Magnetic Resonance Imaging-Only Simulation for Prostate Magnetic Resonance Imaging-Guided Stereotactic Body Radiation...

The stereotactic magnetic resonance (MR)-guided adaptive radiation therapy (SMART) protocol for prostate stereotactic body radiation therapy has demonstrated favorable clinical outcomes using a 3 T MR imaging (MRI) for delineation and a 0.35 T MR-Linac for adaptation, addressing challenges associated with low-field MRI delineation. However, this workflow necessitates 3 distinct simulation scans (computed tomography [CT], 3 T MRI, and 0.35 T MRI), resulting in logistical complexity and inefficiency. To address these limitations, we have developed and validated an end-to-end MR-only workflow to eliminate the need for a planning CT scan (pCT), thereby streamlining treatment delivery and reducing patient burden.

A workflow was developed, wherein 10 prostate cancer patients were simulated on a 3 T 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 delineation, and additionally, a T1-weighted DIXON VIBE sequence was acquired to generate a synthetic CT (sCT) using an FDA-approved deep-learning-based algorithm. This single 3 T simulation data set was used to plan and subsequently deliver treatment on the ViewRay Systems MRIdian 0.35 T MR-Linac. To validate this CT-free pathway, pCT-based plans were retrospectively recalculated on the sCTs. Dosimetric agreement was assessed using dose-volume histogram analysis and 3-dimensional gamma index analysis.

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, 2 mm dose-to-agreement, 10% threshold, local dose normalization) and 99.98%±0.03% (3%dosedeviation,2mm dose-to-agreement, 10%, global). Mean absolute differences in planning target volume D95% were small (0.06±0.14 Gy).

Integrating high-field 3 T 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 3 T delineation and MR guidance that defines the SMART approach.