Millions of women participate regularly in breast screening programs. Some programs are delivered at a state or provincial level, others on a per clinic basis. These programs typically use full field digital mammography as the first step to check for signs of breast cancer, evaluate individual risk, and make decisions about any additional screening modalities that might be beneficial.
There is a large body of research that has established breast density as an independent risk factor for breast cancer. Dense breasts are made up of a high proportion of fibroglandular (dense) tissue; this is the tissue in which breast cancers specifically develop. It has been shown that women with dense breasts have a four- to six-fold increased risk of developing breast cancer. Only age and genetic factors show similarly strong associations with cancer risk. Breast density impacts radiologists’ ability to effectively evaluate mammograms in the breast screening process, and relates to cancer risk in two key respects.
In addition, dense breast tissue has a significant negative impact on the performance of mammography. The dense tissue obscures tumours, micro-calcifications and other information that is vital in detecting and diagnosing cancer. Generally, the sensitivity of mammograms is approximately 80 per cent. When the breast tissue is dense, this sensitivity can decline to as low as 50 per cent.
For these two reasons, women with dense breasts (and other clinical risk factors) may be flagged for additional screening mammography or adjunctive imaging using whole breast ultrasound, MRI, or tomosynthesis.
The current clinical practice of visual assessment of breast density is not standardized, precise or reproducible. Reporting standards vary significantly from radiologist to radiologist, clinic to clinic, and between jurisdictions. There can even be differences in an individual radiologist’s assessment of density from the same mammogram over a period of time.
The accurate assessment of breast density has a wide-ranging and significant impact on the effectiveness and safety of mammography.