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 ((Formula presented.)) 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 ((Formula presented.), chest wall diameters ((Formula presented.) cm), and chest wall to nipple length ((Formula presented.)), aligned with the axis of rotation (AOR) located at 65 cm from the focal spot to determine the (Formula presented.). Three geometries were considered – (Formula presented.) -cm detector with no offset that served as reference and corresponds to a clinical CBBCT system, (Formula presented.) -cm detector with 5 cm offset, and a (Formula presented.) -cm detector with 10 cm offset. Results: For 5 cm lateral offset, the (Formula presented.) ranged (Formula presented.) mGy/mGy and reduction in (Formula presented.) with respect to reference geometry was observed only for 18 cm ((Formula presented.)) and 20 cm ((Formula presented.)) diameter breasts. For the 10 cm lateral offset, the (Formula presented.) ranged (Formula presented.) mGy/mGy and reduction in (Formula presented.) was observed for all breast diameters. The reduction in (Formula presented.) was (Formula presented.), (Formula presented.), (Formula presented.), (Formula presented.), and (Formula presented.) for 8, 10, 14, 18, and 20 cm diameter breasts, respectively. For a given breast diameter, the reduction in (Formula presented.) with offset-detector geometries was not dependent on (Formula presented.). Numerical fits of (Formula presented.) were generated for each geometry. Conclusion: The (Formula presented.) and the numerical fit, (Formula presented.) 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.