Breast density is a known cancer risk – so why aren’t women being told?

Two years ago, Jill Jackson walked away from her mammogram with a sense of relief when it came back showing no sign of breast cancer. But six months later, the then 44-year-old found a lump. She wasn’t particularly concerned, though, because her mammogram had been clear. What the Auckland-based administrator didn’t know was that she had dense breasts, making her mammogram more difficult to read – any early sign of that cancerous lump might have been there but it was probably missed.

Talking to the Listener from her partner’s house in Tauranga, she apologises for her “scrambled brain’', which she blames on the oestrogen blocker she has to take to reduce the risk of her cancer returning.

She’s alive, yes, but she’s one of about 440 women who were diagnosed last year with an “interval cancer” – cancers found after a mammogram, before their next screening is scheduled.

Breast density is recognised as a leading cause of breast cancer: extremely dense-breasted women (7-12% of the population, although New Zealand numbers have never been measured or calculated) have double the chance of getting breast cancer compared with an average woman.

Breast density not only increases the risk a woman will get breast cancer, but also makes cancer harder to detect on a mammogram – like looking for a snowflake in a snowstorm. Though premenopausal women are more likely to have dense breasts, hormone replacement therapy and alcohol also increase density, according to recent studies. The US Mayo Clinic’s research suggests density can also be inherited.

Over the past decade, there have been calls to tell the 550,000 or so women aged 45-69 who get a free mammogram every two years about their breast density. BreastScreen Aotearoa (BSA) is consulting on whether breast density measuring and reporting should be included in our $65 million-a-year nationwide screening programme. After 25 years using the same software, a new ICT system is being rolled out early next year. An announcement on whether density will be screened will come in “due course’', says Dr Alana Ewe-Snow, prevention director at the National Public Health Service.

Internationally, there are moves to try to detect breast cancer much earlier and to offer more tailored screening rather than the current one-size-fits-all approach of our screening programme. There are calls for that here, too – the Breast Cancer Foundation thinks we’ll eventually move away from two-year screening to a system where women are screened according to risk with a broader suite of tools: abbreviated breast MRI and ultrasound for extremely dense-breasted women, for example, and 3D mammograms (tomosynthesis) for women with breasts that are heterogeneously dense (a mix of dense and fatty tissue).

Mammograms are increasingly thought to be imperfect: about four in 10 women probably need additional screening. A game changer could come from artificial intelligence (AI) – but there is caution. Overseas, AI is being used both to detect breast cancers and to assess the risk of developing breast cancer – one programme that will be trialled here if funded can spot a lesion up to five years before it shows up in the breast. AI pilots are happening in breast screening in Australia, and one to measure density has just been given ethical approval here.

Jackson had a lumpectomy last year. But two weeks later, when she was still nursing her wound, she noticed another breast lump that had not been detected on either her mammogram or the follow-up ultrasound. She was whisked in for another biopsy, which confirmed more extensive cancer, and rushed through for a double mastectomy – she wanted to get her good breast off too, so anxious was she that density in her cancer-free breast might mask further disease. Asked how it had been missed in her first screening, she says, “They told me that it wasn’t procedure to do an ultrasound at the same time as a biopsy and I also had just one ultrasound, which highlighted only the lump I could feel.’’