By now, you’ve probably heard the call to Know Your Breast Density! You may have even seen articles about women who are angry because their doctors never told them they have dense breasts or who believe that if they had known they had dense breasts their cancer would have been detected earlier. You may have even applauded new legislation that tells doctors they must inform women about their breast density. But the latest research suggests that it might not be the densest breast tissue we need to be most worried about.
The breast contains ducts, glands, fibrous connective tissue, and fatty tissue. A breast that is considered dense has more fibrous, connective tissue and less fatty tissue, while a very low density breast contain almost all fatty tissue. Virtually all women start out with very dense breast tissue. Over time this changes, and, as a woman ages, her breasts typically become more fatty and less dense. However, there are some women whose breasts remain dense, even after menopause. And there are others whose breasts become completely fatty.
Breast density can only be determined by looking at a mammogram of a woman’s breasts. (A system called BI-RADS is used to report density, which is ranked from 1 (least dense) to 4 (most dense)). For decades, we thought that the problem with dense breasts was that they made it more difficult to detect a tumor: Both dense breast tissue and a tumor appear white on a mammogram, which makes finding akin to looking for a polar bear in the snow. This is one reason why mammography does not find all breast tumors. It is also one of the reasons why mammography is not a very effective screening tool for young women, who typically have dense breasts.
More Than White Tissue
Five years ago, we wrote about a study published in the New England Journal of Medicine that found that breast density was an independent risk factor for breast cancer in women who are close to 50 or who are postmenopausal. This study changed the way we thought about dense breast tissue, because it meant the problem wasn’t just that these tumors were harder to detect.
This study also led some breast cancer advocates to begin promoting the need for women to learn their breast density. This, in turn, led to calls for legislation that would require doctors to tell women their breast density. Four states have already passed this legislation, and more than a dozen other states have bills that could be approved.
But the talk about dense breast tissue often left out some important facts. For example: having dense breasts at 40 is quite normal. The risk increases if your breasts stay dense as you grow older, and after menopause. It also sometimes gave the impression that the only tumors not found on mammography and diagnosed at a later stage were those in dense breast tissue. But that’s not the case. Studies show that about 38 percent of breast cancers are diagnosed at a late stage, but this is not solely due to dense breast tissue. It’s because the tumor’s biology is aggressive.
Why Density Matters
Our need to rethink what we know about breast density again highlighted by findings from a recent study on mammographic density that surprised many by showing that women with dense breast tissue are, in fact, not more likely to die of breast cancer.
This study published in the Journal of the National Cancer Institute looked at 9,232 women who had been diagnosed with breast cancer between 1996 and 2005. About 84 percent of the women were classified as BI-RADS 2 or 3. (Specifically, 560 were BI-RADS 1; 3735 were BI-RADS 2; 4019 were BI-RADS 3 and 918 were BI-RADS 4.)
The researchers followed the women for six years after their diagnosis. During that time, there were 1795 deaths 889 from breast cancer and 810 from other causes. When all breast cancer stages were combined, there was no relationship between BI-RADS density and breast cancer death. However, among women whose tumors were 2.0 cm or more, BI-RADS 1 density was associated with an increased risk of breast cancer death. And, importantly, BI-RADS 1 density the most fatty breast tissue was also linked to obesity.
Tumor Biology Matters
Could it be that dense tissue represents a local microenvironment that promotes cancer development, but not spread? Or that the low breast density seen in obese women creates a microenvironment that makes tumors more aggressive? That is something we need to study. Tumor biology might also explain why the risk factors for developing breast cancer may not necessarily be the same as the factors that influence a woman’s risk of dying from breast cancer. In other words, maybe dense breast tissue increases the risk of getting cancer, but not dying of it. If this is true, it might mean we don’t need to treat the tumors of all women with dense breast tissue the same way, or the same as we would a woman with fatty breasts.
It’s especially important to think about these findings in the context of the mammography debates and the push for breast density legislation. All too often, a recommendation like the one the US Preventive Services Task Force made for routine mammography screening to begin at age 50 is interpreted as a conspiracy to cut health care costs that will ultimately increase breast cancer deaths. Or, we look to legislation, like the laws we’ve seen about breast density, as a way to get women more screening even though there is no evidence that more screening is better.
We have repeatedly bumped up against the limits of what screening can do. We need to focus more on the tumor’s biology and the microenvironment that surrounds it, it we are going to end this disease. The recent study on breast density showed us, yet again, that women who are obese when they are diagnosed with breast cancer are more likely to die of breast cancer than women who are not obese. Doctors need to do more than tell women about their breast density or remind them to get a mammogram. They need to be teaching women the importance of exercising, losing weight (if necessary) and eating a well-balanced diet both before and after a breast cancer diagnosis.