3114 - Interfractional Tumor Volume Variation in MR-Guided Stereotactic Body Radiotherapy for Primary and Metastatic Lung Tumors: A Single-Institution Exploratory Analysis
Presenter(s)
A. Roy1, K. E. Mooney2, C. Ainsley3, M. J. Greenberg4, A. C. Mueller5, P. R. Anne6, M. Werner-Wasik7, C. Jiang6, and H. Cook8; 1Thomas Jefferson University Hospital, Philadelphia, PA, 2Dept. of Radiation Oncology, Sidney Kimmel Medical College and Comprehensive Cancer Center, Thomas Jefferson University, Philadelphia, PA, 3Department of Radiation Oncology, Sidney Kimmel Medical College at Thomas Jefferson University, Philadelphia, PA, 4Department of Radiation Oncology, Sidney Kimmel Medical College & Cancer Center at Thomas Jefferson University, Philadelphia, PA, 5Department of Radiation Oncology, UPMC Hillman Cancer Center, University of Pittsburgh, Pittsburgh, PA, 6Thomas Jefferson University, Philadelphia, PA, 7Department of Radiation Oncology, Sidney Kimmel Cancer Center, Philadelphia, PA, 8Drexel University College of Medicine, Philadelphia, PA
Purpose/Objective(s): Interfractional volumetric changes during MR-guided SBRT (MRgSBRT) for lung tumors may substantially affect target coverage and adaptive planning decisions. MRgSBRT enables daily online adaptation, but the magnitude and clinical significance of interfractional volumetric variation remain poorly defined. We describe fraction-by-fraction GTV and PTV variation in a mixed-histology cohort and explore associations with local control.
Materials/Methods: This is a single-institution retrospective study of patients with lung tumors who underwent MRgSBRT on a 0.35T MR-Linac between 2021 and 2023. Fraction-by-fraction GTV and PTV were compared to simulation volumes. Clinical details and manual adaptive planning utilization were extracted from the electronic medical record (EMR). Point-biserial correlation analysis was performed to assess the relationship between peak per-patient volumetric changes and local control.
Results: We identified nine patients with 10 lung tumors (33.3% primary, 66.7% metastatic). Online adaptive planning was applied in 88.9% of patients across 50 total fractions to increase target coverage and reduce dose to organs at risk. Prescribed doses were 50 Gy/5 fx (n=3), 55 Gy/5 fx (n=2), or 60 Gy in 5 fx (n=3) or 10 fx (n=1). GTV changes ranged from -10.9% to +74.9% (mean +14.8% ± 26.5%), with >10% GTV increase in 32.0% of fractions. PTV changes ranged from -57.9% to +66.1% (mean +10.9% ± 24.7%), exceeding 10% in 30.0% of fractions. The mean GTV change across fractions was +18.2% for metastatic tumors compared to +13.8% for primary tumors. The average peak GTV change per patient in metastatic group was +29.8% and +27.5% in primary group. With a median follow-up of 6.5 months (range: 0–34.6 months), local control was achieved in 7/9 patients (77.8%); time to local failure was 2.0 months and 19.6 months for the remaining two patients. No statistically significant associations were observed between peak per-patient volumetric target changes and local control (max GTV ?%: r = -0.337, p = 0.374; max PTV ?% (r = -0.430, p = 0.249).
Conclusion: Clinically meaningful interfractional GTV and PTV volumetric variation was observed during MRgSBRT for lung tumors, supporting the benefits of manual target review and daily online adaptation. Moderate negative correlations were observed between tumor volumetric changes and local control, though significance was not reached given the small cohort. These findings are hypothesis-generating and warrant further investigation.