The mammograms that DON’T spot breast cancer: Why some women have a condition that makes tumours harder to detect – and puts them at greater risk of getting cancer

Excited about the Greek ­holiday she and her daughter were about to embark on, Deborah King’s thoughts were fixed on last-minute packing as she got out of the shower.

But as she reached for something in her bathroom cabinet, her eye was caught by an indentation on the underside of her left breast. Checking it, she was horrified to feel a few lumps – one that felt as large as a 10c coin – inside the breast as she probed it.

‘I was totally shocked – it was only two months since my mammogram, which had been all clear,’ recalls Deborah, 58, a graphic designer and mother to 15-year-old daughter Grace.

‘My mind was on the nine-day trip Grace and I had planned. We were visiting friends I hadn’t seen for nearly 20 years. I was so looking forward to it.

‘Yet here, clearly, was something worrying. I only spotted the indentation by chance because I happened to have my arms raised. When I put my arm down, I couldn’t see it.’

Though shaken by the discovery, Deborah reasoned it couldn’t be ­’anything serious’, reassuring herself with the fact there was no family history of breast cancer and the result of her most recent ­mammogram.

But she was sufficiently concerned to contact her GP, who immediately referred her for further investigations.

However, the appointment did not come through; Deborah had to wait four weeks for tests in hospital, which took place at the end of August.

This included an ultrasound scan and ­biopsies, which revealed Deborah had ­invasive ductal carcinoma (IDC) — a cancer that starts in the milk ducts and which accounts for up to 75 per cent of all breast cancers. The cancer was stage 1 – meaning it had not spread beyond the breast – but the diagnosis was a terrible blow.

Now undergoing gruelling chemotherapy and anxious about her forthcoming mastectomy, Deborah is angry that tumours, measuring 4cm in total, had been missed only two months earlier on the mammogram.

The cancer also didn’t show on two mammograms at hospital – but was clearly visible on ultrasound.

So how could this have ­happened? It transpires that ­Deborah has ‘dense breasts’ – a term doctors are familiar with, but many women are unaware of.

Crucially, having dense breasts makes it difficult for mammograms to detect small tumours 

The breast itself doesn’t feel different but contains more fibrous tissue, and less fat, than normal.

Women with dense breasts are at an increased risk of developing breast cancer. Research suggests they could be four to six times more at risk than those with low density, but it’s not known why.

Crucially, having dense breasts makes it difficult for mammograms to detect small tumours.

All breasts contain glands, fibrous tissue and fat – but dense breasts have relatively ‘more fibrous tissue than fat,’ explains Cheryl Cruwys, the founder of patient advocacy group Breast Density Matters.

The issue is that both dense breast tissue and tumours show up as white on the mammogram image – so spotting a small tumour can be ‘like looking for a cotton wool ball in a snowstorm’, she says. ‘The denser the tissue, the whiter the mammogram.’

Dr Lester Barr, a consultant ­surgeon specialising in breast cancer, adds: ‘Breast density has nothing to do with how your breast looks or feels, but relates to how your breast tissue looks on a mammogram. The density is caused by the way protein molecules align in a woman’s breasts.’

The reason ultrasound is helpful in women with dense breast tissue is that it uses a different technology, soundwaves, to detect lumps (rather like the echo finder on a submarine), which may not be apparent on X-ray technology.

Deborah was given an ultrasound after she reported a lump – and while two mammograms looked like a mass of white, and no cancer showed up, a dark mass was clearly visible on the ultrasound.

‘If I hadn’t noticed the ­indentation, it could have spread and I would be facing a very ­different prognosis,’ she says. ‘This diagnosis shocked me profoundly as I realised it meant I had been wrong to think I could trust the results of the mammogram.’

It’s not known how many women have dense breast ­tissue. As Dr Barr explains: ‘Breast ­density is a spectrum from very low to very dense. This is assessed subjectively by a radiologist looking at the degree of “whiteness” There is no accepted cut-off on the spectrum that defines when a breast becomes dense.’

Another factor is that the density of a woman’s breast tissue varies with age. Dr Barr says a woman’s breast tissue generally becomes less dense with age – while at age 30-35 almost every woman has dense breast tissue, by the age of 50 this has dropped.

‘It is likely breast density becomes clinically relevant in those with the very densest breasts from the age of 40,’ he says, adding that this group could benefit most from extra screening.

For example, Britain’s National Health Service has a screening program that picks up 16,000 breast cancers every year, says Dr Barr, but ‘there are another 1,200… “missed” by that mammogram’.

Breast density is only one factor in these missed diagnoses.

Another type of cancer difficult to detect on a mammogram is invasive lobular carcinoma – the second most common type. It starts in the glands where milk is made when breastfeeding.

Because the cancer cells can grow in lines – rather than lumps – these are not easily discernible at an early stage.

Inflammatory breast cancer, which accounts for up to five per cent of breast cancers, is also often missed by mammography. It, too, often does not cause a lump – instead, cancer cells block lymph (or drainage) vessels, causing the breast to look swollen.

Britain’s National Health Service has a screening program that picks up 16,000 breast cancers every year, but there are another 1,200 missed by the mammogram

Around a third of breast cancers in women of all ages are detected by mammogram, with the rest spotted by women themselves.

This may be because the woman is younger, so not yet eligible for ­screening, or because of dense breasts and cancers not showing up on ­mammograms. The cancer may also be an interval cancer (one that occurs in between screenings).

Every state and territory in Australia operates a BreastScreen service that invites women aged between 50 and 74 years to have a mammogram every two years.

Most BreastScreen services do not advise people of their mammographic density. BreastScreen in Western Australia and South Australia are the only two states that currently notify people if they are identified as having dense breasts when they have their screening mammograms.

The people who are notified as having dense breasts are given follow-up information, including whether any further care or investigation is necessary.

Campaigners point out that some countries, such as France and Austria, routinely offer follow-up screening (such as ultrasound) to women with especially dense breasts.

They rely on a density scoring system called BI-RADS, developed by the American College of Radiology.

Under this system, there are four categories: A, Fatty; B, Scattered areas of ­fibroglandular density; C, Heterogeneously dense; and D, extremely dense.

In France, you get a standard mammogram but if the ­radiologist considers you are in category C or D they refer you for an ultrasound.

‘However the BI-RADS system is based on subjective judgment and so offers variable accuracy – a computerised assessment of breast density is under development,’ says Dr Barr.

A 2019 UK review of breast screening options concluded there was insufficient evidence to support offering women with dense breasts an ultrasound after a negative mammogram, due to variations in the accuracy of ultrasounds.

Ultrasounds can sometimes ­produce a high number of false positive results (leading to anxiety and unnecessary investigations), and false negative results, explains Dr Barr.

‘Standard hand-held ultrasound has proved too inaccurate for general screening purposes, but it is useful in giving additional information about a lump found by mammography or in a woman coming into the clinic because she has found a lump.’

Around a third of breast cancers in women of all ages are detected by mammogram, with the rest spotted by women themselves 

What’s more, a mounting body of evidence suggests routine screening mammograms may cause more harm than good for some women – regardless of breast density – due to false positives and overtreatment, including surgery on harmless cancers that would never have caused the patient any problems, says Michael Baum, a professor emeritus of surgery and visiting professor of medical humanities at University College London.

‘In my view, routine mammogram screening should be scrapped for all women,’ he told Good Health.

Inflammatory breast cancer, which accounts for up to five per cent of breast cancers, is often missed by mammography. (Pictured: a normal mammogram of a 27-year-old patient)

Professor Baum was responsible for setting up the UK’s breast screening ­program in 1988, but now points to statistics reviewed by the ­independent Cochrane body which suggest current techniques prevent very few deaths.

‘You’d have to screen 2,000 women over a ten-year period to avoid one breast cancer death [compared with not screening the same women],’ he explains.

‘While one woman avoiding breast cancer is of enormous value, this has to be weighed against the risk of over-diagnosis and over-treatment – including needless mastectomies and even an increased risk of death from cancer treatment itself,’ he says.

However, Professor Baum, whose mother died from breast cancer, stresses that doctors must use ‘all the screening tools available’ to ­diagnose women who self-report symptoms such as pain or a lump.

As for Deborah King, who assiduously attended previous screenings, she feels betrayed by the system because no one told her she had dense breasts, or recommended she have a follow-up ultrasound.

‘No one tells you that you have dense breasts — or that if you do, tumours may not show up on a ­mammogram,’ she says.

‘I have always attended screenings and trusted the results. Now I feel my trust was betrayed. How can they call this a screening program when it is not comprehensive?’