About 40 percent of women in the U.S. over age 40 have dense breast tissue, which makes it harder to identify cancer cells on a mammogram. That’s because the breast tissue appears dense on a mammogram compared with fat, which appears nondense.
As a result, 28 states — including California — have passed laws requiring health facilities to notify women when they have dense breasts. Proponents of the law say women can then decide if they want to take further action and undergo other imaging techniques, such as an ultrasound or MRI.
But critics say the laws cause women increased anxiety and cost because there is no consensus if the routine supplemental screening is worthwhile.
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The UCSF-led study evaluated the risk factors of more than 200,000 women ages 40 to 74. About 18,000 of the participants had different stages of breast cancer, while the rest did not.
The researchers found that breast density was the most prevalent risk factor for the disease, and that 39.3 percent of breast cancers in premenopausal women and 26.2 percent in postmenopausal women might have been prevented if all women with higher breast density had been shifted to a lower density category.
The researchers also found that women with a high body mass index have lower breast density, though Engmann said age is also a strong determinant of breast density.
Dense breasts are more common in younger women, said Engmann, and most women experience a sharp decline during menopause that continues in the postmenopausal period.
However, she noted that postmenopausal estrogen and progestin therapy can reverse the decline of breast density with age.
Tamoxifen, an estrogen hormone blocker, is the only treatment known to substantially reduce breast density, and thus the risk of breast cancer. But because the drug can have serious side effects, it is usually recommended only for women at high risk of breast cancer, with guidance from their physician.
“Our study highlights the need for new interventions to reduce breast density for women at average risk,” Engmann said.
At the Fremont-based Cancer Prevention Institute of California, cancer epidemiologist Dr. Ingrid Oakley-Girvan was intrigued by the UCSF findings.
She called the study “exciting because it provides further evidence we should continue investigating ways to reduce the risk of breast cancer.”
For example, along with some of her colleagues, Oakley-Girvan recently submitted a proposal to the U.S. Department of Defense to evaluate whether a nutritional soup product reduces inflammation in the breasts of women with early disease.
“Perhaps we should also consider expanding that work to include women with dense breasts,” noted Oakley-Girvan.
At UCSF, Engmann said that even though one link between dense breasts and breast cancer suggests that carrying extra pounds may protect women against the disease, since overweight and obese women are more likely to have fatty breasts, the study reinforced previous research that has established a link between high body mass index and increased breast cancer risk in postmenopausal women.
The researchers said that almost 23 percent of breast cancers in this group could have been averted if obese and overweight women attained a body mass index of less than 25, the equivalent of 155 pounds for a woman who is 5 feet 6 inches.
By comparison, the study found that factors commonly associated with breast cancer risk — a history of benign breast biopsy, a first-degree family history of breast cancer, and deferring childbirth until after 30 — each accounted for less than 10 percent of cases in the population.
The study was supported by a National Cancer Institute-funded program.