Classification of breast composition by Wolfe [6], is such that in ‘fatty breasts’ almost all of the tissue appears to be fat, and less than 25 % will be fibro-glandular tissues. If a breast has scattered fibro-glandular tissue, 26–50 % volume of the breast is visible as fibro-glandular tissue. Heterogeneously dense tissue will have 51–75 % tissue. Extremely dense breasts will have more than 75 % of fibrous connective tissue [6]. Ten per cent of postmenopausal and 20 % of premenopausal women have a breast density of above 50 %. It is estimated that one in three women have a high mammographic density [7].

Further classifications of breast composition include Boyd’s [8], in which mammographic density is divided into six categories:
and Tabar [9], which classifies the mammograms into five patterns:

The BI–RADS system, (Breast Imaging Reporting and Data System), an American system used to categorise breast density and mammographic abnormalities, has been updated in 2013 to reflect a more recent and relevant method for defining breast density, seen below:

It uses a sequential method to visually define the amount of fibro glandular tissue seen within the breast as opposed to numerical [10]. It is also a widely accepted risk assessment and quality assurance assessment tool used in the USA and parts or Europe.

The sensitivity of mammography in the detection of breast cancer (‘… the number of true positives as a proportion of all those with breast cancer present’) [11] is directly related to the density of the breast tissue. Generally, mammographic sensitivity is higher in older, post-menopausal women because the breast tends to be composed of greater proportions of fatty tissue. There is some evidence to suggest that women with dense breast tissue have a higher than average risk of developing breast cancer. This is because the breast cancer may be obscured in the dense tissue [4, 5].

A high mammographic density is thought to be associated with an increased risk of breast cancer, and is estimated to account for 16 % of all breast cancers [12]. The screening population is also deemed to be at higher risk if breast density is high [13]. However, what is less clearly understood is the breast cancer risk associated with the change in breast density over time; risk increases with age and breast density, but density decreases with age. One study [14] has suggested that the evolution of the density of the breast (as a function of time) may be significant for the risk of developing breast cancer – the faster the changes that occur, the higher the risk. This study does however acknowledge a small sample size and that the cases were not randomly selected. It suggests further research into this. A further study – the PROCAS study – [15] is also presently running, with an aim to recruit 60,000 women to investigate the risk of breast cancer developing over time. This is by using density measurements between routine breast screening, and using these to analyse any risk factors in the screening population.

Nonetheless, regardless of the rate of which breast density changes, sensitivity is reduced in the dense breast [4, 5]. The images, below (Figs. 2.3, 2.4 and 2.5) illustrate a focal lesion which is barely visible in a dense breast, more easily seen in a mixed breast, and clearly seen in a fatty area of the breast.