Enhanced Normal Lung Sparing In MRI-Guided Lung SBRT: A Systematic Comparison with CT-Guided Radiotherapy
Abstract
Purpose
MRI-guided lung SBRT has emerged as a novel treatment modality with the potential to enhance motion management and improve normal lung sparing. This retrospective study evaluates the extent of normal lung sparing achieved by real-time tracking and automatic gating with MRI-guided radiotherapy (MRgRT) and compares it with internal target volume (ITV)-based free-breathing CT-guided radiotherapy (CTgRT).
Methods
Lung cancer patients (n=214) treated on MRgRT or CTgRT systems between January 2020 and September 2023 were pooled and reviewed. A subset of patients treated to 50 Gy in 5 fractions were selected for analysis (MRgRT: n=28, CTgRT: n=28). The planning target volume (PTV) was designed as the GTV plus a 5 mm margin for MRgRT and the 4DCT internal target volume plus a 5 mm margin for CTgRT. The mean, standard deviation, and medians were calculated for tumor volumes, normal tissue dose metrics, and conformity. Differences in plan quality metrics between the two cohorts were assessed using a Wilcoxon rank sum test with a significance level of 0.05.
Results
The V20Gy for total and ipsilateral normal lung were significantly lower for MRgRT compared to CTgRT (3.91±2.49% vs. 6.44±3.75%, p=0.004; 7.70±4.88% vs. 12.8±7.25%, p=0.004). The contralateral normal lung V5Gy was higher for MRgRT compared to CTgRT (5.27±4.43% vs. 2.20±2.96%, p=0.002), and the dose conformality indices (CI100% and CI50%) were less conformal in MRgRT compared to CTgRT (1.15±0.15 vs. 1.003±0.065, p<0.001; 5.61±2.08 vs. 4.57±1.08, p=0.01).
Conclusion
MRgRT spares significantly more normal lung from higher doses than CTgRT, but with reduced conformality (likely due to the step-and-shoot IMRT used by MRgRT). The findings may guide modality selections: MRgRT may be advantageous for substantial organ motion, whereas CTgRT may offer superior plan quality when motion is minimal. Future work will evaluate whether the improved tissue sparing with MRgRT translates into better clinical outcomes.