. Len Lichtenfeld, deputy chief medical officer of the American Cancer Society, agreed. Racial disparity in breast cancer survival did not appear until the mid-1980s, suggesting that much of it is due to lack of screening mammograms and access to care, he said.