Estimating Breast Mass‐Density: A Retrospective Analysis of Radiological Data

To the Editor: Radiological breast density has been widely investigated within the literature due to the reported association with breast cancer risk (1). However, measurements of radiological density do not directly correspond to the mass-density of the breast, which is difficult to assess in vivo (2). The lack of an established mass-density estimate for the female breast has led to the wide approximation of this variable within the literature, with estimates ranging from 780 kg/m (3) to 2,250 kg/m (4). Technological developments within medical imaging have enabled detailed quantitative information to be obtained regarding breast composition, specifically the proportions of fat and glandular tissue within the breast (1). Quantitative radiological data on breast composition could be combined with density values for the breast’s constituent tissues to estimate the mass-density of the breast as a whole. The reported mass-densities of human body fat and cellular matter (representing glandular tissue) are 900 kg/m (5) and 1,057 kg/m (6) respectively. It follows that the massdensity of the breast as a whole should lie between the mass-densities of these tissues, and that previously published estimates that lie outside of this range may not be appropriate. Potentially inaccurate breast massdensity estimates may have implications for several areas of breast research. For example, incorrect massdensity values may limit the accuracy of mathematical breast models used to predict breast deformation during medical imaging or surgery (7). Additionally, calculation of dependant biomechanical variables, such as breast mass, momentum, or force, may be affected by the value used to represent breast mass-density (3). This study represents the first attempt to calculate an improved breast mass-density estimate based on a retrospective analysis of radiological data. Quantitative radiological breast density data were collated from studies published in English using the Medline data base. Studies were excluded if measurements were made qualitatively or using categories; if assumptions regarding density were made; if insufficient data were presented; or if data were obtained on females aged less than 18 years. For longitudinal studies that presented more than one measurement on the same participant, the baseline data values were used for the calculation of mass-density within this study. Estimates of breast mass-density (qB) were calculated using equation 1, where A is the proportion of fat in the breast (8). It was assumed that the breast is composed entirely of fat and glandular tissue (fat: qF = 900 kg/m ; glandular tissue: qG = 1,057 kg/m ) (5,6).