Routine examination or screening with mammography (an x-ray examination of the breast) has been demonstrated in many studies to help reduce deaths from breast cancer by between 30 and 47 per cent. Also, earlier detection can, in some cases, reduce the need to have some of the harsher types of treatment needed when cancers are found later. So instead of mastectomy, lumpectomy surgery may be possible and procedures such as chemotherapy or axillary dissection (surgery in the arm-pit) may be avoided.
Here, in consultation with Dense Breasts Canada, I answer some common questions about breast density and how it affects breast screening for cancer.
What is breast density?
Breast density is a term used to describe the presence of structures inside the breast. These are composed of the glandular tissue responsible for milk production as well as fibrous tissue. The remaining tissue in the breast is mainly composed of fat. Some dense tissue in the breast is absolutely normal. Some women’s breasts are composed of more dense tissue and less fat than average and these breasts are referred to as “dense breasts”.
How do we know how dense the breast is?
Breast density can’t be judged by feel or look but is assessed by a radiologist looking at a mammogram. Dense breast tissue absorbs x-rays more than fatty tissue does and appears white on the mammogram. Typically there are four categories (called BI-RADS density categories) used to describe density. These range from A (fatty) through B (scattered fibroglandular density), C (heterogeneously dense) to D (extremely dense). Often the lower two categories are considered as “low density” and the upper two as “high density.” There are also computer programs that measure breast density automatically from the mammogram.
Why do we care about breast density?
Density is important for two reasons:
- Compared to other women of the same age, women with denser breasts are at greater risk of developing breast cancer in the future than those with less dense breasts. So, density is a risk factor for breast cancer. Women with the most dense breasts have approximately twice the risk for breast cancer compared to those with average density breasts.
It is important to know, though, that density is only one of several possible risk factors, including, mutations in the BRCA1 or BRCA2 genes, a family history of breast cancer, obesity after menopause, use of alcohol and others.
- If a woman has breast cancer, it is less likely to be detected in a mammogram for those with dense breasts. This is because both dense tissue and cancer appear white on a mammogram and this may cause the cancer to be hidden. This is called ”masking”. The BI-RADS density categories C and D specifically refer to this masking risk, which is generally greater for category D than for C.
So there are two types of risks related to breast cancer faced by women with dense breasts – increased breast cancer risk and increased likelihood of masking if a cancer is present.
Should I worry about density?
By and large density is a natural phenomenon and dense breasts are common. To some extent, density is inherited, although it may be affected by some lifestyle factors, such as use of alcohol and hormone replacement therapy. Density is not directly related to the size of your breasts. A higher body mass index is inversely related to density.
There are a lot of different risk factors for breast cancer. But these are risks, not certainties. None of them will guarantee that you will get breast cancer if the factor is high or guarantee that you won’t if the factor is low. So there’s not much point in worrying, but you can take steps to mitigate risk.
What can I do to mitigate risk?
It is useful to know if you have a risk factor like density. This is because there may be new information in the future that can allow you to reduce your risk. And, in the case of density and the masking effect mentioned earlier, it is important to know what your density is because the information can be used right now.
If your breasts are low density, then you can have more confidence that you don’t have breast cancer when you receive a normal result from a mammogram. A mammogram, while still not 100% accurate, should be a very reasonable method for your breast screening exam.
If your breasts have very high density, as judged by your radiologist, then it may be of value for you to receive a “supplemental” screening examination, using an imaging method whose accuracy is not reduced by density. Two possibilities are breast ultrasound and breast MRI. Both of these techniques have been shown to be able to find cancers that are missed by mammography in dense breasts. Another promising method is contrast-enhanced mammography, which appears to provide similar information to MRI.
All screening methods, including breast ultrasound, MRI and contrast mammography have both strengths and limitations and these supplemental methods should currently only be considered for the small fraction (maybe 10 -15%) of women where mammography is not sufficient due to concerns about accuracy. Both MRI and contrast mammography require an injection of a contrast agent. Some suffer from claustrophobia and find the MRI experience challenging.
Mammography screening has been demonstrated in many studies to help reduce deaths from breast cancer. Although there is good evidence that cancers missed on mammography in dense breasts can be found with MRI or ultrasound, no studies have been completed to show that deaths are reduced by screening women with dense breasts using these techniques. But we do know that trials that in the past found a reduction of advanced cancers by earlier detection also demonstrated reduction in breast cancer deaths, so it is probably a good bet that ultrasound MRI, and contrast mammography will do this.
Currently Canadian guidelines do not recommend supplemental screening, but many scientists are convinced that women should be informed about their breast density and, armed with this information, should consider, in discussion with their physician, if in their case it may be a good idea.
In short, I’d recommend that every woman age 40 and over have a conversation with their healthcare provider about breast screening.
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