Evaluation and implementation of an independent Kilovoltage X-ray-based imaging platform for carbon ion radiotherapy

Image-guided particle therapy (IGPT) has significantly advanced in recent years, particularly in the context of proton therapy. However, imaging solutions for carbon-ion radiotherapy (C-ion RT) remain limited.

This study introduces sliding-gantry cone-beam computed tomography (CBCT) and dual-panel digital radiography (DR) systems, both mechanically independent of carbon-ion delivery nozzles. We aim to evaluate the image quality metrics and verify the positioning accuracy of the imaging systems.

Image quality was evaluated in terms of spatial resolution, low contrast resolution, image uniformity, and effective imaging area using a multi-purpose imaging phantom, Catphan 700 phantom, and ImageJ software. The influences of planning computed tomography (CT) slice thicknesses (1-5 mm), radiation quality settings (90-130 kV), and registration algorithms (bony, grayscale, and fiducial marker registrations) on positioning accuracy were assessed using anthropomorphic head-neck and thoracoabdominal phantom images. The clinical feasibility of both systems was validated in 22 enrolled patients.

The CBCT exhibited a lower in-plane spatial resolution (2.50 line pairs per millimeter (lp/mm)) than DR (2.80 lp/mm). Spatial resolution of the CBCT system was measured at 0.90 lp/mm using the CTP 714 module of the Catphan 700 phantom. Both systems achieved a low contrast resolution of 2.30%. DR provided superior image uniformity (1.12%-1.40%) compared with CBCT (2.20%). The effective imaging areas were comparable between the CBCT and DR systems (99.30%-99.50%). Positioning accuracy varied with planning CT slice thicknesses, radiation quality settings, and registration algorithms, showing mean translation displacements ranging from 0.01 to 0.48 mm. CBCT achieved inter-fraction translational positioning errors within 2 mm in 42.3% (22/52) of fractions and rotational positioning errors within 2° in 80.8% (42/52) of fractions, and DR achieved 33.8% (24/71) and 73.2% (52/71), respectively.

The developed CBCT and DR systems achieved superior image quality and sub-0.5 mm positioning accuracy. These findings support the clinical feasibility of integrating CBCT and DR imaging systems into the C-ion RT workflow.