As we have previously reported, “dense breasts” refers to breasts that are composed of a greater proportion of fibrous and glandular tissue compared to fatty tissue. The density can only be determined from a mammogram and cannot be felt by a woman during breast self-examination (seen right). Since dense breast tissue poses up to four times higher risk of developing breast cancer, it is important for women to know their own density status. In the USA, there is a proliferation of mandated written notifications about breast density following a routine mammogram, and yet there appears to be little understanding of the risk.
Researchers from Dartmouth College in New Hampshire set out with the objective to assess women’s perceptions of breast density compared with other breast cancer risks and to explore their understanding of risk reduction. The group’s findings have just been published in JAMA Network Open and the article is freely accessible by clicking here. The authors surveyed 1858 women, aged 40 to 76, who had recently undergone a mammography examination and had no history of breast cancer. All had heard of breast density.
The survey was designed to explore whether the women understood the relative risk of breast density compared to other well known risk factors, such as family history, being overweight or obese, drinking more than one alcoholic beverage per day, never having children, and having had a prior breast biopsy. As seen at left (© JAMA), 93% of the women saw family history as having the greatest risk, when in fact fewer than 15% of breast cancers are of genetic origin.
As seen below right (© JAMA), more than 50% believed that a prior biopsy posed a greater risk for breast cancer than dense breast tissue. A subset of 61 women participated in a telephone interview and only six identified breast density as a contributing factor to breast cancer risk. Interestingly, most of them correctly recognised that breast density could make mammograms harder to read.
While digital breast tomosynthesis (DBT) is fast replacing mammography in the USA, it can still fail to detect tumours in dense tissue and that is why follow-up imaging with ultrasound is advocated. Given that almost half of all women aged 40 and above have either heterogeneously dense or extremely dense breasts, it is obviously a concern that many women – and indeed their doctors – do not fully appreciate the risk posed by dense breast tissue. That is why the authors concluded, “Comprehensive education about breast cancer risks and prevention strategies is needed.”
Breast density as a risk factor for developing breast cancer has been blown out of proportion to its actual importance. Almost 90% of women aged 30 have dense breast tissues. By age 65 this has dropped to 50%. If breast density was a major risk, then the incidence of breast cancer would be highest among young women and drop with increasing age (just the opposite of reality). The major problem with dense breast tissues is that they can hide cancers on mammograms since cancer and fibroglandular tissue have similar x-ray attenuation.
Mammography screening can still find a large number of early cancers in women with dense breast tissue, and DBT can find even more, but the sensitivity of x-ray imaging is reduced. The misunderstanding of claimed large excess risk is likely due to the fact that cancers were likely detected earlier in women with fatty breasts so that studies that claim an increased risk simply had a higher concentration of cancers remaining in dense breasts that were missed earlier (before the data collection began) providing the misperception of increased risk.
At the Massachusetts General Hospital, in a prospective overview, 9% of women had all fat tissue patterns and accounted for 4% of the cancers while 24% had scattered fibroglandular density accounting for 23% of the cancers. Thus 33% had fatty patterns and accounted for 27% of the cancers. Over the same period, 56% of women had heterogenous patterns and 63% of the cancers while 11% had very dense patterns and accounted for 9% of the cancers. Thus women with dense patterns made up 67% of the population and accounted for 72% of the cancers. In other words, the risk is not much higher with the percent of women with various patterns having cancers at about the same rate as the percentage of women with each pattern.
Denes breast tissue can hide cancers on x-ray imaging and tests such as ultrasound and contrast studies such as MRI can detect many of these cancers. As I have said before, MRI with gadolinium is the best way to detect the most cancers at a time when cure is possible, but we should at least have ultrasound in our mammography systems (now DBT systems) as CapeRay has done.
Many thanks, as always, for your thoughtful and considered comments, Dan. I’m not sure if you’ve seen this article that’s just appeared in JAMA Network Open:
I’d be interested in your thoughts on the paper.