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

Upright patient positioning may provide anatomical advantages and more cost-effective proton therapy using a fixed beamline.

To characterize image quality, imaging dose, and dose calculation accuracy for an upright CT scanner with a six-degree-of-freedom patient positioning system.

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.

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.

Upright CT image quality and dose calculation accuracy are acceptable for photon and proton radiotherapy.