Purpose: To develop and experimentally validate a method to create continuous time-resolved estimated synthetic CTs (tresCTs) based on 3DCT and orthogonal cine MRI data for MR-guided radiotherapy.
Methods: A breathing porcine lung phantom was consecutively scanned at a CT scanner and a 0.35T MR-Linac. A mid-exhale breath-hold 3DCT was deformably registered to a mid-exhale 3DMRI. The 3DMRI was deformably registered to orthogonal cine MRI series (sagittal/coronal orientation) at 7.3Hz, intersecting different injected tumor-mimicking gelatin nodules. The time-resolved deformation vector fields were extrapolated to 3D and applied to the deformed 3DCT to create 82s-long tresCTs at 3.65Hz. A 4DCT scan served as ground truth (GT). Ten tresCTs were created for ten tracked nodules with different motion patterns in two lungs. Each intersected nodule, expanded by a 5mm margin, was used as PTV for stereotactic step-and-shoot IMRT treatment planning (8x7.5Gy) on the GT-4DCT mid-exhale image to mimic gated MR-Linac plan generation. Each plan was recalculated on all GT-4DCT phases and one tresCT image per breathing phase, randomly sampled from different breathing cycles. Phase-dependent DVH parameter and dose differences were quantified.
Results: Median nodule motion amplitudes were 8mm (range 3-16mm), with median centroid position deviations between GT and tresCTs of 2.3mm. For all ten nodules and breathing phases (100 dose comparisons), the median absolute differences of D98%, D50% and D2% were 2.1%, 0.8% and 0.5% for the PTV. The dose difference analysis with a 2% prescription dose acceptance level (10% dose threshold) yielded a median pass rate of 97.4%. Dose deviations in the vicinity of the GTVs were observed, where phase-dependent lung density variations were not accurately modeled.
Conclusion: Dose differences between GT and tresCTs were at a clinically acceptable level. Hence, tresCTs could be used for time-resolved dose accumulation to guide inter- and intra-fractional treatment adaptation in the future.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the German Research Foundation (DFG) within the Research Training Group GRK 2274 "Advanced Medical Physics for Image-Guided Cancer Therapy". The Department of Radiation Oncology of the University Hospital of LMU Munich has a research agreement with ViewRay.