Breast screening: Should density matter?

Should women with high breast density be monitored differently to prevent breast cancer screening failures?

THIS year marks the 20th anniversary of the Australian Government’s establishment of the National Program for the Early Detection of Breast Cancer, now known as BreastScreen Australia.

During the past 20 years, knowledge of breast cancer epidemiology has grown, along with the best methods for screening and diagnosis using mammograms and other techniques.

Mounting evidence suggests that highly dense breast tissue is linked to a far greater lifetime risk of breast cancer, more aggressive tumours, and a 50% chance of missed tumours in screening.

Yet women who have breast tissue that appears dense on a mammogram are not monitored any differently from other women under current national breast cancer screening procedures.

And while there are clear processes to follow when abnormal cells are found in mammographic screens of asymptomatic women, there is no strategy in place for when higher breast density is identified.

Is it reasonable, or even feasible, to introduce different screening procedures for women with higher breast density?

Under the Breast Imaging-Reporting and Data System (BI-RADS) breast composition categories developed by the American College of Radiology, breast tissue density is rated from 1 (very low density) to 4 (very high). 

Radiologist, Associate Professor Liz Wylie, medical director of BreastScreen WA, says only about a quarter of all women screened in the 50- to 69-year age group fit into the higher 3–4 BI-RADS categories.

She says there is strong research support suggesting that women with a BI-RADS density of 3–4 have around 4–6 times the risk of developing breast cancer in their lifetime, compared to women with low parenchymal density, BI-RADS 1.

“We also know that in women who have BI-RADS 1 or 2, we have a 98% chance of detecting their cancers in screening, whereas in women who have a BI-RADS category of 4, we have a 50% chance of not seeing a cancer on the mammography.” 

According to Dr Carolyn Nickson, a researcher at the Centre for Women’s Health, Gender and Society at the University of Melbourne, all US breast radiologists are trained in the BI-RADS measurements, and density is recorded against each mammogram.

Breast density is not routinely recorded in Australia at all, apart from in WA, she says – however she believes it should be, though not necessarily using the BI-RADS measurements.

“Most Australian evidence [on breast density] has been collected through research studies and more commonly reports in deciles.”

A 2009 evaluation of the BreastScreen program recommended the identification and management of women according to their breast density, following an evidence review after the introduction of digital mammography. 

National screening programs are currently moving from film to digital mammography, Dr Nickson says, but while overall screening sensitivity will improve, there’s little evidence to show that this move will improve results for women with dense breast tissue.

While several small studies indicate that an extra four cancers per 1000 women could be detected if ultrasound was added to the examination for women with high breast density, according to Dr Wylie it is not that simple.

“There’s no indication of how breast cancer mortality might be impacted,” she says.

While ultrasounds are an obvious next step, Dr Wylie says there’s no sound evidence to support their use as an adjunct to mammograms, and they are also costly (taking 30–40 minutes each) and deliver high false positive results.

MRI has a similar false positive response and is even more costly, she says.

Associate Professor Nehmat Houssami is a principal research fellow at the Sydney School of Public Health, where she is involved in two randomised controlled trials currently under way to look at the effect of ultrasound as an adjunct to breast screening mammography on breast cancer mortality.

She says that while several non-randomised studies have demonstrated that ultrasound detects additional cancers in women with dense breasts, it also causes a lot of false positive results, often requiring needle biopsy or other intervention. 

Professor Houssami, who is also a consultant breast clinician at Sydney’s Royal Hospital for Women, says that while high breast tissue density is a significant and independent risk factor for breast cancer, it’s not the only determinant.

A woman’s risk of developing breast cancer depends on the combination of various risk factors, she says.

“Overall, there is a paucity of high-quality evidence on the most effective strategy to screen women with dense breasts,” she says. 

Professor Houssami says at present, primary care focus should be on awareness of the reduced mammographic sensitivity in women with very dense breasts. 

“If a woman has a ‘normal’ mammographic screen, and she has dense breasts, any new symptoms should be promptly investigated,” she says.

Dr Wylie agrees.

“We want women with very dense breasts to realise that mammography is quite an insensitive test and that they need to be more vigilant than women without dense breasts.”

She doesn’t support calls for more frequent mammograms for women with dense breasts.

“Doing the wrong test more frequently, which involves cost and pain and radiation – is not the answer.” 

However, the limitations of the ‘one size fits all’ method of breast screening are starting to be addressed.

Professor John Hopper is director of research at the Centre for Molecular, Environmental, Genetic and Analytic (MEGA) Epidemiology at the University of Melbourne. 

He says that while women should ideally be screened differently according to their breast density, any change to current procedures needs to be evidence-based – and that isn’t likely to happen for a few years yet.

“At the moment, we advise a screening regime based on age,” he says.

“But we’re working towards identifying women who might benefit from more frequent screens – and also identifying women who can be screened less regularly, which would be a lot more women, and so we would save an enormous amount of money.”

Professor Hopper says current “democratic” approaches to screening need to change.

“Advances in genetics research and in mammographic density research, means in future we can move towards an individualised prevention strategy.”

Randomised trials analysing existing historic data are assessing whether women with high breast density would benefit from more regular screening, or whether less frequent screens are appropriate for women with lower density, he says. However, current analysis of mammographic density is simplistic and subjective, he adds, and a large project is under way to develop automatic mammographic density measurement of digital breast scan images via computer.

Professor Hopper says that evidence to inform a decision on what is best practice will allow every woman to receive a personalised recommendation for their next screening interval, at the time of their first mammographic screen, based on factors like family and medical history and breast density. 

It’s a long-term goal, Professor Hopper admits, with a five-year national collaborative project still in the planning stages. 

Meanwhile, he says, there’s little point giving out individual measurements of breast density.

“The only meaningful interpretation for the woman who’s having the scan is when do you need to come back?”