Author: Austen N. Curcuru, Arash Darafsheh, Winter Green, Yao Hao, Baozhou Sun, Tiezhi Zhang, Tianyu Zhao, Xiandong Zhao 👨🔬
Affiliation: WashU Medicine, Washington University School of Medicine, Department of Radiation Oncology, Washington University School of Medicine, Washington University in St. Louis, University of South Florida, Baylor College of Medicine 🌍
Purpose: Intensity modulated proton therapy (IMPT) requires precise setup imaging due to the sharp dose gradients and rapid distal fall-off seen in proton therapy dose distributions. Additionally, online adaptive proton therapy necessitates accurate volumetric imaging and CT intensity mapping. An in-room CT-On-Rails (CTOR) can provide image quality comparable to a simulation CT scanner for patient setup verification and online daily adaptation. However, an in-room CTOR must be resilient to the neutron and gamma-ray flux generated during IMPT. This work reports the quality assurance stability of an in-room CTOR system over a period of nearly four years.
Methods: Data was collected using a Siemens Quality Control (QC) CT phantom (model 10743595) from January 2021 to November 2024 on a Siemens sliding gantry 64-slice CT scanner installed within a Mevion Hyperscan 250i proton therapy vault. Image analysis was done using the automated QC software included on the scanner. The following parameters were assessed for deviations from baseline values established during CT commissioning: slice thickness constancy, slice homogeneity, image noise, high-contrast resolution, contrast-to-noise ratio, and low-contrast resolution. All images were acquired using typical head and body mode protocols at 120 kV.
Results: No significant changes or trends in any image quality metrics were observed. Maximum deviations from baseline for slice thickness constancy were 0.48mm and 0.51mm for head and body protocols respectively. Slice homogeneity values showed a maximum deviation of 1.15 HU (head) and 1.95 HU (body). Image noise, high and low contrast resolution, and CNR likewise remained within both clinical and vendor specified tolerances.
Conclusion: CTOR image quality remained stable over the time horizon analyzed despite neutron and gamma radiation exposure from the proton therapy unit. CTOR provides a robust and safe method of obtaining simulation quality images for proton therapy.