Step-by-step assessment of MRI-only workflow in pelvic radiotherapy: feasibility and practical implementation

Magnetic resonance imaging-only (MRI-only) radiotherapy workflow capitalizes on the superior soft tissue contrast of MRI while eliminating computed tomography-MRI (CT-MRI) registration uncertainties. Most existing studies focus on optimizing individual components of this workflow, i.e., synthetic CT (sCT) generation, dose calculation accuracy, and the reliability of MRI-based patient positioning, while often neglecting comprehensive evaluation of the whole clinical process and its interdependent technical requirements.

A total of 55 pelvic cancer patients, who underwent the standard radiotherapy workflow based on CT-MRI co-registration and involving a variety of imaging scanners, treatment planning systems (TPSs), and linear accelerators (LINACs), were included. For each patient, a fully integrated MRI-only approach was developed through a step-by-step evaluation of the different radiotherapy phases, including MRI-alone delineation, sCT generation with dose calculation, and cone beam CT-MRI (CBCT-MRI)-guided positioning. Comparative evaluations with the standard workflow were performed for target segmentation (dice similarity coefficient [DSC] and Hausdorff distance [HD]), dose calculation (DVH parameters and gamma analysis), and daily patient positioning (CBCT-CT versus CBCT-MRI registrations).

The proposed MRI-only workflow is applicable throughout all radiotherapy phases and compatible across multiple imaging systems, treatment planning systems, and linear accelerators. MRI-only delineation showed excellent agreement for the prostate (DSC = 0.948 ± 0.028) and cervix (DSC = 0.940 ± 0.015), whereas contour agreement for the rectum and prostate bed (DSC = 0.696 ± 0.037 and 0.689 ± 0.047, respectively) fell within the range of inter-observer variability. Dosimetric comparisons revealed no significant differences between CT- and sCT-based plans (p > 0.05), with median gamma pass rates of 99.0% and 97.9% for the whole body using 3%/3 mm and 2%/2 mm criteria, respectively. CBCT-MRI registration indicated positioning errors comparable to CBCT-CT.

MRI-only pelvic radiotherapy workflows are clinically feasible through customized protocols for target and OAR segmentation, MRI scanner setup and immobilization, sequence selection, sCT-based dose calculation validation, and CBCT-MRI matching reliability.