Author: Yussuf A. Abdelal, Junguo Bian, Michael Delafuente, Sebastien A. Gros, Hyejoo Kang, Luke Layman, M Mahesh, John C. Roeske 👨🔬
Affiliation: Department of Radiology and Medical Imaging, Stritch School of Medicine, Loyola University Medical Center, Department of Radiation Oncology, Stritch School of Medicine, Loyola University Chicago, Johns Hopkins Univ 🌍
Purpose: This study evaluates the feasibility of optimizing 4DCBCT imaging protocols for gated lung SBRT, by reducing dose while maintaining the image quality required for accurate patient alignment.
Methods:
Four 4D_CBCT protocols (Thorax100, Thorax75, Thorax50, Thorax25) with progressively lower dose reductions of 25%, 50%, and 75% were assessed. Imaging dose was measured using weighted Cone Beam Dose Index (CBDIw) for all protocols and compared with the 3D Thorax protocol. 4D_CBCT Images were acquired using a programmable motion phantom with spherical targets (2 cm, 1.5 cm, 1 cm, 0.5 cm) and a sinusoidal motion pattern (2 cm amplitude). Three Radiation Therapists used a 1-5 Likert scale to evaluate gated Average Intensity Projection-AIP and Maximum Intensity Projection-MIP for image quality. Sensitivity analysis of acceptability thresholds (2, 3, 4) was incorporated to assess protocol performance across varying criteria for clinical acceptability, with 3.5 as acceptable threshold for clinical use.
Results:
4D_CBCT protocols Thorax100 (11.50 mGy) and Thorax25 (3.35 mGy) delivered doses 148.9% higher and 27.5% lower than 3D Thorax protocol (4.62 mGy). AIP consistently scored higher than MIP across all protocols, with the largest differences observed for smaller targets (0.5 cm). Larger targets (2 cm, 1.5 cm) achieved higher acceptability percentages across all protocols. At the 3.5 threshold, Thorax100 and Thorax75 maintained high acceptability rates for AIP (100% and 90%, respectively), while Thorax50 and Thorax25 showed reduced acceptability (75% and 60%, respectively). MIP consistently underperformed AIP, particularly for smaller targets and lower-dose protocols, with acceptability rates below 70% for Thorax50 and Thorax25. Sensitivity analysis revealed significant declines in acceptability at higher thresholds, particularly for MIP reconstructions and lower-dose protocols.
Conclusion:
Preliminary results indicate that lower-dose 4D CBCT protocols, such as Thorax25 and Thorax50, are feasible for gated lung SBRT, offering significant dose reductions while maintaining clinically acceptable image quality, particularly for larger targets.