Glandular dose indices using a glandular dose to air kerma volume histogram in mammography

To develop a dose evaluation for heterogeneous glandular dose distributions by using glandular dose indices obtained from a glandular dose to air kerma (GD/K) volume histogram in addition to the existing mean glandular dose (MGD) in mammography.

The mammographic x-ray fluence spectra were created using the EGSnrc/BEAMnrc Monte Carlo code. The combinations of the target and filter were Mo-Mo and W-Rh. The breast phantom was a half-ellipsoidal form, composed of adipose and glandular tissues, and a 1.5-mm-thick outer skin. The compressed breast thicknesses (CBT) were 2, 4, and 6 cm with a glandularity of 15%. Two types of breast voxel phantoms with homogeneous and realistic heterogeneous glandular distributions were modeled. The glandular dose distributions in both breast voxel phantoms were calculated using the simulated x-ray fluence spectra. The GD/K volume histograms for the glandular dose were presented by a relative glandular volume (%) as a function of the ratio of glandular dose (GD) and air kerma (K). Finally, the dose indices of MGD, GD_2% /K (GD_2% the dose covered by a 2% glandular volume), and V_MGD (glandular volume percentage (%) receiving at least MGD), homogeneity index (HI) were obtained from the glandular dose distributions and GD/K volume histograms. The HI indicates the uniformity of GD distributions.

The MGD in a standard heterogeneous phantom (CBT: 4 cm, glandularity: 15%) was 25%-37% lower than that in a standard homogeneous phantom. The lower the tube voltage, the larger was the difference between GD_2% and MGD. The larger the CBT, the greater is the nonuniformity of the glandular dose distribution. The HI values for heterogeneous phantoms were higher than those for homogeneous phantoms. Values of V_MGD were 35%-47% and increased slightly with the tube voltage. Values of V_MGD for heterogeneous phantoms were smaller than those for homogenous phantoms at the same tube voltage.

The GD distribution differs depending on the CBT and tube voltage, even if the MGD has the same value. The new GD indices proposed in this study are useful for the dose evaluation of the heterogeneous GD distributions.