Density: The quality or state of being dense; the quantity per unit volume, unit area, or unit length; the degree of opacity of a translucent medium, or the common logarithm of the opacity.
For more than a decade, federal law in the United States has compelled breast imaging centers to give every mammography patient a letter explaining her result.2
Often, however, the first person a woman speaks to about her findings is her primary care clinician, particularly if she has had a screening mammogram at a center where films are “batch-read” and are not viewed by the radiologist at the time of the appointment. Internal medicine physicians are often called on to help women understand their findings and to order follow-up tests recommended by the radiologist—a not uncommon occurrence. Also, internists often need to address patients’ anxieties about the possibility of breast cancer and provide them with enough information to make an informed decision about an appropriate action plan.
Meanwhile, discussing mammography has become more complicated. In 2009, the United States Preventive Services Task Force stopped recommending that women under age 50 be routinely screened for breast cancer, and instead stated that the decision to begin screening these women should consider “patient context” and the patient’s personal “values”3—with the implication that women’s primary clinicians would play an important role in helping them weigh the test’s potential benefits and harms.
More and more, internists must grapple with the task of how to help women decipher the concept of “breast density,” understand their personal density results, and make an informed decision about whether to undergo additional imaging studies, such as ultrasonography and magnetic resonance imaging (MRI).
LEGISLATION REQUIRING DENSITY NOTIFICATION
The impetus for this change in practice has been spurred in large part by patient advocates, who have argued that women deserve to know their density because mammography is less sensitive in women with dense breasts. So far, at least 12 states have enacted laws requiring breast imaging centers to add information about breast density in the result notification letters they mail to patients. Legislatures in several other states are considering breast density notification laws,4 and federal legislation has been proposed.
Some of the state laws, such as those in Connecticut, Texas, and Virginia, require informing all mammography patients about their density findings, whether or not they have dense breast tissue. Other states, such as California, Hawaii, and New York, require informing only those found to have dense tissue. And some states, such as California, Connecticut, Hawaii, Texas, and Virginia, require specific wording in the density notification letter (Table 1).
The details of all these notification laws may differ in how they specify which patients must be notified and in how the information should be worded, but the goal is the same: to raise women’s awareness so that they can embark on an informed decision with their physician about whether to undergo further testing.
Because of liability concerns, some breast imaging centers in states that currently lack such notification laws have begun informing women about their density results.
Unfortunately, at this point clinicians have no clear guidelines for helping patients with dense breasts decide whether to undergo additional testing. In addition, the evidence is equivocal, and the tests have risks as well as benefits. The patient needs to understand all this by discussing it with her physician. And to discuss this decision effectively, the physician must be well versed in the evolving literature on breast density. Below, we present important points to keep in mind as we foster these discussions with our patients.
BREAST TISSUE DENSITY IS STILL A SUBJECTIVE MEASUREMENT
Breast density limits the sensitivity of mammography. This is widely established. Yet the interpretation of breast density today is subjective. It is determined by the interpreting radiologist based on the Breast Imaging and Reporting Data System (BI-RADS), which defines “heterogeneously dense” breasts as those containing 50% to 75% dense tissue and “dense” breasts as those with more than 75%5 (Figure 1). This subjective measurement is based on two-dimensional imaging, which may underestimate or overestimate the percentage of breast density because of tissue summation. Ideally, density should be measured using three-dimensional imaging with automated software,6 but this technology is not yet widely available.
INCREASED DETECTION OF BENIGN LESIONS
Although adding ultrasonography to mammography in patients with dense breast tissue detects additional cancers,7,8 it also leads to a significant increase in the detection of lesions that are not malignant yet require additional workup or biopsy.
The largest study to examine this was the American College of Radiology Imaging Network Protocol 6666 (ACRIN 6666),7 a multi-institutional study evaluating the diagnostic yield, sensitivity, and specificity of adding ultrasonography in high-risk patients who presented with negative mammograms and had heterogeneously dense tissue in at least one quadrant.7 (High risk was defined as a threefold higher risk of breast cancer as determined by risk factors such as personal history of breast cancer or high-risk lesions, or elevated risk using the Gail or Claus model.) The supplemental yield was 4.2 cancers per 1,000 women (95% confidence interval 1.1 to 7.2 per 1,000) on a single prevalent screen. Of 12 cancers detected solely by ultrasonography, 11 were invasive and had a median size of 10 mm. Of those reported, 8 of 9 were node-negative. Despite this additional yield, the positive predictive value of biopsy prompted by ultrasonography was only 8.9%.7 Other investigators have reported similar findings.8
RELATIONSHIP BETWEEN DENSITY AND CANCER RISK STILL NOT CLEAR
The relationship between breast density and cancer risk is not entirely clear. Higher breast density has been associated with a higher risk of breast cancer,9,10 presumably because cancer usually develops in parenchyma, and not fatty tissue. Yet obesity and age, which are inversely associated with density, are also risk factors for the development of breast cancer. Some prominent radiologists have cast doubt on the methodology used in these density studies, which relied on density measurements calculated by two-dimensional views of the breast, and have called for a re-evaluation of the relationship between density and cancer risk.6