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Women who live in cities have denser breasts than those who live in suburban and rural areas, making them more likely to develop breast cancer, according to a study presented Monday at the annual meeting of the Radiological Society of North America.

Breast tissue in women may be fatty or glandular or a mixture of both. Density decreases with age; on average, women lose about one per cent of their breast density each year.

The only way to know what kind of breasts a woman has is to have a mammogram.

Women with more glandular breasts show denser tissue on a mammogram. They are known to have nearly four times the risk of developing breast cancer than women with fatty breasts.

A Canadian study published earlier this year, in the New England Journal of Medicine, found that women with dense breasts are at about five times greater risk of developing breast cancer compared to those with little or no dense tissue.

The study also found that in addition to increasing risk of breast cancer, dense tissue makes tumours more difficult to spot in a mammogram. That thereby increases the risk of cancer development between tests.

For this latest study, Dr. Nicholas M. Perry, director of The London Breast Institute at The Princess Grace Hospital in London, U.K., says his findings suggest the need for urban women to be more vigilant about their breast cancer screening.

"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."

Perry led a team that set out to determine if there was a relationship between breast density and area of residence.

They 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 team found:

• Among rural women, 69 per cent had dense breasts
• Among suburban women, 74 per cent had dense breasts
• Among urban-dwelling women, 78 per cent had dense breasts

The risk of increased density was twice as great in the 45- to 54-year-old group. The differences by area were more pronounced in women under age 50.

Perry says he had noticed for years that women in rural areas had less dense breasts than women in city.

"I work all my time in the city of London and I read mammograms of very dense breasts. But every time I have gone outside to read mammograms elsewhere in more rural areas or parts of Europe that are rural, I am very impressed with the less dense breast tissue that is there," he told CTV.

"I have talked to some of my colleagues who also read a lot of mammograms in different areas and they agree, so I thought it was about time to do the study."

Causes of breast density unclear

It's unclear why women living in urban areas have denser breasts. There may be environmental factors at work. Or it may be because of differences in lifestyle, such as obesity and exercise rates or stress levels. Hormonal birth control may also play a role.

Dr. Roberta Jong, the head of breast imaging at Sunnybrook Health Sciences Centre in Toronto says the urban-rural differences may also be related to childbearing.

"Having children earlier and having more children may be more common for women who live in rural areas than women who live in urban areas, where they may be more career-oriented. So that may be a factor," she speculates.

But Dr. Kefah Mokbel, a consultant breast surgeon in London, who also contributed to the study, says he doesn't think that a woman's reproductive history is relevant. He says he believes air pollution particles called "polycyclic aromatic hydrocarbons" are causing the problem. These particles mimic female sex hormones and can disrupt normal hormonal functions.

"We have significant evidence that traffic emissions can cause estrogen-like activity on the breast, particularly a city like London with a lot of fog all year," he told Â鶹´«Ã½.

"These particles don't rise high among the atmosphere. We inhale these particles and when we inhale they enter the lungs and the bloodstream."

Perry says more research is needed to identify the different causes.

"But we're left with the fact that women in the city are at high risk of breast cancer, they have denser breast tissue on mammograms, and what can we do about it?" he said. He says there are two things that can be done:

• Firstly, make sure women in the city have access to high quality breast screening programs.
• Ensure that digital mammography is available, since it is superior for examining women with dense breasts.

Jong agrees that digital mammography provides doctors with clearer images, but she says some Canadian women may have trouble getting access to the technology.

"Only a small percentage of facilities have digital mammography, so it is better for a woman to get a mammogram than not get a mammogram at all if she does not have access to digital."

Another study presented on Monday at RSNA 2007 looked at the influence of the Western lifestyle on breast composition.

Dr. Miriam Sklair-Levy from Hadassah Medical Center in Jerusalem compared breast densities of Israeli women and Ethiopian women who immigrated to Israel. Her team found that Ethiopian-born women who have immigrated to Israel had significantly lower breast density than did Israeli-born women.

In addition, past Ethiopian immigrants who had begun to adopt a Western lifestyle (decreased number of children, change in diet or increased hormone use) had significantly higher breast density than recent immigrants.

With a report from CTV's Avis Favaro and producer Elizabeth St. Philip