Density. It's a less obvious feature of the female breast than, say, size. But at least when it comes to good health, it's probably more important.
In fact, it just might be the greatest cancer risk you've never heard of.
Study after study has found that as breast density goes up, so does the risk of breast cancer. "There are very few things we know about that are so reproducibly found," says Dr. Norman Boyd, senior scientist at the Campbell Family Institute for Breast Cancer Research at the Ontario Cancer Institute in Toronto.
And the relationship can be very strong: Evidence shows that for women with extremely dense breasts, the cancer risk can be four to six times higher than for women whose breasts are not dense. By comparison, a family history of breast cancer — long considered an important risk factor — usually only doubles the risk.
The link between breast density and breast cancer was first discovered in the 1970s. "It took a long time for people to accept it," says Malcolm Pike, a professor of preventive medicine at the Norris Comprehensive Cancer Center at USC and an attending epidemiologist at the Memorial Sloan-Kettering Cancer Center in New York. "But nobody's arguing about it now."
Yet as widely accepted as the link is in the scientific community — and despite the fact that many researchers are convinced it could help identify high-risk women and maybe lead to treatments to reduce their risk — to date it has played a small part in the battle against breast cancer.
"Most women don't even know their own breast density," says William Barlow, a senior biostatistician at Cancer Research and Biostatistics in Seattle.
If you've ever had a mammogram, your breast density has probably been measured and recorded — but not reported to you. After all, the measurement has traditionally been considered of interest only to radiologists. It was instituted not as a harbinger of breast cancer risk but rather as an indication of how difficult a mammogram is to read. (The denser the breast, the harder the read.)
Even now, when the link to cancer risk is well established, many health professionals fear that giving women information about their breast density will serve little purpose other than to confuse or worry them, since it's largely determined by factors outside of their control (such as heredity, age and ethnicity).
But Connecticut passed a law last year requiring that patients' mammography reports must include breast-density information. And many doctors and researchers are in favor of more disclosure.
"I think patients should be told as much information as possible — recognizing they may not be able to use all of it. It certainly should be available if women ask for it," Barlow says. "After all, you can't change most of the risk factors for breast cancer."
Besides, Barlow and others argue that there are ways to use breast density information. If you know you have dense breasts, you may choose to have more clinical exams or mammograms than you would otherwise. Or you may choose to have an MRI, which doesn't become harder to read in cases of dense breasts the way mammograms do.
Also, density can change over time, so you may want to track your density from mammogram to mammogram. Two studies reported in April at the American Assn. for Cancer Research 101st Annual Meeting 2010 found that if a woman's breast density changes, so does her risk of breast cancer — if density goes up, so does risk, and vice versa.
One of the studies used data from the 2002 Women's Health Initiative trial that found postmenopausal women using hormone replacement therapy (estrogen and progestin) had a greater risk of breast cancer than women taking a placebo. In the new study, researchers found that in mammograms done a year apart, breast density went up for 85% of the women in the replacement group, and this increase in density could explain the increased cancer risk in that group.
These findings suggest that knowledge about breast density could be important to a woman deciding whether to use hormone replacement therapy for relief of hot flashes or other menopausal symptoms, for example, says Celia Byrne, assistant professor of oncology at the Lombardi Comprehensive Cancer Center at Georgetown University and lead researcher on the study. "If she has dense breasts, she might consider not taking hormones."
Another way to use the breast density-breast cancer link might be to incorporate it in the Breast Cancer Risk Assessment Tool, a method developed by the National Cancer Institute and the National Surgical Adjuvant Breast and Bowel Project that is widely used by health professionals to assess a patient's breast cancer risk. So far, this has not been done.
But researchers have developed other assessment tools that do take breast density into account — and in their studies so far, these tools give better (if only modestly better) results than the standard one. The National Cancer Institute says that additional studies done by independent researchers are necessary before changing the standard tool.
And there's another hang-up. Many researchers acknowledge that implementing such models would not be easy because of a fundamental problem with breast density: measuring it.
The measure in most common use — the one made with most mammograms — is rather imprecise and subjective. It simply rates densities according to four categories from the Breast Imaging Reporting and Data System (or BI-RADS), which is widely used by radiologists, from 1, for predominately fat, to 4, for extremely dense.
Researchers often aim for greater precision by estimating the percentage of dense breast tissue or using computer-based systems to calculate it. MRIs can provide very precise measures too but are probably too expensive for widespread use.
New techniques are on the horizon. Another study reported at the April meeting of the American Assn. for Cancer Research found that a technology commonly used to measure bone density and total body composition — dual energy X-ray absorptiometry, or DXA — can provide breast-density measurements that correlate well with mammographic measurements but with lower radiation exposure. Other researchers are working on ways to make totally radiation-free measurements with ultrasound.
To date, though, no method exists that is precise and objective enough to provide consistent results while at the same time being simple and inexpensive enough for wide use.
Indeed, Boyd says, the fact that density is so closely associated with risk is even more remarkable considering the weaknesses in density measurement. "Probably," he says, "the relationship is much stronger than we know."
Boyd suggests that down the road, the breast density-cancer link may be useful not only in identifying those at high risk but also in helping to lower that risk. Preliminary evidence supports that belief. More than a decade ago, for example, Pike and a number of colleagues found that a particular type of hormonal contraceptive could lead to substantial reductions in breast density. More recently, another study found that the cancer drug tamoxifen can reduce breast density.
"Theoretically," Boyd says, "in the same way that people now take drugs to lower their cholesterol and thus their risk of heart attack or stroke, you could someday take a drug to lower your breast density — and thus your risk of breast cancer."