Are You Dense?
Most women appreciate the importance of regular mammograms in detecting breast cancer at the earliest possible stage. In the last few years, new information has shed light on the value of knowing one’s breast density level, because density affects both risk for breast cancer as well as the ability of mammography to detect any abnormalities.
In particular, researchers have published two statistics that have aroused great concern: a) the finding that women with dense breast tissue are 4.6 times more likely to develop breast cancer than women with fatty breast tissue; and b) that mammography may miss almost half of tumors if a woman’s breast tissue is dense.
Elise Desperito, MD, a breast radiologist at NYP/Columbia’s Clinical Breast Cancer Program, addresses these very important concerns in the following interview.
Q: What should women know about breast density? Should women with dense breasts be concerned?
Dr. Desperito: It is important to first understand what “dense” means. Breast density refers to the relative amount of fibroglandular tissue to fatty tissue. Breast density is the ratio of the area of dense tissue on a mammogram divided by the total area of the imaged breast (percent mammographic density, or PMD).
This ratio is categorized in the mammography report on a scale from 1 to 4.
- Category 1: Least dense, between 0-25% fibroglandular tissue.
- Category 2: 25-50% fibroglandular tissue. The report will say the breast is composed of scattered fibroglandular densities.
- Category 3: 50-75% fibroglandular tissue. The report will say “the breast is composed of heterogeneously dense tissue.”
- Category 4: 75-100% dense. The report will say “the breast is extremely dense.”
This categorization is not based on a precise measurement, but instead is an approximation. Nevertheless, readers should understand that 80-90% of women are in categories 2 and 3. Only about 5-10% have extremely dense breasts, or category 4.
Q: How would a woman know her breast density?
Dr. Desperito: Until recently, she probably would not know it. After a mammogram, the report to her physician would include mention of her breast density according to the scale above, but the letters sent to patients themselves did not notify women that they had dense tissue. Most physicians did not discuss it with their patients because it was not common practice to do so, and the scientific community did not know that breast density in and of itself might confer an increased risk of breast cancer. As a result, some women were told they had a normal mammogram, only to discover later that they had a breast cancer that had not been detected mammographically. Thanks to recent advocacy efforts about this issue, seven states have now passed breast density notification laws: CA, CT, HI MD, NY, TX, and VA. Women who receive mammograms in these states will be notified of their breast density levels in their mammography result letters.
Q: Why are we hearing so much about breast density now, when mammography has been in use for decades?
Dr. Desperito: A study in 2006, published in Cancer Epidemiology by McCormack, concluded that breast density can increase the likelihood of developing breast cancer by 4.6-fold. That caused a lot of concern, which has spurred a lot of advocacy efforts by women who want to educate others about the importance of breast density.
However, it is very important to understand what the results of that study really mean. McCormack’s team compared breast cancer incidence in women in category 1 with women in category 4. When comparing extremely dense breast tissue to extremely fatty tissue, dense tissue was found to be 4.6 times more likely to develop breast cancer. While the study compared the two extremes of the spectrum, it did not include women from categories 2 and 3, which account for 80-90% of women. When you look at the risk difference between categories 2 and 3 compared to average risk, it is less than 1.5 times.
Q: Should women be concerned about the ability of mammography to detect cancers in dense breast tissue?
Dr. Desperito: Sensitivity of mammography refers to the ability of the test to detect cancers, and like many of our tools in medicine, the mammogram is not a perfect tool. Mammography is the best instrument we have, but it misses 10% of cancers, a fact which highlights the importance of the clinical breast exam both by the individual and by the primary care doctor. If she or her doctor feels something, we take this very seriously, even when the imaging study is negative.
In women with fatty breasts, sensitivity of mammography is about 90%. For extremely dense tissue, the sensitivity drops to about 60%, because dense tissue appears white, as do tumors. There is a common analogy that looking for a tumor in very dense tissue is like trying to see a snowball in a snowstorm. Knowing the patient’s breast density is important. I can walk away from reading a screening mammogram with fatty breast tissue and be very confident that the patient does not have cancer, whereas when looking at a mammogram with dense breast tissue, it is harder for me to be sure.
Q: If mammography may miss a percentage of cancers, what other testing is available?
Dr. Desperito: Two are used as additional screening tools – ultrasound and MRI. Both ultrasound and MRI have a significant rate of false positives (in other words, an abnormality is detected, but it is not a significant finding, i.e. it is not abnormal, atypical, or cancer, but rather proves to be a benign finding). The high rate of false positives increases patient anxiety and can lead to biopsies and other followup that may turn out to be unnecessary. That is the down side of additional screening with ultrasound and MRI.
MRI testing is more sensitive than ultrasound in detecting breast cancers, but it is also much more expensive. Studies show that MRI plus mammography is about 93% sensitive in detecting breast cancer, compared to mammography plus ultrasound, which is about 52% sensitive. It is equally important to understand that these studies were performed in high risk women, not in normal risk women
Q: Does Columbia recommend that women with dense breasts receive additional screening beyond mammography?
Dr. Desperito: This is a difficult and controversial question. One of the important and primary pieces of clinical history to ascertain is whether the woman has average risk or elevated risk of breast cancer. If we know the patient is at elevated risk, we follow the guidelines of the American College of Radiology, or ACR. For any woman who has a 20-25% or higher lifetime risk of developing breast cancer, our center will recommend she has MRI testing. For patients at high risk, MRI is usually covered by insurance. If a patient is at average risk and her mammogram is normal, insurance will typically not pay for screening with MRI.
Q: How should women manage all of this information about breast density and limits of mammography as a screening tool?
Dr. Desperito: A helpful starting point for every woman is to find out her density when she has a mammogram and determine her lifetime risk of developing breast cancer. Online tools are available that calculate your risk simply and quickly, such as the National Cancer Institute’s Risk Calculator, which uses the common GAIL model. After answering questions about your age, age when you had your first child, ethnicity, relatives with breast cancer, etc. the calculator will determine your lifetime risk. If that number is 20% or higher, that would be a good reason to go back to your primary care doctor and request additional testing, and there will be a good chance that insurance will cover such testing.
Bottom line, if you are at high risk for breast cancer, then a second screening tool is a strong consideration. If you have dense tissue but are not at high risk, insurance will likely not cover additional screening with ultrasound or MRI, but paying out of pocket for ultrasound may be a reasonable thing to do, because ultrasound does increase the chances of detecting cancers.
If you feel any lump, then testing is no longer considered screening but is diagnostic, and ultrasound or other testing will automatically be covered.