Study Finds Dense Breast Tissue Increases Breast Cancer Risk
January 31, 2007 Ever since mammography began to be widely used in the 1970s there has been a suspicion that women with breasts that appear dense on a mammogram are at higher risk of developing breast cancer.
Many people attributed this problem to the fact that dense breasts can more easily hide a cancer. That's because dense breast tissue appears white on a mammogram. And that makes it hard to spot a cancer, which also appears white. In contrast, fatty tissue is dark on a mammogram, which makes it easier to spot a white cancer.
Others, however, felt that there might be additional explanations for why women with dense breasts had a higher breast cancer risk. Norman Boyd, MD, a senior scientist at Toronto's Princess Margaret Hospital, in Canada, is one of the researchers who persisted in pursuing the hypothesis that dense breast tissue might do more than just hide a cancer. His most recent research, published in the January 18, 2007, issue of the New England Journal of Medicine, suggests that he and others who thought we needed to pay more attention to dense breast tissue were right.
Boyd and his team recruited women from three previous mammography screening trials for their study. First they identified 1112 women (the cases) who were found to have breast cancer during the screening trial they had been enrolled in. Each of the cases was matched with a woman of a similar age who had not been found to have breast cancer during the screening trial (the controls). The researchers then used special computer tools to read the mammograms of all 2224 women and to assess their mammographic density.
The study showed that, as a group, the women who had developed breast cancer had more of the known risk factor for breast cancer than women in the control group did. This meant that they were more likely to have their period start at an earlier age, to have a first birth at a later age, to not have given birth, to have had a smaller number of children, to have a later age at menopause, to have a family history of breast cancer, and to have used hormone therapy.
The study also found that the women who had developed breast cancer had, on average, a baseline mammographic density that was 5.8 percentage points greater than the women who had not developed breast cancer.
The researchers then compared women who had extensive breast density (defined as density in more than 75 percent of their breast) with women who had only a small amount of breast density (defined as density in less than ten percent of their breast). They found that the women with the most dense breasts (75% or more) had 4.7 times the odds of being diagnosed with breast cancer less than 12 months after a negative screening test then did the women with the least dense breast tissue. This could be because the mammogram missed the cancer because it was hidden by the dense breast tissue. It is also possible that cancer grows faster in dense breast tissue, which would mean that the problem wasn't necessarily that the mammogram missed the cancer, but rather that the cancer wasn't visible when the mammogram was done.
Based on their findings the researchers concluded "attention should be directed to the development and evaluation of alternative imaging techniques" for women who have very dense breast tissue and who have a greater risk of having a breast cancer detected at screening or between screening.
Susan says: A number of previous studies have found that women with dense breasts have a higher breast cancer risk. This study provides important new information about breast density and breast cancer risk and provides evidence in support of the use of breast density as an independent risk factor for breast cancer in women who are close to 50 or who are postmenopausal.
Why only these women? It is normal for young, premenopausal women to have dense breast tissue, and this is one reason why mammography is not used as a screening tool in women under 40. (It's also not recommended because of the low rate of breast cancer in this age group).
Why would breast density increase breast cancer risk? Recent studies indicate that there are two aspects to developing breast cancer. First, you need the abnormal cell (seed). Second, the seed has to be in an environment (soil) that supports its growth and development. Many people now believe that dense breast tissue represents a "soil" or environment that is more supportive of breast cancer cells than less dense breast tissue. Why isn't yet known, but research that is now underway may soon provide us with some answers.
[It is interesting to note that some women who start on hormone therapy to treat menopausal symptoms find that their mammograms become denser. We also know that women who start taking tamoxifen (an estrogen blocker) to prevent or treat breast cancer will show a decrease in density. Furthermore, these changes in density have been found to happen within weeks. Combining these observations with the findings of this current study on breast density might suggest that women whose mammograms become denser because of hormone therapy should consider stopping their use of this menopausal treatment. At the very least, it is worth studying.]
How do you know if your breasts are dense? Ask your doctor. The mammography reports doctors receive include a measure of breast density. The technique used is not as accurate as newer techniques now being developed or that which was used in the NEJM study, but it still provides useful information. If you learn that you are postmenopausal and still have very dense breasts, this should be factored into your breast cancer risk assessment.
How much greater is a woman's risk because she has dense breasts? As breast cancer specialist Karla Kerlikowske, MD, explains in her editorial accompanying the NEJM study, mammographic density appears to increase risk more than hormonally-related risk factors like late menopause, HRT use, and not having children, but less than age or carrying one of the BRCA mutations.
Boyd's study found that women with the densest breast tissue are 4.7 times as likely as women of similar age with the least dense breast tissue to have a breast cancer detected less than 12 months after a negative screening test. Further, for women younger than age 56, 26 percent of all breast cancers and 50 percent of all cancers detected less than 12 months after a negative screening test were attributable to density in 50 percent or more of the mammogram.
This tells us that a number of breast cancers can be attributed to breast density. However, because of how the statistics in this study were reported and because of the type of study that was done, it's hard to say precisely what that number is. However, you should not make the mistake in thinking that if you have dense breasts your risk is nearly five times greater than that of the average woman. That is not what this study tells us. As Boyd explained in an interview with a Canadian reporter, the average 50-year-old woman has a 2.5 percent risk of developing breast cancer over 10 years. If she has extensive breast density, he says, the risk doubles to five percent.
As you can see, when you think about it this way, the risk is much less than you would think when you hear "nearly five times as likely to get breast cancer"—as many papers reported. Also, as Boyd underscored in interviews, it's important that women understand that even though having dense breasts increases breast cancer risk most women with dense breasts do not get breast cancer.
What should you do if you learn you have very dense breast tissue? Many people think that breast density is inherited—which would mean that there is little that you can do about it. However, a recent study found that overweight postmenopausal women who exercised reduced their breast density, so if you have dense breasts you may want to start exercising, if you don't already.
There is no evidence that more frequent screening (more than once a year) would be beneficial. However, the type of mammography performed may make a difference, as studies have found that digital mammography appears to detect more cancer in women with dense breasts than does film mammography. This means that finding a provider who uses a digital machine should be a starting point.
Women with very dense breasts may also want to be seen at a high-risk clinic. Breast specialists at these clinics can help you assess your risk and discuss with you whether you should consider some type of breast cancer prevention.
In the future, there will be more research done on, and we will know more about, breast density and how it should be managed in the risk assessment setting. We will need to wait for the answers from these studies to determine the best options for postmenopausal women with dense breasts.
Irwin ML, et al. Physical Activity, Body Mass Index and Mammographic Density in Postmenopausal Breast Cancer Survivors. Journal of Clinical Oncology. Published online January 27, 2007. http://www.jco.org/cgi/content/abstract/JCO.2006.07.3965v1
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