Radiation dosimetry of a clinical prototype dedicated cone-beam breast CT system with offset detector.

Abstract

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 ( ) that provide air kerma-to-mean glandular dose conversion factors using Monte Carlo simulations. The clinical prototype CBBCT system uses 49kV x-ray spectrum with 1.39mm 1st half-value layer thickness. Monte Carlo simulations (GATE, version 8) were performed with semi-ellipsoidal, homogeneous breasts of various fibroglandular weight fractions ( , chest wall diameters ( cm), and chest wall to nipple length ( ), aligned with the axis of rotation (AOR) located at 65cm from the focal spot to determine the . Three geometries were considered - -cm detector with no offset that served as reference and corresponds to a clinical CBBCT system, -cm detector with 5cm offset, and a -cm detector with 10cm offset. For 5cm lateral offset, the ranged mGy/mGy and reduction in with respect to reference geometry was observed only for 18cm ( ) and 20cm ( ) diameter breasts. For the 10cm lateral offset, the ranged mGy/mGy and reduction in was observed for all breast diameters. The reduction in was , , , , and for 8, 10, 14, 18, and 20cm diameter breasts, respectively. For a given breast diameter, the reduction in with offset-detector geometries was not dependent on . Numerical fits of were generated for each geometry. The and the numerical fit, 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.