Single-Fraction Sterotactic MR-Guided Adaptive Radiotherapy: Evaluation of Plan Efficiency and Quality
Abstract
Purpose
Efficacy, feasibility, and safety of extracranial, single-fraction MR-guided adaptive radiotherapy (SMART-1fx) was reported in the SMART ONE trial (NCT04939246). This work evaluates SMART plan quality relative to RTOG-0813 standards for an expanded cohort.
Methods
A retrospective analysis was performed on patients treated with SMART-1fx between 2020-2025. Step-and-shoot IMRT was delivered on a 0.35 T MR-Linac. Efficient beams/segments were prioritized. Online adaptation was performed when necessary to meet OAR constraints. Plan quality was compared to RTOG-0813 no/minor/major deviation thresholds for PITV (prescription isodose volume/target volume), D2cm (Dmax 2cm from PTV) and R50% (50%Rx IDL volume/target volume). Additionally, magnetic field (Bo) impact in low density medium by Lorentz Force was investigated for lung plans with a single re-optimization at Bo=0 T.
Results
87 plans were evaluated. Lesions were in the lung (n=27, 30-34 Gy), liver (n=29, 30-40 Gy), pancreas (n=12, 20-30 Gy), adrenal gland (n=9, 25-34 Gy), abdominal/pelvic lymph nodes (n=8, 25-30 Gy), celiac plexus (n=1, 25 Gy), and kidney (n=1, 26 Gy). Median total in-room time was 66 minutes. ART was used in 45% of cases and median PTV was 16.9cc (3.7-273.5cc). Average PITV, D2cm, and R50% were 1.10+/-0.17, 53%+/-7%, and 5.70+/-2.32, respectively. For RTOG-0813 thresholds, two plan quality metrics (PITV, D2cm) for most plans showed no (75%, 68%) or minor (25%, 30%) deviations. In general, R50% performed poorly with 20%/41%/39% of plans having no/minor/major deviations, respectively. Lung plans had larger mean R50% of 7.24+/-1.31, with 81% of plans classified as major deviation. Re-optimizing with Bo=0 T did not significantly decrease lung R50% (p=0.45).
Conclusion
To our knowledge, this is the largest evaluation of single-fraction stereotactic MR-guided treatment plans. Favorable plan quality metrics were reported with higher R50% values not attributed to the Lorentz Force. Further study is warranted to evaluate whether R50% deviations were associated with adverse patient outcomes.