Accuracy and inter-observer variability of 3D versus 4D cone-beam CT based image-guidance in SBRT for lung tumors (Radiation Oncology)

Background: To analyze the accuracy and inter-observer variability of image-guidance (IG) using 3D or4D cone-beam CT (CBCT) technology in stereotactic body radiotherapy (SBRT) for lungtumors.Materials and methodsTwenty-one consecutive patients treated with image-guided SBRT for primary and secondarylung tumors were basis for this study. A respiration correlated 4D-CT and planning contoursserved as reference for all IG techniques. Three IG techniques were performed independentlyby three radiation oncologists (ROs) and three radiotherapy technicians (RTTs). Imageguidanceusing respiration correlated 4D-CBCT (IG-4D) with automatic registration of theplanning 4D-CT and the verification 4D-CBCT was considered gold-standard. Results werecompared with two IG techniques using 3D-CBCT: 1) manual registration of the planninginternal target volume (ITV) contour and the motion blurred tumor in the 3D-CBCT (IGITV);2) automatic registration of the planning reference CT image and the verification 3DCBCT(IG-3D). Image quality of 3D-CBCT and 4D-CBCT images was scored on a scale of1-3, with 1 being best and 3 being worst quality for visual verification of the IGRT results. Results: Image quality was scored significantly worse for 3D-CBCT compared to 4D-CBCT: theworst score of 3 was given in 19 % and 7.1 % observations, respectively. Significantdifferences in target localization were observed between 4D-CBCT and 3D-CBCT based IG:compared to the reference of IG-4D, tumor positions differed by 1.9 mm +/- 0.9 mm (3Dvector) on average using IG-ITV and by 3.6 mm +/- 3.2 mm using IG-3D; results of IG-ITVwere significantly closer to the reference IG-4D compared to IG-3D. Differences between the4D-CBCT and 3D-CBCT techniques increased significantly with larger motion amplitude ofthe tumor; analogously, differences increased with worse 3D-CBCT image quality scores.Inter-observer variability was largest in SI direction and was significantly larger in IG using3D-CBCT compared to 4D-CBCT: 0.6 mm versus 1.5 mm (one standard deviation). Interobservervariability was not different between the three ROs compared to the three RTTs. Conclusions: Respiration correlated 4D-CBCT improves the accuracy of image-guidance by more precisetarget localization in the presence of breathing induced target motion and by reduced interobservervariability.