ESTRO 2025 - Abstract Book

S3193

Physics - Inter-fraction motion management and offline adaptive radiotherapy

ESTRO 2025

4590

Digital Poster Computed tomography number characterization and dose calculations using an on-board imager and reconstruction algorithm for c-arm linacs Kenneth W Gregg 1 , Theodore Arsenault 1,2 , Atefeh Rezaei 1 , Rojano Kashani 1 , Lauren Henke 1,3 , Alex Price 1,3 1 Radiation Oncology, University Hospitals Seidman Cancer Center, Cleveland, USA. 2 Biomedical Engineering, Case Western Reserve University, Cleveland, USA. 3 School of Medicine, Case Western Reserve University, Cleveland, USA Purpose/Objective: Current cone-beam computed tomography (CBCT) on-board c-arm linacs lack CT number precision sufficient for accurate dose calculation due to increased scatter from the cone beam geometry. Recent advancements in CBCT imaging panel hardware and reconstruction algorithms show potential for direct-to-unit dose calculation. In this investigation, we evaluated the CT number and dose calculation accuracy in-phantom on an on-board CBCT unit. Material/Methods: 8 head (20-cm diameter cylindrical) and 8 body (30x40-cm diameter elliptical) configurations of an electron density phantom using 16 materials were imaged with a clinical CT-simulator and CBCT (Varian TrueBeam v4.1, HyperSight) imager using four vendor-supplied and one user-customized techniques. CBCT acquisitions were reconstructed using conventional Feldkamp-Davis-Kress (FDK) and iterative CBCT with metal artifact reduction (Acuros iCBCT MAR). The FDK and iCBCT-MAR reconstructions of each acquisition were used to create a CT number vs electron density curve for treatment planning system dose calculations. CBCT images of heterogeneous, anthropomorphic head and thorax phantoms were acquired, and three treatment plans were generated per phantom emulating single brain met (SRS), hippocampal avoidance whole bran radiotherapy (HAWBRT), head and neck (H&N) IMRT, Lung SBRT, Spine SBRT, and Rib SBRT treatment plans. All image datasets were registered per phantom, and dose was recalculated. 3D gamma analysis (Python 3.9, Library: PyDicom) was performed (10% dose threshold, local, 1% dose difference, 2mm distance to agreement), and PTV D98% was evaluated on each treatment plan for target coverage comparisons. Results: CT numbers for materials ≤1.08g/cc showed high agreement between CT-simulator and CBCT acquisitions reconstructed with Acuros iCBCT-MAR, and depreciating precision and agreement in materials >1.08g/cc. The FDK reconstruction method showed worse agreement and precision in CT number across all materials compared to Acuros iCBCT-MAR. Plans calculated on CBCT reconstructed with Acuros iCBCT-MAR compared to CT-simulator showed minimum 97.8% voxels passing in H&N, and maximum deviation of -3.3% PTV D98% in lung SBRT. As illustrated in Figure 1 and presented in Table 1, plans calculated on FDK-reconstructed CBCT compared with CT simulator worst-case showed 83% gamma passing rate in spine SBRT and maximum deviation of -11.7% PTV D98% in lung SBRT.