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

Abstract

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 ( ${\mathrm{DgN}}^{\mathrm{CT}}$ ) 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 ( $f_g=0.01,0.15,0.5,1)$ , 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 ${\mathrm{DgN}}^{\mathrm{CT}}$ . Three geometries were considered - $40\times 30$ -cm detector with no offset that served as reference and corresponds to a clinical CBBCT system, $30\times 30$ -cm detector with 5 cm offset, and a $30\times 30$ -cm detector with 10 cm offset.

Results: For 5 cm lateral offset, the ${\mathrm{DgN}}^{\mathrm{CT}}$ ranged $0.177-0.574$ mGy/mGy and reduction in ${\mathrm{DgN}}^{\mathrm{CT}}$ with respect to reference geometry was observed only for 18 cm ( $6.4\%\pm 0.23\%$ ) and 20 cm ( $9.6\%\pm 0.22\%$ ) diameter breasts. For the 10 cm lateral offset, the ${\mathrm{DgN}}^{\mathrm{CT}}$ ranged $0.221-0.581$ mGy/mGy and reduction in ${\mathrm{DgN}}^{\mathrm{CT}}$ was observed for all breast diameters. The reduction in ${\mathrm{DgN}}^{\mathrm{CT}}$ was $1.4\%\pm 0.48\%$ , $7.1\%\pm 0.13\%$ , $17.5\%\pm 0.19\%$ , $25.1\%\pm 0.15\%$ , and $27.7\%\pm 0.08\%$ for 8, 10, 14, 18, and 20 cm diameter breasts, respectively. For a given breast diameter, the reduction in ${\mathrm{DgN}}^{\mathrm{CT}}$ with offset-detector geometries was not dependent on $f_g$ . Numerical fits of ${\mathrm{DgN}}^{\mathrm{CT}}\left(d,,,l,,,f_g\right)$ were generated for each geometry.

Conclusion: The ${\mathrm{DgN}}^{\mathrm{CT}}$ and the numerical fit, $Dg{N}^{\mathrm{CT}}\left(d,l,f_g\right)$ 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.

Keywords: Monte Carlo; breast CT; breast cancer; mean glandular dose; offset detector; radiation dose; truncated detector.