Feasibility of artificial-intelligence-based synthetic computed tomography in a magnetic resonance-only radiotherapy workflow for brain radiotherapy: Two-way dose validation and 2D/2D kV-image-based positioning

Background and purpose: Magnetic Resonance Imaging (MRI)-only workflow eliminates the MRI-computed to-mography (CT) registration inaccuracy, which degrades radiotherapy (RT) treatment accuracy. For an MRI-only workflow MRI sequences need to be converted to synthetic-CT (sCT). The purpose of this study was to evaluate a commercially available artificial intelligence (AI)-based sCT generation for dose calculation and 2D/2D kV -image daily positioning for brain RT workflow. Materials and methods: T1-VIBE DIXON was acquired at the 1.5 T MRI for 26 patients in RT setup for sCTs generation. For each patient, a volumetric modulated arc therapy (VMAT) plan was optimized on the CT, then recalculated on the sCT; and vice versa. sCT-based digitally reconstructed radiographs (DRRs) were fused with stereoscopic X-ray images recorded as image guidance for clinical treatments. Dosimetric differences between planned/recalculated doses and the differences between the calculated and recorded clinical couch shift/rotation were evaluated. Results: Mean Delta D50 between planned/recalculated doses for target volumes ranged between -0.2 % and 0.2 %; mean Delta D50 and Delta D0.01ccm were-0.6 % and 1.6 % and-1.4 % and 1.0 % for organ-at-risks, respectively. Dif-ferences were tested for clinical equivalence using intervals +/- 2 % (dose), +/- 1mm (translation), and +/- 1 degrees (rota-tion). Dose equivalence was found using +/- 2 % interval (p < 0.001). The median differences between lat./long./ vert. couch shift between CT-based/sCT-based DRRs were 0.3 mm/0.2 mm/0.3 mm (p < 0.05); median differ-ences between lat./long./vert. couch rotation were -1.5 degrees/0.1 degrees/0.1 degrees (after improvement of RT setup: - 0.4 degrees/ -0.1 degrees/- 0.4 degrees , p < 0.05). Conclusions: This in-silico study showed that the AI-based sCT provided equivalent results to the CT for dose calculation and daily stereoscopic X-ray positioning when using an optimal RT setup during MRI acquisition.