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Session: MRI for Adaptive Treatment Planning and Delivery [Return to Session]

Time-Resolved Estimated Synthetic CTs Based On Orthogonal Cine MRI for Low-Field MR-Linac Treatments

M Rabe1*, C Paganelli2, H Schmitz1, G Meschini2, M Riboldi3, J Hofmaier1, J Dinkel4, M Reiner1, K Parodi3, C Belka1,5, G Landry1,3, C Kurz1,3, F Kamp1,6 (1) Department of Radiation Oncology, University Hospital, LMU Munich, Munich, GER (2) Dipartimento di Elettronica, Informazione e Bioingegneria, Politecnico di Milano, Milano, IT (3) Department of Medical Physics, Ludwig-Maximilians-Universitaet Muenchen (LMU Munich), Garching (Munich), GER (4) Department of Radiology, University Hospital, LMU Munich, Munich, GER (5) German Cancer Consortium (DKTK), Partner Site Munich, Munich, GER (6) Department of Radiation Oncology, University Hospital Cologne, Cologne, GER


WE-C-TRACK 6-5 (Wednesday, 7/28/2021) 1:00 PM - 2:00 PM [Eastern Time (GMT-4)]

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.



    MR, Phantoms, Target Localization


    IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking

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