Purpose: In-room CT-on-rails (CTOR), used for patient setup, provides CT images on a par with planning CT that can be used for online and offline plan adaptation. However, the in-room CTOR system is exposed to stray radiation, mainly neutrons and prompt gamma rays during the proton beam delivery. This work reports the images quality reproducibility of the CTOR over one year time period since its installation.
Methods: Data acquisition phantom studies were conducted using a CIRS electron density (Model 062) and Catphan phantoms (Model 604). Both phantoms were scanned on a Siemens sliding gantry 64-slice CT scanner using a TwinBeam Dual Energy (Au and Sn filters), Dual Spiral Dual Energy (80 kV and 140 kV), and clinical protocol (120 kV helical) over a 1-year period since the installation of the CTOR inside of a Mevion Hyperscan proton therapy vault. Image quality was studied for these six different datasets: 80 kV, 140 kV, Au filter, Sn filter, Au & Sn filters, and clinical 120 kV. Image matrices include scaling discrepancy, geometric distortion, spatial resolution (50% MTF), uniformity, contrast, CNR, HU, and effective atomic number (Z). Electron density and Catphan phantoms images were processed by using Siemens syngo.via and Varian DoseLab software, respectively.
Results: No noticeable change in image quality matrices was observed. For example in the clinical 120 kV datasets, scaling discrepancy, spatial resolution (50% MTF), uniformity, contrast, and 20% bone HU intervals are 0.2-0.4 mm, 0.41-0.54 lp/mm, 98.7%-98.8%, 5-5.3, and 207-215, respectively. Water insert effective Z, calculated from 80 kV and 140 kV varies from 7.19 to 7.42.
Conclusion: In-room CTOR image quality did not change from the baseline after a one-year follow up. The high-quality helical CTOR images can be safely used for proton online and offline adaptation.
Dual-energy Imaging, Image Analysis, Protons