Mammographic Breast Density and Utility in Breast Cancer Screening and Clinical Decision-Making

Mammographic breast density (MBD), the proportion of the breast composed of fibroglandular tissue has become a subject of contention, advocacy, and legislations over the last few decades. This chapter provides background information on MBD and examines its clinical usefulness in breast cancer risk assessment, early detection through to prognosis, and outcome assessment. Evidence demonstrates that MBD increases breast cancer risk and that age and body mass index influence the relationship between MBD and risk. It also shows that high MBD contributes to between 17% and 50% of interval breast cancers by providing cells and tissues that support breast cancer development, growth, and progression or the masking of cancers in mammograms. Furthermore, extremely dense breasts limit the sensitivity of mammography as a screening tool. Supplemental imaging modalities detect small and invasive cancers that may be concealed in mammograms, potentially allowing for earlier treatment. Supplemental screening tools such as ultrasound, digital breast tomosynthesis (DBT), molecular breast imaging, and magnetic resonance imaging (MRI) detect cancers in women with dense breasts whose mammograms were normal. Whilst DBT reduces the recall rate, ultrasound and MRI increase the recall rate and false-positive recall to assessment clinics. Available data validate DBT and ultrasound as assessment tools for lesions recalled by mammography, demonstrating their effectiveness in identifying both invasive cancers and lesions <1 cm. It also suggests that ultrasound and DBT can reduce the benign biopsy rate. High MBD has also been shown to increase the risk of contralateral breast cancer and recurrence. Importantly, MBD is potentially modifiable, and its reduction is associated with reduced risk of metastasis, contralateral breast cancer, and breast cancer recurrence. MBD information is clinically useful for breast cancer risk assessment and selection of imaging intervals and pathways; MBD can also support the prediction of adverse events from breast cancer.