Use of Partial Cone Beam CT (pCBCT) Reduces Total Setup Images and Estimated Radiation Exposure for Breast Radiation Verification Simulation

Abstract

Electronic portal imaging (EPI) is standardly employed for patient setup verification in breast radiotherapy (RT). However, such imaging can be associated with substantial radiation exposure, particularly when repeated imaging is required to optimize patient positioning. This study aims to evaluate the impact of an institutional protocol using partial cone beam computer tomography (pCBCT) in addition to standard EPI on overall imaging-related radiation exposure during verification simulation (vsim) in breast RT. In 3/2016, our institution implemented a protocol for vsim in breast RT that newly utilized kilovoltage pCBCT for alignment at isocenter prior to obtaining standard megavoltage EPI. Patient demographic and treatment data including number of treatment fields, technique, laterality, supine vs. prone positioning, and use of breath hold procedure was collected for 2 temporal cohorts: pre-implementation of the protocol from 12/2013-2/2016, and post-implementation from 3/2016-4/2018. Number of EPI for each field taken and corresponding monitor unit (MU) settings were recorded for each patient’s vsim. Chi square tests compared the percentage of patients who received ≥2 double exposure EPI (one open and one shaped field) per treatment field during vsim between cohorts. Overall radiation exposure from all imaging modalities used during vsim was estimated. Data from 768 vsims for 3D conformal breast cancer treatments were collected from 12/2013 to 04/2018. 330 underwent vsim with EPI guidance only, and 440 were underwent vsim following implementation of the protocol with the use of pCBCT in addition to EPI. The addition of pCBCT was associated with significantly fewer patients receiving ≥2 EPI per field during vsim as compared to the pre-pCBCT cohort (35% vs. 64%, respectively, p<0.001). Most patients (94%) required only 1 pCBCT. The mean number of EPI per field was 2.9 in pre-pCBCT cohort and 2.4 with use of pCBCT protocol. Number of treatment fields, breast laterality, supine vs. prone positioning, and use of breath hold procedure were not significantly associated with ≥2 EPI per field; all subgroups required fewer images per field with the addition of pCBCT. Given that a single pCBCT confers an estimated scan dose of 1.8cGy and a double exposure EPI with 2 MU per field confers 2 cGy, we estimated that use of the CBCT protocol reduced imaging-related radiation exposure by at least 2.2cGy for 2-field, 4.2cGy for 3- field, and >6.2 cGy for >3 field treatments across patients during vsim. Our study demonstrates that the addition of pCBCT to vsim procedures may reduce the number of EPI required for patient setup in breast RT. These data support implementation of such CBCT protocols as a means to streamline setup and reduce imaging-related radiation exposure in this patient population.