Breast Density and What Does It Mean?

I requested a patient return for a screening ultrasound of her breasts.  When she returned, she wanted to know what it meant to have dense breasts and if she needed to have ultrasound ever year.  We hear about breast density quite often and how mammography is limited in people with dense breasts.  But what does that really mean?  How does it affect mammography?

To make it simple for people to understand, I compare reading mammograms with hunting for deer (no, I do not actually hunt):

I am the hunter looking for deer (breast cancer).   If I am looking for a deer in a field with no trees, the deer is easily seen.  In fact, I will be able to see a small deer easily in a field.  If I am looking in a densely wooded forest, I may have more difficulty seeing deer, particularly smaller deer.

If you have fatty (not dense) breasts, you have the “field”.  If you have dense breasts you have the “forest”.  The density of your breasts depends on whether you have mostly fields or forests in your breast.

We grade density on what percentage of your breast is filled with “trees”.  If 0-25% is filled with trees, then you are predominantly fatty;  25-50% is graded as scattered densities, 50-75% heterogeneously dense, and 75-100% is extremely dense.

In reality, it is not that simple.  The distribution of “trees” in the breast can vary significantly and makes classification difficult.  Suppose one acre has 100 trees but they are spread out over the entire acre (100%).   There is enough space between the trees that you would probably be able to see a small deer between the trees.  But what if the 100 trees were packed into 1/10th of an acre (10%)?  If the deer happens to show up in that 1/10th of an acre, you might have difficulty seeing it.  The first scenario would be classified as dense (>50%) and the second would be classified as not dense (<50%).  And what does it mean if we are around 50% (borderline scattered versus heterogeneously dense)?  Our current system of grading breast density doesn’t necessarily correlate with our ability to find breast cancers.

Unfortunately, determining the density of your breasts and the likelihood of finding a breast cancer is not a perfect science.  It is left at the discretion of the radiologist.  Not every radiologist may necessarily agree on whether or not you need additional testing, such as MRI or ultrasound.  So what should you do?

1.  Have a mammogram done.  You cannot tell density by how firm your breasts feel.  There is no correlation.  If your breasts are fatty, then it mammograms may be a really good screening test for you and you may not need anything else.

2.  Have your mammogram read by a breast imaging expert.  The radiologists who read more mammograms are going to be better at it.  An easy way to tell who is an expert is to see if they have received additional training (“fellowship”) in breast imaging.

3.  Have a discussion with the radiologist when you can.  If you get called back for additional imaging, ask to speak with the radiologist.  That is the best person to tell you about your breast density and whether or not you may benefit from additional testing.

4.  If you have family history, consider seeing a breast surgeon.  If you are at increased risk of developing breast cancer, you may benefit from having breast MRIs for screening.  The breast surgeon can determine your risk and also provide you with genetic testing.  If your risk is high enough, you may benefit from additional imaging, even if your breasts are not dense.

I spent about 10 minutes with the patient I described above explaining what her breast density was and what it meant for her.  She was borderline dense and although I felt that I would probably be able to find a small cancer in her breast, her ultrasound was negative and posed no risks (such as short interval followups or benign biopsies).  I told her I was not sure she needed the ultrasound every year, but that if she wanted it, there would likely be no downsides to having it done.  So at the end, we decided together that she should have a mammogram and ultrasound together every year.

As she was walking out the door, my patient turned around and said to me, “Thank you for what you do.  I know you are trying to save lives.”  I was incredibly touched by what she said.  It makes every minute I spend talking to a patient worthwhile.


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