Feasibility of Low-Field 0.55T MRI-Based Synthetic CT for Simulation-Free Adaptive Stereotactic Prostate Radiotherapy on Ethos with Hypersight
Abstract
Purpose
The 0.55T MRI offers potential advantages including reduced susceptibility artifacts and improved accessibility. Separately, HyperSight cone-beam CT on the Ethos enables direct dose calculation during adaptive radiotherapy. This study evaluates the feasibility of integrating 0.55T MRI-based synthetic CT (SynCT) into a simulation-free adaptive workflow for prostate stereotactic adaptive radiotherapy (STAR), with daily adaptive replanning performed on HyperSight CBCTs.
Methods
Eight patients were currently enrolled in this retrospective study. Five initial patients were used to optimize the MRI protocol for synCT generation. Three subsequent patients underwent workflow validation and dosimetric comparison. Reference treatment plans were generated on MRI-based SynCT using the Ethos 2.0 emulator. For each patient, three adaptive sessions were simulated using their daily HyperSight CBCTs. Emulator-generated adaptive plans were compared with clinically delivered sessions. Dosimetric endpoints included target coverage (D99%) and organ-at-risk (OAR) dose constraints.
Results
Nine treatment fractions were successfully simulated using the proposed workflow. Adaptive plan selection matched clinical sessions in 7 of 9 fractions. Target coverage variability (D99%) was 1.3% ± 2.5% across all target structures. OAR constraint differences averaged 0.20 ± 0.25 Gy with Dmax variability was 0.03± 0.04 Gy for bladder, 0.08±0.11 Gy for rectum and 0.03±0.03 Gy for urethra.
Conclusion
Simulation-free adaptive prostate SBRT using 0.55T MRI-based synCT for reference plan generation and Ethos HyperSight for daily adaptation is deemed feasible. Adaptive plan quality remained robust despite reference plans generated without conventional CT simulation. This workflow may streamline the treatment pathway while maintaining dosimetric accuracy for prostate STAR.