The researchers identifed factors common to high-risk patients. (CBC)

Researchers have found a new way to calculate which women with cancer in one breast are at highest risk of developing a tumour in the other breast.

Women with breast cancer often consider having their second breast removed "not because of medical recommendation, but because they fear having their breast cancer return," said Dr. Kelly Hunt, a surgical oncology professor at the University of Texas M. D. Anderson Cancer Center and lead author on the study.

"Currently it is very difficult to identify which patients are at enough risk to benefit from this aggressive and irreversible procedure. Our goal was to determine what characteristics defined these high-risk patients to better inform future decisions …"

In the March 1 issue of the journal Cancer, Hunt and her colleagues reviewed the cases of 542 women who had a mastectomy at the cancer centre for invasive tumours from January 2000 to April 2007, and chose to get a mastectomy on their second breast as a precaution.

Three factors were more common among women with cancer in the breast that they had removed as a precaution, the researchers found:

• Having more than one tumour in the breast that was first diagnosed.
• Having invasive lobular cancer, which starts in the milk-producing glands called lobules rather than in the milk ducts, in the breast that was initially diagnosed.
• Being at high risk for breast cancer, according to the Gail model.

The Gail model is typically used to calculate invasive breast cancer risk over the next five years in people without breast cancer. The model evaluates factors such as age at first menstrual period, age when first child was born and whether a mother or sister had the disease.

'Rational choice'

The team concluded that contralateral prophylactic mastectomy, or removal of the second, unaffected breast, "may be a rational choice" for certain breast cancer patients who score high on the Gail model or have the other factors identified by the study.

Tests showed that immediately after the contralateral prophylactic mastectomy, about 95 per cent of women had no cancer in that second breast, and 1.5 per cent had an invasive tumour in that breast.

The researchers also looked at another 1,574 women who had a mastectomy to remove a cancerous breast but chose not to have a preventive mastectomy in their second breast. Over the next four years or so, 2.4 per cent of the women developed breast cancer in their remaining breast. It's not clear how many of those cancers were invasive tumours.

According to the researchers, approximately 2.7 per cent of women diagnosed with breast cancer choose to have CPM. Recent statistics have shown that the rate of CPM in women with stage I-III breast cancer increased by 150 per cent from 1998 to 2003 in the United States.

Other options

Breast cancer survivors may choose to remove their second breast to reduce their risk, and address concerns such as symmetry or balance.

"As we begin to clarify the specific risk factors, the number of women undergoing CPM may decrease and those with a low to moderate-risk may be more open to less extreme options for risk reduction, such as hormonal therapy and newer agents for prevention of breast cancer," Hunt said.

In the study, patient race and hormone receptor status were not associated with increased risk.

Breast tumours are tested for estrogen and progesterone receptors that can bind hormones or drugs in the the blood. For example, some tumours are estrogen-receptor positive. If a patient is found to be ER/PR positive (estrogen receptor/progesterone receptor), then anti-hormone drugs are prescribed.

The researchers did not look at whether mutations in the genes called BRCA1 and BRCA2 that raise the risk of breast cancer also increased the risk of having cancer later develop in the second breast.