Removing a non-cancerous breast, called prophylactic mastectomy, in women with a diagnosis of breast cancer in the opposite breast, prevents the second cancer 95% of the time, according to doctors at the Mayo Clinic.

The report, published in a recent issue of the Journal of Clinical Oncology, describes the results after prophylactic mastectomy in 745 women.

Women who have breast cancer have a higher than average risk of developing cancer in the opposite breast. For most women, that risk is less than 1% a year.

But for women with a family history of breast cancer, the risk is much higher. For example, for women with the BRCA gene mutation, the risk can range from 2.5% to 6% a year.

For other women with a family history who lack the mutation, the risk is lower, but not as low as in women without a family history of breast cancer.

In this situation, many women prefer the peace of mind that comes from having the breast surgically removed to the anxiety and risk of a second cancer.

The Mayo doctors reviewed the records of 745 women who had a prophylactic mastectomy of the opposite breast after being diagnosed with breast cancer. Over half the women were 10 years past their prophylactic mastectomy.

They all had a family history of breast cancer. About 40% of the time, the affected relative was a parent, 20% of the time it was a sibling, and the other 40%, it was a more distant relative.

With the knowledge of which relative had breast cancer, the researchers could predict that 169 women would have developed breast cancer in the opposite breast. But only eight women, or 5% actually did have breast cancer in either the chest wall or lymph nodes on the side of the prophylactic mastectomy.

This meant that 95% of the women avoided developing breast cancer. The cancer that was found in the eight women developed in breast tissue left behind after the surgery.

The results were not changed by whether the women were pre- or postmenopausal at the time of their prophylactic mastectomy.

One surprise was that the type of mastectomy did not make a difference. Because it is much more cosmetically appealing, 40% of the women had a subcutaneous mastectomy. This preserves the nipple but also allows some breast tissue to remain.

Yet these women did not have a higher cancer rate than the other 60% who had a total mastectomy, which removes all breast tissue.

The recurrences in women who had a total mastectomy means that even when performing a total mastectomy, a surgeon may not be able to remove every bit of breast tissue.

But the Mayo doctors still prefer total mastectomy. The procedure can remove breast tissue more completely than subcutaneous mastectomy; and, in this modern era, breast and nipple reconstruction techniques have improved enough to produce a good cosmetic result.