Like many other women, I have a love-hate relationship with my breasts. Sure, they could nourish an infant, they’re a go-to destination for sexual arousal, and most important, they’re healthy. But my breasts are small, barely a B-cup, which would be fine if I were a supermodel. In reality, I’m a slightly plump, middle-aged woman who wears padded bras to avoid looking like a box when I put on a sweater. The silver lining: My breasts are perky, aka firm. And that firmness led me to assume that they are, in medical parlance, dense.
So I was surprised when I learned, from the radiologist’s report on my most recent mammogram, that my breasts aren’t all that dense. In fact, they are categorized as a bit fatty. Which in this case is a good thing.
Knowing your density matters because it can affect your odds of getting breast cancer. For women with the densest breasts, the chance of developing the disease is four to six times as high as it is for those with the least dense breasts. As a risk factor, density is third in importance, coming behind gender (being a woman) and age (the older you are, the higher your chances of getting breast cancer), says Karla Kerlikowske, MD, professor of medicine, epidemiology and biostatistics at the University of California, San Francisco.
You can’t look in the mirror and figure out your density. The determining factor is how much of your breasts consists of fibroglandular tissue (the milk ducts and the fibrous tissues that support them) and how much of fat tissue. About 10 percent of women have -extremely dense breasts (more than 75 percent fibroglandular tissue), and 40 percent have fairly widespread density (51 to 75 percent fibroglandular tissue), according to the American College of Radiology. Of the rest, 10 percent have almost completely fatty breasts, and 40 per-cent have only scattered areas of density.
Most women, after a normal mammogram, receive a letter that says “Congratulations!” and also gives their breast-cancer risk assessment. But some breast-cancer activists are calling for mammogram reports to give women more information. As of this writing, five states—New York, Connecticut, California, Virginia and Texas—have mandated that mammography providers inform women of their density in a written report and advise women with dense breasts that they may benefit from additional testing via ultrasound and MRI. (In other states, you can find out your density by asking your physician or radiologist.) Here’s what you need to know about why some experts and activists think breast density is so important.
The cancer connection
On a mammogram, fat looks gray-black, and normal fibroglandular tissue, malignant lesions and some benign conditions look white. A radiologist examines the proportion of gray-black and white on the mammogram image and categorizes breast density, typically assigning a BI-RADS (short for Breast Imaging-Reporting and Data System) composition classification. In this system, number 1 refers to the fattiest breasts, and number 4 the densest. (Don’t confuse the BI-RADS composition scoring with the BI-RADS assessment number, which runs from 0 to 6 and indicates your risk of having cancerous cells based on your mammogram results.)
The BI-RADS composition number sounds solid and scientific, but there’s one problem: It is “very subjective,” says Peter M. Jokich, MD, director of breast imaging at Rush University Medical Center in Chicago. Observational studies reveal that a radiologist, when shown the same breast image at various times, may categorize the image differently, giving it a BI-RADS 2 classification at one point and a BI-RADS 3 at another. “Doctors are good at classifying very fatty breasts and very dense breasts, but sometimes it’s more difficult for women whose breasts are somewhere in the middle,” explains Jokich. The middle matters because, according to one study, the risk of breast cancer increases as breasts become denser. In theory, then, women with a BI-RADS score of 3 (51 to 75 percent fibroglandular tissue) would have higher odds of getting cancer than women with a BI-RADS 2 score (25 to 50 percent fibroglandular).
For years, medical experts believed that the association between breast cancer and breast density stemmed from the difficulty of spotting cancerous lesions (white) against dense tissue (also white). “It can be tough, like finding a polar bear in a snowstorm,” says Jokich. Mammograms in general miss 20 percent of cancers. But in women with the densest breasts, this screening can fail to spot half the cancers later found on ultrasound.
“Women need to understand that a ‘normal’ mammogram doesn’t always mean you don’t have breast cancer,” says Stacey Vitiello, MD, a breast--imaging and biopsy specialist in Montclair, New Jersey, who has worked closely with the advocacy group Are You Dense? (areyoudense.com). “Women don’t come to me for a mammogram. They come for early detection of breast cancer, and for women with dense breasts, mammograms alone are really a coin toss.”
To some experts, new findings have made density seem even more important: Researchers now know that women with denser breasts not only are at high risk of having cancers missed on mammograms but also have a greater chance of developing cancer in the first place. Why? One potential link stems from the larger number of stromal cells, a kind of connective tissue, in dense breasts. Researchers have found a relationship between stroma and an enzyme that increases the estrogen in breast tissue, which could contribute to the rise of cancer. Good news: A study published last year in the Journal of the National Cancer Institute discovered that while women with dense breasts are more likely to be stricken with breast cancer than other women, their risks of dying from it are similar.
That study suggests that women with fatty, rather than dense, breasts may also benefit from knowing their BI-RADS composition score. After following 9,232 women diagnosed with invasive breast cancer for about six and a half years, researchers were surprised to find that a specific group of breast-cancer patients had higher odds of dying: women with fattier breasts who were also obese. “Breasts with a higher percentage of fat may contribute to a tumor microenvironment conducive to cancer growth,” explains lead researcher Gretchen Gierach, PhD, of the National Institutes of Health.
What to do once you know your score
While most doctors believe that density is an important concern, they are less in step when it comes to advising women after they are notified of their density classification. Although five states require that mammogram providers disclose density as part of routine reports, many experts do not promote follow-up testing. One problem is that ultrasound and MRIs pick up too many false positives, abnormalities that look like cancer but turn out to be false alarms, says Otis W. Brawley, MD, chief medical and scientific officer of the American Cancer Society. Says UCSF’s Kerlikowske: “Some women may undergo tests that are not helpful.”
Kerlikowske believes that information about density should be used in the context of other breast-cancer risks. “Density is just another factor to weigh, along with other situations that help determine overall risk,” she says.
Monica Morrow, MD, chief of Memorial Sloan-Kettering Cancer Center’s Breast Surgical Service in New York, gives an example of the importance of considering other risk factors, based on the estimate that density increases the chances of getting cancer by four times. “In absolute terms, if a woman is otherwise low risk—young, had children before 30, no family history of breast cancer—so that her 10-year risk of breast-cancer development is 1 percent, that fourfold increase in risk due to density translates to 4 percent, a difference that clinically is not very meaningful,” she explains. “In contrast, if risk is already 5 percent, the increase to 20 percent is much more significant.”
A large, multicenter trial published in the Journal of the American Medical Association showed benefits from adding ultrasound—for women at the highest risk. The trial involved some 3,000 women with denser breasts (BI-RADS 3 and 4), who also had one other risk factor. Results showed that 7.6 cancers were found per 1,000 women who had had a mammogram and no other testing. But 11.8 cancers were found per 1,000 women among those who had had a mammogram and a screening ultrasound, a 55 percent increase in sensitivity. So it’s worth discussing all your risk factors with your doctor.
You can lower your odds of developing breast cancer by losing weight if you need to and exercising regularly, advises Kerlikowske. The American Cancer Society recommends working out moderately for 150 minutes or vigorously for 75 minutes a week.
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