3417 - Beyond Prescription Dose: Target Coverage and Volume Extremes Predict Local Control after Lung SBRT
Presenter(s)
A. J. Allen1, M. J. Ferris1, M. A. L. Vyfhuis1, M. A. Hamza2, H. R. R. Cherng3, J. W. Assif2, C. Eggleston1, K. Marter4, K. Krudys4, C. Gahagan4, S. A. McAvoy1, J. K. Molitoris1, P. Mohindra5, and Z. H. Rana1; 1Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, MD, 2Univ of Maryland School of Medicine, Radiation Oncology, Baltimore, MD, 3Department of Radiation Oncology, University of Maryland Medical Center, Baltimore, MD, 4University of Maryland School of Medicine, Baltimore, MD, 5Case Western Reserve University, Cleveland, OH
Purpose/Objective(s): We hypothesized that SBRT planning quality metrics—specifically target volume and minimum internal gross tumor volume (iGTV) coverage—are primary determinants of oncologic outcomes, exceeding traditional clinical predictors in early-stage non–small cell lung cancer (NSCLC). The primary objective was to identify modifiable dosimetric drivers of local control (LC) and progression-free survival (PFS).
Materials/Methods: We retrospectively analyzed 195 lesions in 185 patients with T1–T3N0 NSCLC treated with lung SBRT (1–5 fractions) from 2015–2021 across a single academic healthcare system, with updated follow-up through February 2026. Endpoints included cause-specific LC (event = local failure), PFS, overall survival (OS), and toxicity. Planning variables included planning target volume (PTV), iGTV volume, minimum iGTV dose (D99%), and prescription isodose volume. Kaplan–Meier and cumulative incidence methods were used. Univariable Cox regression evaluated associations with outcomes.
Results: Median tumor diameter was 1.6 cm (IQR 1.2–2.2). Lesions were peripheral (n=143), central (n=46), and ultracentral (n=6). Fractionation consisted of 5 fractions (54%), 4 fractions (41%), and =3 fractions (5%). Median prescription dose was 50 Gy (median BED10 105.6 Gy). Median PET SUV was 4.1 (IQR 2.5–7.1; range up to 25). Median follow-up was approximately 38 months. Cause-specific LC at 12, 24, and 36 months was 98.4% (95% CI 95.1–99.5), 94.5% (89.7–97.1), and 90.1% (83.6–94.1), respectively. Two-year PFS and OS were 81.9% (75.2–86.9) and 78.1% (71.5–83.4). Local failure risk increased with PTV volume (HR 1.013/cc, p<0.001), iGTV volume (HR 1.024/cc, p=0.002), and prescription isodose volume (p=0.003), translating to an approximately 25–30% increase in failure risk per 10 cc increase in target volume. Synchronous lung primaries were also associated with higher local failure risk (HR 2.59, p=0.030). Minimum iGTV coverage independently predicted improved PFS (HR 0.98 per 1% increase in D99%, p=0.019). PET SUV, tumor centrality (including ultracentral disease), and fractionation schedule were not associated with LC or PFS. The brain represented the most common first site of distant failure. No grade =3 toxicity occurred; grade 2 toxicity was 6%, primarily fatigue and dyspnea.
Conclusion: Following lung SBRT, local failure is predominantly volume-driven, while higher minimum tumor coverage independently improves progression-free survival. PET activity, tumor location, and fractionation do not predict outcomes. These findings establish target size and minimum iGTV dose as key planning determinants and support routine incorporation of tumor coverage metrics into SBRT workflows, with brain-predominant distant failure informing post-treatment surveillance strategies.