Breast composition and radiographic breast equivalence

The composition of the female breast is an elusive variable in screen-film mammography that has a vital role in determining both the radiographic image quality and breast radiation dose. Despite the large volume of research in mammography only two studies known to the author define the chemical composition of the two major breast constituents, fibrous glandular tissue and adipose tissue. Unfortunately there is not complete agreement between these two studies. Upon this basis, a variety of breast phantom materials have been suggested and developed. This paper examines the radiographic equivalence of some of these materials dealing notably with formulations given by Hammerstein et al and ICRU Report 44, as well as BR12, water, and lucite. Radiographic equivalence was calculated using a computer simulation of mammographic spectra (after the example of Birch and Marshall and Tucker et al, and fitted to available FDA data) applied to attenuation data of phantom materials, in turn based on elemental linear attenuation coefficient data. A software algorithm was written that iteratively calculates the thickness of a nominated material required to match the attenuation of another material of particular thickness. The equivalence of 2-8 cm of lucite to various phantom materials was calculated for molybdenum target, molybdenum filter (Mo/Mo) x-ray beams from 25 to 35 kVp and for Mo/Rh and Rh/Rh beams from 27 to 35 kVp. It was found that for Mo/Mo beams the equivalent thickness of breast phantom material for certain thicknesses of lucite was largely independent of x-ray beam energy with the range of thickness equivalences being less than 3% at 8 cm of lucite and less than 0.5% at 2 cm of lucite. The RMI 156 breast phantom was found to be equivalent to 42 mm of Hammerstein et al. 50% glandular/50% adipose phantom material for Mo/Mo beams at 27 kVp, increasing to 43 mm for the range of Rh/Rh beams. While some questions remain on the accuracy of the present composition of breast constituents, it is clear that various breast compositions can be successfully simulated by using an appropriate thickness of lucite material. The use of lucite breast phantoms is then seen as an effective and reliable way of evaluating mammographic image quality and for calculating breast dose for varying required breast compositions.