Published: 08 May 2023

Breast density – we answer your most-asked questions

By Dr Alan Smith, Specialist in Public Health Medicine, National Screening Service

We have received a few questions recently asking what the HSE is doing about breast density and why we don’t report on breast density to women who come for screening. This blog is aimed at building an understanding of the current scientific approach to breast density and its impact on population breast cancer screening programmes.

So what exactly is breast density?

Breasts are made up of 2 types of tissue – fatty and non-fatty tissue. How ‘dense’ your breasts are is dependent on how much of the non-fatty tissue type that you have. In medicine we are interested in breast density because the risk of breast cancer is higher in women with the densest breasts compared to those with less dense breasts.

Important characteristics of BreastCheck

It is worth recapping on a couple of important points relating to BreastCheck – The National Breast Screening Programme. Firstly, the programme offers population-based screening mammography to women aged 50-69 every two years. Our aim is to increase the opportunity for an earlier diagnosis for an individual woman and to reduce mortality in the total population. In general terms a population approach to screening means a ‘one-size’ fits all approach to the whole of the population using a proven screening test. This follows best international practice. In contrast, an individual approach is what you would experience if you went to your GP or a hospital Consultant when you can talk about your problems in more detail and together agree on further investigations or treatment specifically for you.

Secondly, BreastCheck as with every population cancer screening programme is not able to find all signs of cancer. This is heartbreaking if you are that person diagnosed with a cancer despite participating in the programme. It is perfectly normal for someone to feel that they did everything right, but the programme ‘missed’ their cancer. Unfortunately, this cruel ‘why me’, ‘what if’, ‘if only’ realisation, sometimes with life changing consequences, is and will always remain an integral part of all population screening programmes and BreastCheck is no different.

Finally, and perhaps one of the most difficult things to understand about a screening programme is that we knowingly implement screening programmes when it is a reality that not every individual within a screened population will benefit from participation. Some individuals will be harmed. All screening programmes have a difficult balancing act to perform, which involves balancing the known benefits with the known harms. See below for information on the benefits and harms of screening.

What does all this have to do with breast density?

High breast density is a non-modifiable risk factor for breast cancer, a risk factor you cannot change. You can’t tell how dense your breasts are by looking at them or by feeling them. Dense breast tissue appears white on a mammogram. But, and here’s the big but. Cancer also appears white on a mammogram. This can make it more difficult to spot breast cancer in the mammogram of women with dense breasts. So essentially if you are someone with dense breasts a screening mammogram may be less effective for you.

BreastCheck, mammograms and research into breast density

As is often the case in medicine nothing is ever simple. While women with dense breasts in a population have a higher risk of breast cancer, not all women with dense breasts in that population will get breast cancer. And equally, having dense breasts does not mean you are less likely to have a good outcome if you are diagnosed and treated for breast cancer. Frustratingly we just don’t have all the answers to these challenges……yet.

Internationally over the last decade, there has been an unprecedented amount of advocacy and attention surrounding the issue of breast density in relation to mammography screening. There is a particular question about the requirement for and the type of any additional screening in a population screening programme for breast cancer.

You will remember from earlier that a population approach to breast cancer screening involves a one size fits all type approach. This means that we need to know all the answers for all women with all types of breast density. It isn’t good enough to just have some answers for some women. Key gaps or uncertainties in our knowledge at the moment includes:

  • Identifying the breast density unit of measurement most strongly associated with breast cancer.
  • Identifying the method of measurement most strongly associated with cancer
  • Identifying the measure and method of measurement most suitable for use in a population screening programme that maximises benefits and minimizes harms (see below for more information)
  • The extent to which density can and/or should contribute to placing a screened population into different at-risk groups for follow up screening
  • The management of screened populations identified at increased risk and/or different types of risk. For example, should we increase the frequency of screening or should we use further tests such as digital breast tomography, MRI or ultrasound; and do we do this for all women or just for some women with dense breasts
  • Last, but by no means least, the best means of communicating breast density risk information safely and coherently in a population screening programme setting.

Does BreastCheck collect breast density information?

No. It is important to know that no breast density information is currently captured for women attending population breast screening in Ireland. As outlined above this is an area of considerable ongoing international research and many ongoing clinical trials will inform us how breast density can be incorporated safely and effectively to inform breast screening strategies in a way that is equitable for all.

Has Europe not already made a recommendation on breast density?

The actual answer is a somewhat annoying ‘Yes’ and ‘No – not yet’. From a yes perspective, the European Society of Breast Imaging (EUSOBI) has recently published recommendations that state “women should be appropriately informed about their individual breast density - and the diagnostic and prognostic implications of having dense breasts - by all (European) organisations that offer breast screening in order to help them make well-balanced choices.”

But crucially, and what is often not publicised is that EUSOBI “acknowledges the fact that before a population-wide use of non-mammographic screening methods such as screening ultrasound and screening MRI is put to practice in women with extremely dense breasts, the necessary quality assurance systems and benchmarks must be established for these non-mammographic screening methods similar to those that are in place for mammographic screening. This will take some time to prepare and to implement.”

From the ‘no – not yet’ perspective the European Commission Initiative on Breast Cancer (2021) (ECIBC), the accepted evidence-based voice for population screening, does not currently recommend the use of either MRI or ultrasound in the context of an organised screening programme for asymptomatic women with high mammographic breast density. This position is being kept under constant review.

BreastCheck’s approach to breast density

Having in place the necessary quality assurance systems and benchmarks are paramount for BreastCheck. The programme is one of only two programmes in Europe with ‘Reference Centre’ accreditation status from EUREF. Ireland follows the evidence and the guidance from the ECIBC (2021) as do most other countries that offer population breast cancer screening.

However, that said, all screening programmes are dynamic and part of our role is to monitor evidence and practice and respond when the evidence is sufficient to warrant change safely and effectively.

How do changes to BreastCheck happen in Ireland?

Any changes to population screening programmes in Ireland, including BreastCheck, must now be first evaluated by the National Screening Advisory Committee (NSAC). The Committee is independent of BreastCheck and makes recommendations to the Minister for Health and the Department of Health, and if accepted, these recommendations are communicated to the HSE and the National Screening Service begins the planning phase for their implementation.

Is a change likely in BreastCheck because of breast density?

NSAC is currently evaluating an application for the reporting of breast density in our population screening programme, an application that was submitted in 2021. More information about the Committee and its own work programme can be found on the NSAC website.

What is BreastCheck doing now?

There are approximately 3,500 cases of breast cancer diagnosed in Ireland every year. BreastCheck will screen about 170,000 women every year and will detect about 1,100 of these cases, about a third (34%). Some people are surprised at this relatively low percentage but it is important to remember that most cancers are still detected when women present with symptoms or a concern to their own GP or hospital Consultant.

BreastCheck has also created information materials about the things that increase your chance (risk) of getting breast cancer. This information was created in conjunction with the HSE National Cancer Control Programme (NCCP) and is available in leaflet format, on our website, and has been communicated to our advocacy partners, GPs and practice nurses, and across our annual communication campaigns.

What are the benefits and harms of screening?

Benefits of Screening:

  • Reduced severity and morbidity
  • Less invasive treatment
  • Reduced incidence
  • Reduced deaths

Harms of Screening:

  • Overdiagnosis
  • Overtreatment
  • False negatives and false positives
  • Harmful physical and psychological effects of screening test