WASHINGTON (Reuters) - It may be possible to predict which breast cancer patients will be helped by tamoxifen based on changes in so-called breast density, researchers reported on Saturday.

Women with dense breasts -- a term meaning they have more non-fatty tissue -- are known to have a higher risk of breast cancer and the study suggests that lowering density using tamoxifen also lowers the chances tumors will come back.

Women whose breasts became noticeably less dense after a year or so of taking tamoxifen had a 63 percent reduced risk of breast cancer, the team of British researchers told a breast cancer meeting in San Antonio, Texas.

“It is important to find a way to predict who will respond to tamoxifen, and changes in breast density may constitute an early indicator of benefit,” said Jack Cuzick of the Wolfson Institute of Preventive Medicine in London.

He said about 10 percent of women have dense breasts. Such breasts are harder to read on mammograms but there is evidence the tissue in their breasts may be more cancer-prone.

“Women with dense breasts are typically at four to five times the risk of developing breast cancer than women without dense breasts,” Cuzick told reporters in a telephone briefing.

Cuzick’s team had conducted one of many studies that showed high-risk women who took tamoxifen were at least 40 percent less likely to either develop breast cancer, or to have it come back.

They went back and looked at all the mammograms of the more than 1,000 women who took part in the study.

If a woman’s breast density did not change during the treatment, she was much more likely to develop cancer despite taking tamoxifen, Cuzick told the meeting.

TWO-THIRDS REDUCTION

“Women who lost 10 percent or more in breast density -- 40 percent of the women getting tamoxifen -- had a 63 percent, almost a two-thirds, reduction in all breast cancer,” he told the briefing.

Cuzick stressed the study only looked at tamoxifen and not at other treatments, such as the newer drugs known as aromatase inhibitors. But he said it validated the idea of using breast density as a way of telling whether a treatment is working, whether tamoxifen or something else.

“If a woman doesn’t show breast density dropping within a year or so, you might want to consider other therapy,” Cuzick said.

In another report, a team at the Mayo Clinic in Rochester, Minnesota said they found some clues as to why dense breast tissue is more likely to develop tumors.

Dense breast tissue contains more cells believed to give rise to breast cancer, Dr. Karthik Ghosh told the meeting.

Dr. Celine Vachon and colleagues found that dense breast tissue has more aromatase enzyme than non-dense tissue. This is significant because aromatase helps convert hormones into estrogen, which can fuel breast cancer development.

“If aromatase is differentially expressed in dense and non-dense breast tissue, this could provide one mechanism by which density may increase breast cancer risk,” Vachon said in a statement.