According to a new law in California, radiologists who read mammograms will soon need to inform patients if their breasts are dense as seen with the mammograms.  The reason is that breast cancer occurs more often in women with dense breasts than in women with low-density or fatty breasts.  What should you do if you get a letter like this? First, please don’t panic!

Dense breasts are the normal starting point.

Virtually all breasts start out dense.  We don’t usually do mammograms on young women, but when we do, they are invariably dense.  It is how breasts grow during adolescence.  For this reason, breast density is better understood as persistence of how breasts grew in the first place than as a change.   The reason is that, as women age, the gland tissue is often replaced with fat, even though the breasts can stay the same size.  Fat is less dense on mammograms, so breasts often become less dense after breastfeeding, after menopause, or after stopping hormone replacement therapy at menopause.  Whatever the cause, it is the low-density, fatty breast that has changed, not the other way around. Persistent breast density is literally the lack of change.

Cancer starts more easily where there is more gland tissue.

Density of breast tissue does not make the tissue bad – it’s just that dense breasts have more tissue that can go bad because of other factors.  When breast tissue is examined, there are some types of fibrocystic change that predict cancer, and some types of fibrocystic change that indicate no increase in risk. Years ago, we did a study to see if dense breasts had more of the kind of fibrocystic change that predicted cancer.  We found that dense breasts do not have more high-risk fibrocystic change.

So, you might ask, why is breast density related to breast cancer risk?  The answer is that breast cancer has to start in gland tissue, and when there is more gland tissue – for example, when gland tissue persists and makes the breast dense – there is more tissue where cancer can begin. We’ve actually known for a long time that cancer occurs where there is more breast gland tissue:  if a woman is lying down, there is much more tissue in the part of her breast toward her armpit, and that part toward the armpit is also where the most cancers occur.  So it is pretty logical that cancer would be more likely if breast gland tissue persists.  And similarly, there will be less cancer if there is less gland tissue in which cancer can begin, like in a fatty replaced breast. 

The risk is real, but it often decreases over time.

What is the risk that a woman with no family history of breast cancer and no history of any breast biopsy – two factors that independently predict higher risk of breast cancer – will develop breast cancer in the next five years?  This table shows the percent of women who develop breast cancer in the next five years, for women without a family history and who have not had a breast biopsy*.  Women with family history of breast cancer, or who have had a biopsy, have an increased risk even if they do not have dense breasts.  The most important thing to remember when looking at this table is that breast density tends to decrease over time, so a woman who has dense breasts at age 45 can have less dense breasts, and thus less risk, when she reaches age 65.

Understanding the risk with dense breasts.

To understand the effect of density from this table, compare the percent of women with fatty breasts who get breast cancer to the percent of women with very dense breasts who get cancer.

For example, the women who are age 60 to 64 with fatty breasts get cancer 0.7% of the time over five years, but the women age 60 to 64 with very dense breasts get cancer 2.7% over the same time.  This can be summarized two ways, and both are correct:  1. women with fatty breasts have about one-quarter the risk of women with very dense breasts (0.7 ÷ 2.7 = 0.26); or 2. women with dense breasts have about four times the risk of women with fatty breasts (2.7 ÷ 0.7 = 3.9). It all depends on which perspective you want to take. 

The table is also useful to compare relative risk and absolute risk.  If you look at the same women who are age 60 to 64, the absolute difference in risk is 2 percent (2.7 – 0.7 = 2 percent).  This is the largest difference for any age group in the table.  Thus, it is correct to say that the risk of cancer is “four times higher” with dense breasts, but the absolute difference is only 2 percent. This is a real difference – especially for the women who are affected – but emphasizing the “four times higher” relative risk description may cause more anxiety than fits the facts.

An important additional observation is that many women lose density over time so that a person who is at higher risk now, will tend to lose gland tissue (it gets replaced by fat) and thus experience reduced risk over time. 

Fatty (low-density) breasts are not risk free.

* These percentages were published in 2008 in an article by Tice et al, “Using Clinical Factors and Mammograpic Density to Estimate Breast Cancer Risk: Development and Validation of a New Predictive Model.”  Annals of Internal Medicine, 2008; 148: 337-347, Table 5.  Percentages apply only to women who do not have the risk factors of family history and/ or a previous breast biopsy.