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There are many factors that influence a woman's risk of developing breast cancer, including genetics, age, diet, exercise, income and a number of reproductive and hormonal milestones such as age at first menstruation, age when she gives birth, late menopause and using birth control pills or hormone replacement therapy.

A new study suggests that where you live matters too - specifically that women living in big cities are markedly more likely to develop breast cancer than their suburban and rural counterparts.

The research, presented yesterday at the annual meeting of the Radiological Society of North America, actually shows that women living in London have significantly denser breasts than those living outside the city or in the countryside.

"Women in the city have denser breasts than women outside the city, and there is a gradient in that the further you go outside in areas that are less dense in population, breast density diminishes further," said Nicholas Perry, director of the London Breast Institute at the Princess Grace Hospital.

He said the precise reason for this phenomenon is unknown, but speculated that stress, environmental factors such as pollution and lifestyle may contribute.

Breast density is one of the most significant risk factors for breast cancer.

A landmark Canadian study showed that women with dense breasts were five times more likely to develop breast cancer than those whose breasts had a lot of fatty tissue.

Norman Boyd of the Campbell Family Institute for Breast Cancer Research at Princess Margaret Hospital in Toronto, and principal investigator on that study, expressed skepticism about the new findings.

One of the things that influence breast density the most is body weight, he said. Dr. Boyd said obesity is far more prevalent in rural areas than in urban ones.

Regardless of why big-city women have dense breasts, Dr. Perry said, it is important for them to realize they are at higher risk and to be more vigilant.

"Women living in cities need to pay more attention to having regular breast screening," he said.

"Currently, women who live in urban areas are known to have lower attendance for breast-screening programs than women in outlying areas."

Cancerous tumours are more difficult to detect in women with dense breasts.

Women with dense breasts were also 18 times more likely to have a cancerous tumour detected within 12 months of a negative mammogram, according to the Canadian research.

For this reason, Dr. Perry recommends that women with dense breasts undergo digital mammography rather than traditional X-ray mammography.

City dwellers should insist on this more sophisticated technology, he said.

Some screening programs, such as the one administered by Cancer Care Ontario, now recommend that women with dense breasts undergo X-ray mammography every year after the age of 50. (The general recommendation is that women be tested every two years.) They are also considering moving to digital mammography for high-risk women.

The size of a woman's breasts has no bearing on density, and women cannot determine this measure on their own.

Density is defined as the percentage of breast tissue that is not clearly visible on X-ray mammography. Breast tissue is made up of fat, supporting tissue (stroma) and the epithelium that forms ducts and lobules. X-rays pass through these tissues differently. As a result, fat appears dark on a mammogram while denser tissues appear light - making it difficult to spot tumours.

Density decreases with age and, on average, women lose about 1 per cent of their breast density each year. Density is also affected by hormones and hormone replacement therapy in particular, which probably explains why taking HRT increases the risk of cancer.

The researchers analyzed digital mammograms of 972 women from urban, suburban and rural areas. They discovered that women who lived in London had significantly denser breasts than those living outside the city. The risk of increased density was twice as great in the 45- to 54-year-old group. Age-specific analyses suggested that overall differences by area were more pronounced in women younger than 50.

A second, unrelated study also presented yesterday at the radiology conference suggests that a Western lifestyle has a big influence on breast density.

Miriam Sklair-Levy from Hadassah Medical Center in Jerusalem compared breast densities of Israeli women and Ethiopian women who immigrated to Israel.

The study found the Ethiopian-born women had significantly lower breast density than the Israeli-born women. In addition, Dr. Sklair-Levy found that past Ethiopian immigrants who had begun to adopt a Western lifestyle (decreased number of children, change in diet and increased hormone use) had significantly higher breast density than recent immigrants.

An estimated 22,300 women will be diagnosed with breast cancer in Canada in 2007, and about 5,300 will die of the disease, according to the Canadian Cancer Society.