Mammographic breast density | ISSUE OF CONCERN

You may have seen discussion in the media recently about the role that breast density plays in breast cancer.

Mammographic density, which is also sometimes called breast density, describes how breasts look on a mammogram. It isn’t a measure of how the breasts feel and it cannot be detected in a clinical exam by a doctor. Breasts are made up of several kinds of tissue, including fat, glandular tissue (the milk ducts and lobules) and connective tissue (which helps hold everything in place). Glandular and connective tissue are denser than fat, and this difference shows up on a mammogram.

A woman is said to have a high mammographic density if her mammogram shows her breasts have a lot of glandular and connective tissue and less fatty tissue.
Mammographic density varies from woman to woman. It is thought that about 50 per cent of women undergoing a mammogram will have a mammographic density of 51 per cent or greater. Of these, around 10 per cent are thought to have ‘extremely dense’ breasts, which means their breasts are made up of more than 75 per cent dense tissue.

Researchers think that a woman’s level of mammographic density is determined when her breasts first form, and that it is largely due to genetic factors. However, breast density can change a little over a woman’s lifetime due to factors such as body weight, age and hormonal changes after menopause.

One of the main concerns with mammographic density is that there is a greater chance of breast cancer not being seen on a mammogram. This is because small cancers can be hidden or ‘masked’ by glandular and fibrous tissue. Because of the masking issue, some women with high mammographic density may be recommended to have extra screening, such as a breast ultrasound or MRI.

Research is also beginning to show that having dense breasts can itself be a risk factor for developing breast cancer. Women with high mammographic density are one to two times more likely than average to develop breast cancer. This risk is even greater if you compare the two ‘extremes’ of mammographic density.

The 10 per cent of women with extremely dense breasts have a four to six times greater risk of developing breast cancer than the 10 per cent of women with mostly fatty breasts. These risks should be taken into account alongside other known breast cancer risk factors, such as being over the age of 50, having a high body mass index or having a strong family history of breast cancer.

At present, breast density is still not generally used in Australia to assess a woman’s risk of breast cancer and there are no guidelines on the best way to screen women with dense breasts. Most breast screening clinics in Australia also do not routinely advise women of their mammographic density.
More research is needed to understand why mammographic density is a risk factor and to find out whether the risks could be reduced through treatments or lifestyle changes.

There are also problems with the methods currently available to radiologists that help them measure mammographic density. Although radiologists can use scoring systems to rate mammographic density, most of these scores depend on how radiologists interpret what they see in the woman’s mammographic images.
This means that a woman’s mammographic density score can vary from one radiologist to another.

There are some new technologies that use computer models to help give a more reliable measurement of mammographic density. However, these technologies are more expensive and not widely used at present.

Currently, the BreastScreen Australia national screening program is the ‘gold standard’ for identifying breast cancer early in women. Biennial mammograms are still the only screening method proven through clinical trials to reduce the risk of death from breast cancer in women over the age of 45.

However, alternative screening methods – including breast ultrasound, breast MRI and breast tomosynthesis (each used in combination with mammography) – are now being studied to learn whether they improve detection in women with dense breasts compared to mammograms alone.

If you are interested in knowing about your mammographic breast density, you may be able to ask your radiology clinic, breast surgeon or GP.

If you have dense breasts, you may like to talk to your breast specialist about the best follow-up screening protocol for you. This may include adding ultrasound and/or breast MRI to your regular follow-up mammograms, although MRI is not currently covered by Medicare for this purpose. This means there can be significant out-of-pocket costs.

We have information about breast density on our website, bcna.org.au. You can also read our position statement on the advocacy pages of the website. Mammographic density is an issue that will continue to evolve as new evidence comes to light.

We will keep our members up to date as new knowledge emerges.

Issue 79
Winter 2017