MRI-Only Simulation for Prostate MRI-Guided SBRT.

PURPOSE: The SMART (Stereotactic MR-Guided Adaptive Radiation Therapy) protocol for prostate SBRT has demonstrated favorable clinical outcomes using a 3T-MRI for delineation and a 0.35 T MR-Linac for adaptation, addressing challenges associated with low-field MRI delineation. However, this workflow necessitates three distinct simulation scans (CT, 3T-MRI, and 0.35T-MRI), resulting in logistical complexity and inefficiency. To address these limitations, we have developed and validated an end-toend MR-only workflow to eliminate the need for a planning CT scan (pCT), thereby streamlining treatment delivery and reducing patient burden. METHODS: A workflow was developed wherein ten prostate cancer patients were simulated on a 3T Siemens MAGNETOM Vida MRI scanner. Consistent with the SMART protocol, high-resolution T2-weighted BLADE/DWI sequences were used for definitive urethra/dominant intraprostatic lesion (DIPL) delineation, and additionally a T1weighted DIXON VIBE sequence was acquired to generate a synthetic CT (sCT) using an FDA-approved deep-learning-based algorithm. This single 3T-simulation dataset was used to plan and subsequently deliver treatment on the ViewRay Systems MRIdian 0.35T MR-Linac. To validate this CT-free pathway, pCT-based plans were retrospectively recalculated on the sCTs. Dosimetric agreement was assessed using DVH analysis and 3D gamma index analysis. RESULTS: Clinical implementation of the full workflow was successful. Dosimetric validation demonstrated high fidelity between sCT and pCT calculations. Computed γ-indices were 96.80±1.33% (2% dose deviation (DD), 2 mm dose-to-agreement (DTA), 10% threshold, local dose normalization) and 99.98±0.03% (3%DD,2mm DTA, 10%, global). Mean absolute differences in PTV D95% were small (0.06±0.14 Gy). CONCLUSIONS: Integrating high-field 3T MR simulation with 0.35 T MR-Linac delivery via an sCT pathway is clinically feasible and dosimetrically robust. MRI-only simulation reduces patient burden and improves workflow efficiency, while maintaining the high-quality 3T delineation and MR-guidance that defines the SMART approach.