Public Release: 

Breast cancer risk overestimated by women, says University of Toronto study

University of Toronto

Many women who undergo prophylactic mastectomy of both breasts have an exaggerated perception of their breast cancer risk before surgery, says a study by University of Toronto researchers.

"We asked the women in our study group what they thought their risk of developing breast cancer was on a scale of zero to 100 per cent," says Professor Kelly Metcalfe of nursing at U of T and lead author of a paper in the Oct. 16 issue of the Journal of the National Cancer Institute. "Looking at all the 75 women in the study, 30 per cent of them estimated their risk to be 100 per cent. We don't even allot that level of risk to women who carry a mutation in one of the two breast cancer susceptibility genes. The highest risk we would give is 80 per cent. So these women's risk estimates were very elevated, particularly for those who did not carry the genetic mutation."

Prophylactic bilateral mastectomy is a preventative option for women who are at high risk of developing breast cancer. From 1991 to 2000, Metcalfe - who did this work as part of her PhD thesis - and Dr. Steven Narod of the Centre for Research in Women's Health at Sunnybrook and Women's College Health Science Centre and the University of Toronto attempted to contact all the women in Ontario who had prophylactic bilateral mastectomies.

The researchers identified about 120 women; 75 agreed to take part in the study. The women provided detailed family histories of the number and type of cancers within their family and the ages of onset. With that information, they determined an objective risk estimate for each woman and the likelihood of developing breast cancer. "We wanted to compare the perceptions of breast cancer risk among women with objective estimates of what their risks actually were," says Metcalfe.

The researchers found that all the women in the study significantly overestimated breast cancer risk except those who carried the mutation of the breast cancer susceptibility genes, BRCA1 and BRCA2.

Prior to surgery, the women estimated their perceived lifetime risk of developing breast cancer as 76 per cent, on average. After surgery, their estimate dropped to 11.4 per cent. Those figures were then contrasted with the objective risk estimates. Of the 14 women who reported that they carried the BRCA1 or BRCA2 gene mutation, the objective risk estimate for developing breast cancer averaged 59 per cent. Of the remaining 61 women (43 of whom had a strong family history of breast cancer and 18 with a limited history), the objective risk was 17 per cent.

"It's concerning that they thought their risk was that high," says Metcalfe. "In fact, we found that the women in the study with the lowest risk actually estimated their risk as the highest, higher even than those women with a BRCA1 or BRCA2 mutation. These women are somehow getting the idea that they're at high risk of developing breast cancer and they're opting for prophylactic bilateral mastectomies when perhaps they shouldn't be. We don't know what is driving these perceptions, whether it's from the media, their families or physicians. But it is troubling."

With the advent of genetic testing in 1996, the researchers noticed differences between the groups of women who had surgery before and after that year. The women continued to overestimate their risk even after genetic counselling but to a lesser degree.

Metcalfe and Narod believe that any woman considering prophylactic mastectomy should undergo formal genetic counselling. "Having a great-aunt who developed breast cancer in her 80s is probably not a high risk," says Metcalfe. "But numerous family members - like your mother, sister, aunt and grandmother - diagnosed with breast cancer at a very young age likely would constitute a significant family history.

"Previous research has shown that women come into genetic clinics thinking they're at really high risk, then go away with a better understanding of what their risk actually is after speaking with trained professionals. Genetic counselling helps women make an informed choice."


This research was supported by the Canadian Breast Cancer Foundation.

Additional Contact:
Kelly Metcalfe
Faculty of Nursing

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