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

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 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 ( $$ , chest wall diameters ( $$  cm), and chest wall to nipple length ( $$ ), aligned with the axis of rotation (AOR) located at 65 cm from the focal spot to determine the $$ . Three geometries were considered - $$ -cm detector with no offset that served as reference and corresponds to a clinical CBBCT system, $$ -cm detector with 5 cm offset, and a $$ -cm detector with 10 cm offset.

For 5 cm lateral offset, the $$ ranged $$  mGy/mGy and reduction in $$ with respect to reference geometry was observed only for 18 cm ( $$ ) and 20 cm ( $$ ) diameter breasts. For the 10 cm lateral offset, the $$ ranged $$  mGy/mGy and reduction in $$ was observed for all breast diameters. The reduction in $$ was $$ , $$ , $$ , $$ , and $$ for 8, 10, 14, 18, and 20 cm 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.