The 2012 CTRC-AACR San Antonio Breast Cancer Symposium, 4-8 December
Mammographic density and the risk and detection of breast cancer
Dr Norman Boyd – University of Toronto, Canada
Mammograms in normal healthy women show variation in their appearance and they vary in the amount of white tissue that one can see and the amount of black tissue. So the black tissue essentially is reflecting fat in the breast and the white tissue is reflecting what we call fibroglandular tissue, the glandular ductal structures that make milk and the supporting tissue around them. So these variations vary quantitatively; some women have a breast that’s almost entirely white, mostly fibroglandular tissue, some have a breast that’s almost entirely dark or almost entirely fat and these variations, we’ve found, are related, strongly related, to the risk of future breast cancer.
What are the implications of this for developing breast cancer?
The risk of breast cancer in people with extensive density is about five times higher than that of women with little or no density. This is stronger than most other risk factors that we know for the disease, like parity and age at menarche, age at menopause and so on, which have risk ratios of around two. So it’s a very strong risk factor, it’s also common, so a lot of women have it and that means it may account for a large fraction of the disease. The current estimates are about a third, perhaps, of breast cancer might be explained by this one risk factor. The third reason it’s interesting is that it can be changed, so a drug like Tamoxifen can reduce it as it reduces breast cancer risk; a drug like hormone therapy, combined hormone therapy, can increase it and, as we know, that’s associated with a small increase in breast cancer risk. So these drugs, I’m not suggesting that we would use them in this way, but they illustrate the principle that changing this risk factor may change the risk of disease subsequently. So there’s a lot of research going on as to find out how to change it and what is the meaning of change and can it be exploited in this way as a biomarker by which one can change the risk of future disease.
30% of breast cancers occurring in the population may be explained by this one risk factor so the frequency of the risk factor varies with age. The average level of density, if we took, say, fifteen year olds, as we’ve done, and examined their breasts using not X-rays but magnetic resonance without any X-rays, the average level of density in them is about 50%. If you examine women of age eighty, the average level of density is about 15%, so density goes down, the average goes down as people get older. But at all ages there is huge variation so you can find eighty year olds in whom the breast is still completely dense and you can find twenty year olds in whom there’s no density at all.
What are the implications for diagnosis?
Density, as you might expect, makes it more difficult for a radiologist to see breast cancer so the density has some effects on detection and detection is less efficient in people with extensive density than in those without. It’s become common practice among at least some radiologists to refer women with extensive density for additional imaging modalities – ultrasound or magnetic resonance if it’s available.
How will using an MRI fit in with breast density?
MRI is much less likely to be deceived, as it were, by the presence of density in the breast. Cancers that can be missed by mammography may be detected by MRI.
What advice would you give women doing regular screenings with a dense breast?
Talk to a radiologist. Not all facilities have access to MRI, or easy access to it, and the type of ultrasound that’s available also varies a great deal geographically so it depends on the resources that are available and, to some extent, on the experience of the personnel running the screening units.
