Technical assessment of a novel vertical CT system for upright radiotherapy simulation and treatment planning

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Abstract Background Upright patient positioning may provide anatomical advantages and more cost‐effective proton therapy using a fixed beamline. Purpose To characterize image quality, imaging dose, and dose calculation accuracy for an upright CT scanner with a six‐degree‐of‐freedom patient positioning system. Methods CT imaging dose (CTDI vol ) was measured at 120 kVp for head and thorax protocols. Image quality was evaluated using an ACR‐464 phantom. Mean CT number accuracy was assessed within inserts of known material, and uniformity as the difference in values at the center and periphery of uniform phantoms. High‐contrast resolution was assessed by visible line pairs and modulation transfer function (MTF). Low‐contrast performance was quantified by contrast‐to‐noise‐ratio (CNR). Spatial integrity was evaluated between fiducials 100 mm apart. Hounsfield unit to mass density and stopping‐power‐ratio calibrations were performed. Proton and photon plans were optimized on upright CT scans of a thorax phantom in heterogeneous and homogeneous regions. Dose was forward computed on a registered recumbent CT scan and agreement evaluated using 3D gamma analysis. Results CTDI vol was 23.5 ± 0.02 mGy for the 16 cm head and 10.1 ± 0.01 mGy for the 32 cm body phantoms. Mean CT numbers (HU) were within the expected range for water (1.7) and acrylic (120.8). CT numbers were slightly [5–27 HU] out‐of‐range for air (−950.4), polyethylene (−78.8), and bone (823.0). Image uniformity was 20.2 HU and 35.0 HU for 20 and 48 cm diameter phantoms, respectively. Eight high‐contrast line pairs were visualized. The MTF equaled 4.4 cm −1 at 50% and 7.1 cm −1 at 10%. The median CNR was 0.93, below the ≥1.0 tolerance. Spatial integrity was ≤0.36 mm. Gamma pass rates were ≥99.8% for photon and ≥90.6% for proton plans with 1%/1 mm criteria, and ≥98.0% for all plans with 3%/2 mm criteria. Conclusion Upright CT image quality and dose calculation accuracy are acceptable for photon and proton radiotherapy.

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