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. Conclusions: 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.