Risk-adapted SBRT ($$\textrm{BED}_{10} < 100~\textrm{Gy}$$) for recurrent lung-only metastases in patients with frailty or ultracentral anatomy

PURPOSE: Stereotactic body radiation therapy (SBRT) is widely used for pulmonary oligometastatic disease. Guideline-based regimens target a biologically effective dose [Formula: see text]; However, this threshold may not be achievable in patients with limited physiological reserve or ultracentral lesions. We examined outcomes after SBRT with [Formula: see text] versus [Formula: see text] in a consecutive cohort of patients with recurrent lung-only metastases. METHODS AND MATERIALS: Consecutive patients treated with SBRT for recurrent lung-only metastases between January 2019 and December 2022 were retrospectively reviewed. Primary endpoints were overall survival (OS), progression-free survival (PFS), and freedom from local-regional progression (FFLP). Kaplan-Meier estimates were compared using log-rank tests, and univariable Cox proportional hazards models estimated hazard ratios (HRs) with 95% confidence intervals (CIs). A propensity-score inverse probability of treatment weighting (IPTW) analysis adjusted for age, Eastern Cooperative Oncology Group (ECOG) performance status, Charlson Comorbidity Index (CCI), lung primary status, and number of metastases. RESULTS: Fifty-three patients were included. Median OS was 47.3 months for [Formula: see text] versus 34.3 months for [Formula: see text] (HR 1.45; 95% CI 0.66-3.16; [Formula: see text]). Median PFS was 22.8 months versus 14.7 months (HR 1.59; 95% CI 0.82-3.11; [Formula: see text]). For FFLP, the HR was 2.67 (95% CI 0.60-11.93; [Formula: see text]). In IPTW-weighted models, [Formula: see text] was not significantly associated with OS (HR 0.58; 95% CI 0.25-1.32; [Formula: see text]) or PFS (HR 0.59; 95% CI 0.24-1.40; [Formula: see text]). CONCLUSION: In this retrospective cohort, [Formula: see text] was delivered to selected higher-risk patients on the basis of frailty, ultracentral anatomy, or both. Differences in OS, PFS, and FFLP compared with [Formula: see text] did not reach statistical significance, and IPTW-weighted estimates were directionally consistent. These observations are consistent with the feasibility of sub-ablative SBRT in selected higher-risk patients when ablative dosing is not safely deliverable, and they define the patient population and dose parameters that warrant prospective, risk-stratified evaluation.