SAN DIEGO (Reuters) - New research shows a sharp drop in U.S. breast cancer cases in recent years was limited to white women, possibly because they abandoned hormone replacement therapy in greater numbers than minority groups.

A radiologist examines breast X-rays after a cancer prevention medical check-up at the Ambroise Pare hospital in Marseille, southern France, on April 3, 2008. REUTERS/Jean-Paul Pelissier

Many women stopped using hormone replacement therapy after a large study suggested in 2002 that the combination of estrogen and progestin used to treat menopause symptoms raised the risk of breast cancer and heart disease.

White women had been more likely to use hormone therapy, and were also the most likely to abandon the drugs after U.S. regulators warned about the cancer link in 2003, according to Dr. Dezheng Huo of the University of Chicago and the study’s lead investigator.

“The sharp reductions seen in Caucasians aged 50 to 69 years were not seen among other ethnic groups,” Hou told the American Association for Cancer Research.

According to the American Cancer Society, the overall incidence of female breast cancer fell 3.9 percent a year from 2001 through 2004.

The researchers said the decline has been mainly among women older than 50 with estrogen-receptor positive cancer.

Using data from the National Cancer Institute’s database, they calculated breast cancer rates between 2000 and 2004 to determine whether or not the trends were similar across racial and ethnic groups.

The researchers found that in the first two years, the rate of invasive breast cancer among whites was stable.

However, toward the end of 2003, it started falling by as much as 2.4 percent per quarter while continuing to grow by 0.7 percent per quarter among black women.

Rates for American Indians and Alaskan Natives fell 0.14 percent and among Asian Americans by 0.46 percent.

“The finding ... suggests that exogenous estrogen serves as a promoter rather than an initiator of breast cancer,” the researchers said.