Tumor size as a key determinant of treatment strategy and prognosis in stereotactic body radiotherapy for pulmonary metastases: a retrospective study

Background: Stereotactic body radiotherapy (SBRT) is widely used for pulmonary metastases, but the optimal dose-fractionation strategy remains uncertain. Tumor size may critically influence both treatment selection and prognosis. Materials and methods: We retrospectively analyzed 173 metastatic lung lesions in 152 patients treated with SBRT between February 2018 and August 2023. Tumor characteristics, dose-fractionation schedules, and outcomes were assessed. Associations between tumor size, treatment choice, and prognosis were evaluated using the Kruskal–Wallis test, Kaplan–Meier method, Cox proportional hazards models, and receiver operating characteristic (ROC) analysis. Results: Median tumor size was 15 mm. Larger tumors more frequently received fractionated regimens such as 60 Gy in 10 fractions (p = 0.028). Tumor size was an independent predictor of overall survival (OS) [hazard ratio (HR) = 1.041; 95% confidence interval (CI): 1.001–1.083; p = 0.045)]. Receiver operating characteristic (ROC) analysis identified 16.75 mm as the optimal OS cutoff [area under the curve (AUC) = 0.651], with tumors above this threshold showing significantly worse OS (p = 0.009). Local control (LC) did not significantly differ by size or regimen, although larger tumors showed a non-significant LC decline. Primary tumor histology was not associated with OS or LC. Grade ≥ 2 radiation pneumonitis occurred in 5.3% of cases. Conclusion: Tumor size is a decisive factor shaping both dose-fractionation and survival in SBRT for pulmonary metastases. Larger tumors tend to receive more fractionated, lower BED regimens, potentially compromising survival. Prospective trials incorporating tumor size–based stratification and BED optimization are warranted to improve outcomes.