By Will Dunham

WASHINGTON (Reuters) - A small but growing number of women with cancer in one breast are deciding to have the other one surgically removed to avoid the possibility that a tumor develops there in the future.

Women diagnosed with cancer in one breast are at heightened risk for a tumor in the other breast as well, but doctors have struggled to determine which women are at highest risk of such an outcome.

Based on a study involving 542 women with breast cancer, U.S. researchers on Monday identified three factors that make cancer in the other breast much more likely:

-- having more than one tumor in the same breast when the initial cancer is diagnosed;

-- having invasive lobular breast cancer, which begins in the milk-producing glands called lobules rather than in the milk ducts, and then invades surrounding tissues;

-- and having a high score in the so-called Gail model that calculates breast cancer risk and considers things such as age at first menstrual period, age when first child was born and whether close relatives like mother or sister had the disease.

"Not all patients are equal in that risk," Dr. Kelly Hunt of the University of Texas M.D. Anderson Cancer Center in Houston, who led the study published in the journal Cancer, said of the risk for cancer in the other breast.

"Perhaps we can help patients to really more carefully understand their risk and make decisions in a more informed way," Hunt said in a telephone interview.

Breast cancer is the leading cause of cancer death among women worldwide, with an estimated 465,000 dying annually, according to the American Cancer Society. About 1.3 million women are diagnosed with breast cancer per year.

'RATIONAL CHOICE'

Women may have the other breast removed before any sign of cancer appears. Called a contralateral prophylactic mastectomy, these women view it as a potentially life-saving move.

The researchers wrote that for women who had the three risk factors identified in the study, such surgery "may be a rational choice." The women in the study had both their breasts removed between 2000 and 2007.

Hunt said doctors typically are cautious about recommending such a step. "It's an irreversible thing, obviously. It's a pretty drastic procedure," Hunt added.

The researchers found that a woman's race or the tumor's hormone receptor status did not help determine risk of developing cancer in the future in her other breast.

Breast tumors are tested for estrogen and progesterone receptors -- proteins on the outside of a cell that can bind to chemicals, hormones or drugs traveling through the blood. For example, some tumors are estrogen-receptor positive.

The study did not focus on whether mutations in the genes called BRCA1 and BRCA2 that raise the risk of breast cancer also raised the risk of having cancer later develop in the initially unaffected breast.

Some women with these mutations or a strong family history of breast cancer get preventive mastectomies even before any tumor has developed in either breast.

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