What is breast density? The denser the material is, the lighter it appears on an X-ray. Conversely, the less dense the material is, the darker it appears. On a breast X-ray — a mammogram — dark areas correspond to fat in the breast. Lighter areas correspond to denser, fibrous and glandular tissues. (Your breasts, in other words, are one place where fat seems to be good for you.)
What determines breast density? Mostly your genes. Other important factors include your stage of life (density goes down if you have children and goes down even further after menopause) and your ethnicity (on average, Asian women have denser breasts than other women).
Why is cancer risk higher in dense breasts? No one knows exactly. One idea is simply that dense breasts (of the same size and age as less-dense breasts) have more cells and therefore more chances for things to go wrong. Another theory is that cancer grows better in dense tissue cells than in other cells. There are other theories too.
Why is it harder to find cancer in dense breasts? Cancerous tissue appears light, just like other dense tissues. So the denser the breast, the harder it may be to see any cancer in it. But evidence shows that the increased breast cancer risk associated with density cannot be explained by the mere fact that it's harder to find.
How common are dense breasts? The Breast Imaging Reporting and Data System (or BI-RADS) has four density categories, with 1 being predominantly fat to 4 being extremely dense. There aren't many 1s and there aren't many 4s, says William Barlow, senior biostatistician at Cancer Research and Biostatistics in Seattle. "Most women are 2s and 3s." The risk differential is greatest between 1s and 4s, of course. But, Barlow says, "the risk of 3s versus 2s is still enough to notice the difference."
If high density leads to high risk, why is risk highest after menopause, when density is lowest? You'd think it would be the other way around. The theory goes that risk keeps adding up over time. So if, say, your breast density starts out very high (e.g., a 4) but goes down after menopause (e.g., to a 3), your risk will still go up after menopause — it just won't go up as much as it would if your density stayed at a 4.
What risk factors are included in the Breast Cancer Risk Assessment Tool? Factors included are medical history of breast cancer, age, age at your first menstrual period, age when you had your first child, family history of breast cancer, biopsy history (Have you had any? What were the results?), and race or ethnicity. Some scientists believe breast density should be added, but this has not happened yet.