CHICAGO—Women who live in urban areas have increased breast density compared with their suburban and rural counterparts, a finding that may help to explain their increased risk for breast cancer, researchers reported in a study here at the Radiological Society of North America Annual Meeting.
Women age 45 to 54 who lived in central London were more than twice as likely to have dense breasts with more than 25% fibrotic tissue as women who lived outside the city.
“We know that women in cities have an elevated risk of breast cancer. And many studies have shown that women with denser breasts have higher rates of breast cancer,” said Nicholas M. Perry, MBBS, Director of the London Breast Institute at Princess Grace Hospital. “So these findings fit in nicely, offering one reason women in cities have higher rates of breast cancer.”
Dr. Perry said his observations, both as a practicing urban physician and as advisor to the European Commission's Europe against Cancer program, led him to undertake the study. Launched in 1985, the program aims to reduce deaths due to cancer throughout Europe.
“I read thousands of mammograms in the mid-1990s and early 2000s and it struck me that in the screening programs in cities, women had denser breasts. Colleagues in similar positions said they were observing that as well,” he said.
Breast density on mammography depends on the relative proportion of dark fatty tissue to white fibroglandular tissue, he said.
Studies of breast density are important, due to its clear association with an increased risk of breast cancer. In one recent meta-analysis, researchers looked at percentage density measured using prediagnostic mammograms and found that women with a breast density of 75% or more were 4.64 times more likely to have breast cancer than those with a breast density of less than 5%, he said.
Compared with women with a density of less than 5%, those with breast densities of 5% to 24%, 25% to 49%, and 50% to 74% were 1.79, 2.11, and 2.92 times more likely to have breast cancer, the study showed (McCormack et al: Cancer Epidemiol Biomarkers Prev 2006;15:1159–1169).
Additionally, dense breast tissue has a masking effect, reducing the sensitivity of mammograms. As a result, radiologists may miss early lesions in women with dense breasts, which lessens their chance of cure, he said.
The study included 972 women aged 27 to 87 who underwent digital mammograms; 318 of them had a city of London zip code.
Breast density was graded into four categories: Less than 25%, 25% to 49%, 50% to 75%, and more than 75% fibrotic tissue.
Results showed that 24% of women whose zip codes correlated with addresses in central London had extremely dense breasts with more than 75% fibrotic tissue. This compared with 19% of women living outside the city, Dr. Perry said.
The city dwellers were also less likely to have low breast density. A total of 26% of non-Londonites had a breast density of less than 25%, compared with 21% of women who lived downtown.
“The difference between 21% and 26% may not seem like much, but it is more than a 25% increase based on the denominator figure [of 318 for the city dwellers and 654 for the non-city dwellers],” he said.
The researchers then did an analysis by age group and found no significant association between living in the city and having a breast density of over 25% fibrotic tissue for women who were under 45 years or over 54 years.
But women ages 45 to 54 who lived in central London were 2.2 times more likely to have that higher density than women who lived outside the city were.
The researchers then used zip codes to identify 128 women, ages 40 to 60, who worked within the square mile of the city of London. “Even though about half of them lived outside central London, these women had the densest breasts of all,” Dr. Perry said.
Specifically, 94% of women who worked in the city of London had dense breasts with more than 25% fibrotic tissue. This compared with 33% to 79% of those who worked outside the city of London, with the figure increasing as zip codes corresponded to areas further from the inner city.
Also Apply to Other Places
While only London and its surrounding areas were studied, Dr. Perry said that he believes the findings also apply to women living in other cities, including those in North America.
“There's clearly something about exposure to the urban environment. I am not persuaded that London is any different from any other urban population—urban is urban,” he said.
While the study was not designed to investigate why women living or working in urban areas have denser breasts, “it must be essentially lifestyle.” He said he suspects that women who live in cities are more likely to be on hormone-replacement therapy (HRT), to not have children or to delay childbirth, and to be younger and slimmer than non-urban women.
Lower breast density is associated with increasing age, postmenopausal status, and hormone suppression, he said. And increased breast density is associated with low body mass index, current use of HRT, and multiparty, Dr. Perry said.
Implications for Practice
So how does that explain why women who work in the city have the densest breasts of all? There have been some studies that suggest that stress is associated with increased breast density, he said, noting one study in particular: Kruk J, Aboul-Enein HY: Cancer Detect Prev 2004;28:399–408).
Also, research suggests that fossil-fuel breakdown products have an estrogenic effect (Wang J et al: Atmos Environ 2003; 37:3225–3235) and that high exposure to traffic emissions at the time of first birth can more than double the risk of postmenopausal breast cancer (Nie J et al, Cancer Causes Control 2007;18:947–955).
The findings have several implications for clinical practice, he said. “First, women in urban areas show lower response to population screening programs, but these data suggest they have increased risk because of breast density. So we need to promote increased participation of urban women in breast screening programs.”
His feeling, he said, was that digital mammography is known to be more effective for women than traditional film mammography is. “I think all women living or working in urban areas should get digital mammograms.”
Still, about that issue, Philip O. Alderson, MD, Chairman of the Department of Radiology at Columbia University and Director of the Radiology Service at Columbia-Presbyterian Medical Center and the James Picker Professor of Radiology at the College of Physicians and Surgeons of Columbia University, pointed out that the Digital Mammographic Imaging Screening Trial (DMIST), a study of nearly 50,000 women who underwent both digital and film mammography, found no difference in their overall diagnostic accuracy as a means of screening for breast cancer (Pisano E et al, New Engl J Med 2005;353:1773–1783).
“But Dr. Perry is correct that among the subset of women with dense breasts, digital mammography appeared to be superior,” Dr. Alderson said.
Both Dr. Alderson and Joseph H. Tashjian, MD, President of St. Paul (MN) Radiology, said they did not know whether the findings are applicable to cities outside London since neither of them know of any other study linking urban life to dense breasts.
Dr. Tashjian said he would like to see the findings corroborated. “It's well known that women with dense breasts are at higher risk of breast cancer. So this may be a very good explanation for why younger women in urban areas are at higher risk. The reasoning fits,” he said.
He added that the fact that women who worked in the city also had denser breasts makes him think that environmental factors are at play: “When I visit London, I notice there is a lot more particulate matter in the air than where I live in Minneapolis-St. Paul,” Dr. Tashjian said. “But at this point, all the theories Dr. Perry mentioned—HRT use, hormones, etc.—are reasonable, but still speculative.
“If we can find out why women in urban areas have denser breasts, it could be a new way of preventing breast cancer,” he said.