The proportion of women having both breasts removed when breast cancer appears in one has increased more than ten-fold over a 10-year period, despite a limited amount of evidence showing a survival benefit for the procedure, researchers reported Wednesday.

Nearly one in every 20 women now has the second breast removed in an effort to forestall the development of a tumor in it, Dr. Katherine Yao of the NorthShore University HealthSystem in Evanston, Ill. and her colleagues reported in the October issue of the Annals of Surgical Oncology.

Removal of both breasts can reduce the risk of breast cancer by as much as 85% for women who have the BRCA1 or BRCA2 genes, which confer a high risk of contracting cancer, but less than 1% of the general population carries either of these genes.

Yao and her colleagues studied nearly 1.2 million breast cancer cases recorded in the National Cancer Data Base of the American College of Surgeons. The women had cancer diagnosed in one breast between 1998 and 2007. The researchers found that the rate of prophylactic mastectomies in the nonaffected breast increased from 0.4% in 1998 to 4.7% in 2007. The greatest increase was among white women under the age of 40, more than 10.5% of whom had the procedure. Women with private insurance were more than twice as likely to undergo prophylactic mastectomy as those who were uninsured or who had Medicare. Similarly, women in the highest-income ZIP codes were twice as likely as those in the lowest income areas. Women in the Midwest had the highest rates, 6.4% in 2006-07, followed by the South with 5.6%. The Northeast was lowest at 3.3%

The records did not indicate whether or not a woman carried a gene that predisposed her to breast cancer. However, noted senior author Dr. David P. Winchester of the NorthShore HealthSystem and medical director of cancer programs at the American College of Surgeons, a woman’s own cancer history is her No. 1 risk factor. For a woman with early-stage breast cancer and a family history, he said, the risk of another tumor grows by 2% per year. In a 40-year-old woman with a 40-year life expectancy, “you’re looking at a very, very high risk,” he said, so it is not surprising that many women are choosing a proactive stance.