Evaluation of radiation dose delivered by cone beam CT and tomosynthesis employed for setup of external breast irradiation

Abstract

A systematic set of measurements is reported for evaluation of doses to critical organs resulting from cone-beam CT (CB-CT) and cone-beam tomosynthesis (CB-TS) as applied to breast setup for external beam irradiation. The specific focus of this study was on evaluation of doses from these modalities in a setting of volumetric breast imaging for target localization in radiotherapy treatments with the goal of minimizing radiation to healthy organs. Ion chamber measurements were performed in an anthropomorphic female thorax phantom at the center of each breast and lung and on the phantom surface at one anterior and two lateral locations (seven points total). The measurements were performed for three different isocenters located at the center of the phantom and at offset locations of the right and left breast. The dependence of the dose on angle selection for the CB-TS arc was also studied. For the most typical situation of centrally located CB-CT isocenter the measured doses ranged between 3 and 7 cGy, in good agreement with previous reports. Dose measurements were performed for a range of start/stop angles commonly used for CB-TS and the impact of direct and scatter dose on organs at risk was analyzed. All measured CB-TS doses were considerably lower than CB-CT doses, with greater decrease in dose for the organs outside of the beam (up to 98% decrease in dose). Remarkably, offsetting the isocenter towards the ipsilateral breast resulted on average to additional 46% dose reduction to organs at risk. The lowest doses to the contralateral breast and lung were less than 0.1 cGy when they were measured for the offset isocenter. The biggest reduction in dose was obtained by using CB-TS beams that completely avoid the critical organ. For points inside the CB-TS beam, the dose was reduced in a linear relation with distance from the center of the imaging arc. The data indicate that it is possible to reduce substantially radiation doses to the contralateral organs by proper selection of CB-TS angles and imaging field sizes. Our results provide the first systematic study on CB-TS doses from setup imaging for external breast irradiation and can be a useful resource for estimating anticipated radiation doses as a function of the conditions chosen for imaging breast setup.