The 37,305 women 65 and older examined in the study had undergone a procedure called a lumpectomy in which doctors removed just the tumor and spared the breast, a procedure less radical than a mastectomy, which removes the entire breast.

Standard care for a woman after a lumpectomy is a series of radiation treatments to destroy any remaining cancer cells. While 74 per cent of white women in the study were given radiation therapy, only 65 per cent of black women got it.

The study was one in a long line showing US blacks get inferior care for cancer and a variety of other ailments than whites. Experts concerned about the disparities have struggled to understand why they persist.

Dr. Grace Li Smith of the University of Texas M.D. Anderson Cancer Center, who led the study released ahead of a breast cancer meeting sponsored by the American Society of Clinical Oncology and other groups, listed some possible explanations.

It is possible doctors offer fewer black women radiation therapy after lumpectomy, but also possible they decline such treatment or are deemed unable to complete it due to other health problems, Smith said.

"This is a very important area of follow-up research," Smith told reporters in a conference call.

Dr. Eric Winer of Harvard Medical School and Dana-Farber Cancer Institute in Boston said there is no reason why black women should be less likely to get radiation therapy.

"Lumpectomy, or conservative surgery, plus radiation has been shown to be equivalent to mastectomy for the vast majority of women with early-stage breast cancer. But the key is that radiation is a treatment that goes after lumpectomy. And for all but the oldest and sickest women, radiation should be considered a standard after lumpectomy," Winer said.

While other research has looked at racial differences in breast cancer treatment, Smith called this the first study of disparities nationwide in radiation after lumpectomy. Of the women in the study, 34,024 were white and 2,305 were black.