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Some women with suspected cancer may have had their breasts removed unnecessarily, a study suggests.

Researchers in Canada have found that women significantly overestimate their risks of developing the disease.

They believe some women may agree too easily to surgery to remove their breasts in an effort to reduce their chances of developing cancer.

The researchers described the findings as "troubling" and urged hospitals to offer counselling to patients to ensure they made the correct decision.

Professor Kelly Metcalfe and colleagues at the University of Toronto examined the cases of 75 women who had both breasts removed between 1991 and 2000.

Each woman provided detailed histories of the number and type of cancer within their families and estimated their chances of developing breast cancer.

High risk

Almost all of the women significantly overestimated their risk.

One in three suggested they were 100% likely to develop the disease. Overall the average risk was estimated at 76%.

After surgery, the women said their chances of developing breast cancer had dropped to 11%.

However, nobody is 100% certain to develop breast cancer - not even those carrying the genes that have been linked to the disease. The highest risk is usually in the region of 80%.

Calculations by the researchers suggested that the vast majority of women in the study only had a 17% chance of developing breast cancer, based on family medical history.

Even those who said they carried the BRCA1 or BRCA2 genes had a 59% risk.

"It's concerning that they thought their risk was that high," said Professor Metcalfe.

"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.

"I don't think you can compare what happened then to what happens today "

Dr Michelle Barclay, Breakthrough Breastcancer

"We don't know what is driving these perceptions, whether it's from the media, their families or physicians. But it is troubling."

Professor Metcalfe suggested that women considering surgery should undergo formal genetic counselling, which examines the likelihood of developing disease based on family-medical 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.