CT Simulation–Free, MRI-First Workflow for Stereotactic Radiosurgery
Abstract
Purpose
Studies show measurable changes in brain metastasis over 1-2 weeks, with delays between MRI and radiosurgery delivery reducing local control. We aim to minimize this gap by developing a multi-target radiosurgery workflow that eliminates need for CT simulation, with pre-planning, immobilization, and image guidance carried out using the diagnostic MRI.
Methods
We address three challenges: (1) immobilization and skull orientation for planning, (2) dosimetric accuracy for calculation using MRI, and (3) localization accuracy when IGRT consists of MRI to kV-CBCT match rather than simulation CT to kV-CBCT. To address (1) the mask is created on a 3D printed head phantom designed from the MRI and oriented using a custom mount; the same rotation is applied to the MRI for treatment planning. To address (2) virtual CT datasets were generated from MRI and dose was compared with standard CT-based plans (n=10). To address (3) fusions were carried out by 3 physicists for 4 patients, and variation in PTV centroid position compared between standard IGRT (MRI to simulation-CT, simulation-CT to kV-CBCT) and an MR planning based process (MRI to kV-CBCT).
Results
MRI-derived immobilization achieved reproducible head orientation with translational variability under phantom torque testing of ≤0.26cm across all axes (compared to ≤0.62cm for conventional masks). Virtual CT reproduced multi-target radiosurgery dosimetry with minor differences in PTV mean dose (1.3% ± 0.4%), D95 (1.3% ± 0.4%), and D5 (1.2% ± 0.4%). Direct MRI to kV-CBCT localization demonstrated sub-millimeter agreement, with variation between physicists (mean±standard deviation) being 0.1±0.0mm, 0.2±0.1mm, and 0.2±0.1mm in right-left, ant-post, and sup-inf axes, respectively. For comparison, these values for simulation CT based IGRT were 0.1±0.0mm, 0.3±0.1mm, and 0.2±0.1mm, respectively.
Conclusion
We demonstrate feasibility of a CT simulation–free stereotactic radiosurgery workflow incorporating MRI-derived, pre-fabricated immobilization, virtual CT–based dose calculation, and direct MRI to kV-CBCT localization.