Evaluation of Magnetic Resonance Imaging-Derived Synthetic Computed Tomography in Prostate Cancer Carbon Ion Radiotherapy: Impact of Fixation Shell

Purpose: Magnetic resonance imaging (MRI) offers superior soft-tissue contrast for prostate delineation, and MR-only radiotherapy eliminates computed tomography (CT)-MRI registration errors. Although synthetic CT (sCT) has been validated for photon and proton therapy, its clinical feasibility in carbon ion radiotherapy (CIRT) remains unclear, particularly because dose accuracy in CIRT is highly sensitive to stopping-power ratio (SPR) errors. This study evaluated the dosimetric accuracy of MRI-derived sCT for prostate CIRT, including the effect of a thermoplastic fixation shell used clinically but not visualized on MRI. Methods: Twenty patients with prostate cancer underwent same-day planning CT (pCT) and MRI. sCT images were generated using the MRCAT algorithm (Philips Healthcare). Bone pseudo-Hounsfield units were replaced with institution-specific SPR values to improve range accuracy. Treatment plans were created on the pCT and recalculated on the sCT using identical beam parameters. Dose agreement was assessed using carbon-ion depth and lateral dose profiles, 3D gamma analysis (3%/3 mm, 2%/2 mm, 1%/1 mm), and key Dose-volume histogram indices. To evaluate the fixation-shell impact, a 3-mm virtual shell was added to the sCT and compared with a corresponding pCT containing the physical shell. Results: The sCT-based distal range was longer than that of the pCT by 2.7 ± 1.6 mm, reduced to 1.2 ± 1.6 mm with shell inclusion. Lateral dose differences were