Indices for the evaluation of glandular dose heterogeneity in full-field digital mammography

This study aims to evaluate the indices of glandular dose heterogeneity in full-field digital mammography. The distributions of GD in a breast phantom with a skin layer of 4 mm were determined using the Monte Carlo method with simulated x-ray fluence spectra. First, the GD to air kerma (GD/K_air) volume histogram was obtained from the GD distributions, which were indicated by the glandular volume (%) as a function of GD/K_air. The GD indices, namely, the maximum glandular dose (GD_2%) and glandular volume percentage (%) receiving at least the mean glandular dose (MGD) (V_MGD) were calculated from the GD/K_airvolume histogram. Next, the scatter plots of GD_2%/MGD andV_MGDwere drawn as functions of the normalised mean glandular dose (DgN). Finally, (GD_2%)and (V_MGD)were obtained from the relationship between the GD indices and DgN for 596 clinical irradiation cases based on individual irradiation conditions. The values of GD_2%/MGD were more affected by breast thickness than glandularity and tube voltage, and they decreased according to the power law of DgN for all the target/filter combinations. The values ofV_MGDwere proportional to DgN and decreased with increase in the compressed breast thickness. The values of (MGD)and (GD_2%)for 596 clinical irradiation cases were estimated to range from 0.6-3.0 mGy to 1.1-7.0 mGy, respectively, and (V_MGD)was in the range 32%-48%. (GD_2%)and (V_MGD)are mainly affected by breast thickness. These indices are useful for the evaluation of glandular dose heterogeneity in mammography.