A comprehensive analysis of DgN(CT) coefficients for pendant-geometry cone-beam breast computed tomography

The use of a computed tomography (CT) scanner specifically designed for breast imaging has been proposed by several investigators. In this study, the radiation dose due to breast CT was evaluated using Monte Carlo techniques over a range of parameters pertinent to the cone-beam pendant geometry thought to be most appropriate. Monte Carlo dose computations were validated by comparison with physical measurements made on a prototype breast CT scanner under development in our laboratory. The Monte Carlo results were then used to study the influence of cone angle, the use of a beam flattening ("bow-tie") filter, glandular fraction, breast length and source-to-isocenter distance. These parameters were studied over a range of breast diameters from 10 to 18 cm, and for both monoenergetic (8-140 keV by 1 keV intervals) and polyenergetic x-ray beams (30-100 kVp by 5 kVp intervals. Half value layer at 80 kVp = 5.3 mm Al). A parameter referring to the normalized glandular dose in CT (DgN(CT)) was defined which is the ratio of the glandular dose in the breast to the air kerma at isocenter. There was no significant difference (p = 0.743) between physically measured and Monte Carlo derived results. Fan angle, source-to-isocenter distance, and breast length have relatively small influences on the radiation dose in breast CT. Glandular fraction (0% versus 100%) for 10 cm breasts at 80 kVp had approximately a 10% effect on DgN(CT), and a 20% effect was observed for an 18 cm breast diameter. The use of a bow-tie filter had the potential to reduce breast dose by approximately 40%. X-ray beam energy and breast diameter had significant influence on the DgN(CT) parameters, with higher DgN(CT) values for higher energy beams and smaller breast diameters. DgN(CT) values (mGy/mGy) at 80 kVp ranged from 0.95 for an 8 cm diam 50% glandular breast to 0.78 for an 18 cm 50% glandular breast. The results of this investigation should be useful for those interested computing the glandular breast dose for geometries relevant to dedicated breast CT.