S2139
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
treatment on 0.35T MR-Linac with online adaptive planning and motion-gated therapy. Results: Clinical implementation of the full MOST SMART Prostate workflow was successful. Dosimetric validation demonstrated high fidelity between sCT and pCT calculations. Computed γ -indicies were 96.56 ± 1.14% (2% dose deviation (DD), 2mm dose-to- agreement (DTA), 10% threshold, local dose normalization) and 99.98 ± 0.03% (3% DD, 2 mm DTA, 10% threshold, global normalization). Mean absolute differences in PTV D95% were small (0.06 ± 0.14 Gy).
Scientific Computing (IWR), Heidelberg University, Heidelberg, Germany. 3 Department of Physics and Astronomy, Interdisciplinary Center for Scientific Computing (IWR), Central Institute for Computer Engineering (ZITI), CZS Heidelberg Initiative for Model- Based AI, Heidelberg University, Heidelberg, Germany. 4 Mannheim Institute for Intelligent Systems in Medicine (MIISM), Medical Faculty Mannheim, Heidelberg University, Mannheim, Germany Purpose/Objective: The SMART (Stereotactic MR–Guided Adaptive Radiation Therapy) protocol for prostate stereotactic body radiotherapy (SBRT) has demonstrated favorable clinical outcomes using a 3T-MRI for delineation and a 0.35T MR-Linac for adaptation, addressing challenges associated with low-field MRI delineation including dominant intraprostatic lesion (DIPL) and urethra visualization [1,2]. 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 advancement of this protocol, MOST (MRI-Only Simulation and Treatment) SMART Prostate, which employs 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. Material/Methods: A workflow was developed wherein nine prostate cancer patients were simulated on a 3T Siemens MAGNETOM Vida MRI scanner. Consistent to the SMART protocol, high-resolution T2- weighted/DWI/ADC sequences were used for definitive urethra/DIPL delineation, and additionally a T1- weighted DIXON 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.
Figure 2: Poor DIPL/urethra visibility on 0.35T MR-Linac TrueFISP images (a, b) contrasts with clear delineation on 3T planning scans. 3T shows the DIPL on DWI/ADC (c) and urethra on T2-weighted BLADE images (d). Conclusion: Integrating high-field 3T MR simulation with 0.35T MR- Linac delivery via an sCT pathway is clinically feasible and dosimetrically robust. Utilizing 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. References: 1.Leeman JE, Cagney DN, Mak RH, Huynh MA, Tanguturi SK, Singer L, Catalano P, Martin NE, D'Amico AV, Mouw KW, Nguyen PL, King MT, Han Z, Williams C, Huynh E. Magnetic Resonance-Guided Prostate Stereotactic Body Radiation Therapy With Daily Online Plan Adaptation: Results of a Prospective Phase 1 Trial and Supplemental Cohort. Adv Radiat Oncol. 2022 Mar 6;7(5):100934. doi: 10.1016/j.adro.2022.100934. PMID: 35847547; PMCID: PMC9280019.2.Pham, Jonathan; Savjani, Ricky R.; Yoon, Stephanie M.; Yang, Tiffany; Gao, Yu; Cao, Minsong et al. (2022): Urethral Interfractional Geometric and Dosimetric Variations of Prostate Cancer Patients: A Study Using an Onboard MRI. In Frontiers in Oncology 12, p. 916254. DOI:
Figure 1:Initial Patient Visit: Initial imaging uses a 3T MRI instead of CT for simulation. Pre-treatment Workflow: Uses 3T MRI to generate synthetic CT and define key structures; the 0.35T scan provides reference images for planning and motion management. Daily Fraction Workflow: Delivers
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