Computed tomography sequence integration for enhanced proton therapy in patients with metallic implants

Objective. Metallic implants introduce significant uncertainties in proton therapy by distorting computed tomography (CT) images and compromising dose calculations. This study evaluated single-energy CT (SECT), dual-energy CT (DECT) with iterative metal artifact reduction (IMAR) and megavoltage CT (MVCT) to optimize imaging and dosimetry during pencil beam scanning proton therapy.Approach. Controlled phantom studies were conducted using titanium and cobalt alloy spinal rods and a titanium plate embedded in solid water. Imaging protocols included SECT with MAR, DECT with IMAR and MVCT. DECT datasets were post-processed to generate virtual monoenergetic images (VMI), relative electron density (RED) and stopping power ratio (SPR) images. Geometric accuracy was assessed using the Dice similarity coefficient, the Hausdorff distance and the mean distance to agreement. CT number calibration was performed for each modality. Proton range and dose accuracy were validated using Gafchromic EBT3 films and ionization chamber arrays. Optimal protocols were applied to complex implants involving dental posts, a dental crown, spinal screws and hip prostheses. Treatment plans were generated in RayStation using robust optimization and range errors were compared between single field uniform dose (SFUD) and intensity modulated proton therapy (IMPT).Main results. MVCT achieved the highest geometric fidelity (Dice = 0.89, distance-to-agreement = 0.19 mm). SECT provided the most accurate density estimate for titanium (4.34 g cm^-3) and RW3 (1.01 g cm^-3). SPR or RED with IMAR yielded superior dose accuracy (γ>98%, dose deviation⩽1.1%, range error ≈ 1.1 mm). With complex implants, range errors varied between 0.8 mm (titanium stem) and 7.6 mm (cobalt screw head). SFUD plans demonstrated better range control than IMPT.Significance. SPR or RED, with IMAR, enabled accurate dose and range estimations for small titanium implants. For larger or high-Zimplants, combining MVCT with SPR or RED, with IMAR, improved treatment precision due to their higher geometric and dosimetric accuracies, respectively.