Radiation Dosimetry Of A Clinical Prototype Dedicated Cone-Beam Breast Ct System With Offset Detector

Purpose A clinical-prototype, dedicated, cone-beam breast computed tomography (CBBCT) system with offset detector is undergoing clinical evaluation at our institution. This study is to estimate the normalized glandular dose coefficients (DgNCT) that provide air kerma-to-mean glandular dose conversion factors using Monte Carlo simulations.Materials and methods The clinical prototype CBBCT system uses 49 kV x-ray spectrum with 1.39 mm 1st half-value layer thickness. Monte Carlo simulations (GATE, version 8) were performed with semi-ellipsoidal, homogeneous breasts of various fibroglandular weight fractions (fg=0.01,0.15,0.5,1), chest wall diameters (d=8,10,14,18,20 cm), and chest wall to nipple length (l=0.75d), aligned with the axis of rotation (AOR) located at 65 cm from the focal spot to determine the DgNCT. Three geometries were considered - 40x30-cm detector with no offset that served as reference and corresponds to a clinical CBBCT system, 30x30-cm detector with 5 cm offset, and a 30x30-cm detector with 10 cm offset.Results For 5 cm lateral offset, the DgNCT ranged 0.177-0.574 mGy/mGy and reduction in DgNCT with respect to reference geometry was observed only for 18 cm (6.4%+/- 0.23%) and 20 cm (9.6%+/- 0.22%) diameter breasts. For the 10 cm lateral offset, the DgNCT ranged 0.221-0.581 mGy/mGy and reduction in DgNCT was observed for all breast diameters. The reduction in DgNCT was 1.4%+/- 0.48%, 7.1%+/- 0.13%, 17.5%+/- 0.19%, 25.1%+/- 0.15%, and 27.7%+/- 0.08% for 8, 10, 14, 18, and 20 cm diameter breasts, respectively. For a given breast diameter, the reduction in DgNCT with offset-detector geometries was not dependent on fg. Numerical fits of DgNCT d,l,fg were generated for each geometry.Conclusion The DgNCT and the numerical fit, DgNCT d,l,fg would be of benefit for current CBBCT systems using the reference geometry and for future generations using offset-detector geometry. There exists a potential for radiation dose reduction with offset-detector geometry, provided the same technique factors as the reference geometry are used, and the image quality is clinically acceptable.