Only in America can we find a way to scare the bejesus out of a woman with normal breasts and a normal mammogram. Because that’s exactly what happened when New York Times reporter Roni Caryn Rabin read her entirely normal mammogram results letter: “A sentence in the fourth paragraph grabbed me by the throat. ‘Your breast tissue is dense.'”

I can’t really blame Rabin for being afraid. The information about breast density in her mammogram letter was mandatory verbiage crafted by legislators as part of a law that all women be told if they have dense breasts on mammogram:

Your mammogram shows that your breast tissue is dense. Dense breast tissue is very common and is not abnormal. However, dense breast tissue can make it harder to find cancer on a mammogram and may also be associated with an increased risk of breast cancer. This information about the result of your mammogram is given to you to raise your awareness. Use this information to talk to your doctor about your own risks for breast cancer. At that time, ask your doctor if more screening tests might be useful, based on your risk. A report of your results was sent to your physician.

Raise awareness? More like raise the alarm. The information mandated by the law is just enough to scare any women who happens to have dense breasts, but not enough to help her understand what this really means.

If you’ve gotten a letter telling you your breasts are dense, don’t be afraid. Having dense breasts is entirely normal, especially if you are under age 60.  Here’s what you need to know.

What is breast density?

Breast density is a radiologic assessment of how well x-rays pass through the breast tissue. It is a surrogate for how much of the breast is composed of glandular tissue and how much is fat. The radiologist reading the mammogram classifies the breast composition as one of the following:

• almost entirely fat (<25% glandular)
• scattered fibroglandular densities (25-50%)
• heterogeneously dense breast tissue (51-75% glandular)
• extremely dense (>75% glandular)

Breast density is subjective.

Different radiologists may give the same mammogram different ratings. Use of computerized density measurement could alleviate inter-observer variability, but there is not yet a standardized computer rating system. For the purposes of the law, dense breasts are defined as those that are heterogeneously dense or extremely dense.

Breast density can vary across a woman’s menstrual cycle and over her lifetime.

The same women being scanned at a different time of month or at a later year can land into a higher or lower breast density category, and may or may not get that extra statement in her mammogram letter. Recent research suggests that a single breast density reading may not be the best way to predict breast cancer risk, and that the risk may be confined to those women whose breast density does not decrease with age.

Dense breasts are extremely common, especially in younger women.

According to a recent report of mammograms here in New York City, 74% of women in their 40s, 57% of women in their 50s, 44% of women in their 60s and 36% of women in their 70s have dense breasts.

Increased breast density may be a risk factor for getting breast cancer. 

The mechanism is unknown, but it may be that breast density is just the end result of other factors that increase breast cell proliferation and activity — factors like genetics and postmenopausal hormone use.

How much of a risk? Well, it depends on what study you read and who you compare to whom. If you compare the two extremes of breast density in older women, those with extremely dense breasts have a three to five-fold higher cancer risk than those with mostly fatty breast. The risk is lower than that in those in the middle category of breast density and in younger women, though not well-defined.

The truth is, we really have no way to translate individual breast density into individual risk. Researchers are trying to see if breast density can be incorporated into current risks assessments such as the Gail Model, but at this point, breast density has not been shown to add much more than we already know about a woman’s risk from using these models.

The problem with breast density as a risk factor is that most women at some point in their lives have dense breasts. Should we really consider 75% of women in their 40′s to be at increased risk for breast cancer?

I don’t think so.

Dense breasts can obscure a cancer on mammogram.

This makes mammogram less reliable in women with dense breasts. Digital mammograms may be better at finding breast cancers in women with dense breasts who are also perimenopausal or less than age 50, but it is not known if this translates into better outcomes. Additional testing with ultrasound and MRI can find cancers that mammograms miss in women with dense breasts. Unfortunately, breast ultrasound and MRI screening tests are less specific than mammograms — three times as many biopsies will be done, most of which will not be cancer.

Breast cancer patients with dense breasts are not at increased risk of death compared to those without dense breasts.

In a study of over 9,000 women with breast cancer, no association between increased density and death from cancer was found. In fact, it was obese women with lower breast density who had the higher risk of death, possibly because their fatty breasts may be a more favorable environment for tumor growth.

We do not know if additional breast cancer screening beyond mammograms saves lives.

Sonogram and/or MRI for breast cancer screening is currently not recommended based on breast density alone. Additional screening beyond mammography is only used in women at highest risk for breast cancer — those with cancer in a first degree relative with a high risk gene mutation, a family history suggesting one of these mutations, a Gail model or other combined lifetime breast cancer risk assessment >25% or a history of chest irradiation. Even in this group, declines in morality with the additional screening have not yet been shown, and the false positive rate of this additional testing is extremely high — only 20% of abnormals are cancer when biopsied.

There are no recommendations to use sonogram and MRI in otherwise low risk women, and none that have shown that using it based on breast density alone saves lives.

Additional screening beyond mammograms adds significant costs to breast cancer screening.

For some women, this additional cost may not be covered by insurance. While Connecticut has passed a law mandating that insurers cover additional sonograms, New York State has not.

What should you do if you’ve been told your breasts are dense on mammography? 

If you are at increased risk for breast cancer due to personal or family history, you may want to consider adding ultrasound or MRI screening.

Otherwise, at the point there is no recommendation that you do anything other than continue screening at whatever interval you and your doctor have decided is right for you. If having a sonogram will reassure you that you’ve done everything you can to screen for breast cancer, and are willing to accept the additional false positives and biopsies that may results from this additional screening, and understand that the additional screening has not been shown to decrease deaths from breast cancer in women at average risk, then ask your doctor to order you a breast sonogram.

Margaret Polaneczky is an obstetrician-gynecologist who blogs at The Blog That Ate Manhattan.