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Clinical Update on Breast Density
University of Vermont Medical Center 
Sally Herschorn,MD Radiologist, Director of  Breast Imaging    
What it means for Breast Cancer Screening & Risk

There are 4 breast densities described by radiologists: entirely fatty, scattered fibroglandular (these 2 are considered non-dense), heterogeneously dense and extremely dense (both considered dense)
see below.

Approximately 50% of women undergoing mammography will have dense breast tissue but the distribution varies by age. Breast density affects both the mammogram's ability to detect breast cancer and the woman's risk of developing breast cancer. Having dense breasts is the #1 reason for a missed cancer on a mammogram. A woman with heterogeneously dense breasts has 1.2-1.5 x the relative risk of developing breast cancer (compared with a woman with scattered fibroglandular tissue) and a woman with extremely dense breasts has 2.1-2.3 x the relative risk.1  
While these relative risks do not approach those of carrying a BRCA1 or BRCA2 mutation or having a biopsy that showed a high risk lesion, they are similar in magnitude to having a first degree relative with premenopausal breast cancer. 

On a population basis, having dense breasts is the most prevalent risk factor for breast cancer.1 Substituting 3D mammography for traditional 2D mammography will improve cancer detection for women with dense breasts and reduce the recall rate.2 However, the detection rate for those with extremely dense breasts is not significantly or adequately improved with 3D because of the lack of fat.3 Adding ultrasound or MRI to 3D 
mammography screening improves detection of invasive cancers in women with dense breasts. 4