Background <p>It is not clear how frailty may affect the outcomes of stereotactic radiosurgery (SRS) for brain metastasis. This study aimed to evaluate the impact of frailty on clinical outcomes in patients ≥ 60 years old who underwent SRS for brain metastasis from a population-based perspective.</p> Materials and methods <p>Data were extracted from the National Readmission Database (NRD), 2016 to 2020. Inclusion criteria were ≥ 60 years old with brain metastasis who underwent SRS. Frailty was assessed using the modified Frailty Index (mFI), derived from 11 clinical conditions. The primary outcomes were in-hospital mortality, length of hospital stay (LOS), total hospital costs, and 30-day and 90-day readmission rates. Logistic and linear regression models were used to assess the association between frailty and outcomes.</p> Results <p>A total of 904 patients (mean age: 71 years, 53% male) were included, of which 17.5% were defined as frail. After adjusting for demographic, clinical, and hospital-related factors, frailty was significantly associated with increased in-hospital mortality (adjusted odds ratio [aOR] = 2.39, 95% confidence interval [CI]: 1.16–4.92), longer LOS (adjusted Beta [aBeta] = 2.61 days, 95% CI: 1.95–3.28), higher total costs (aBeta = $36.04 thousand USD, 95% CI: 28.84–43.23), and higher 30-day readmission rate (aOR = 1.47, 95% CI: 1.02–2.11).</p> Conclusion <p>Frailty independently predicts poorer outcomes in older adults undergoing SRS for brain metastasis, including higher mortality, longer hospital stays, increased hospital costs, and increased 30-day readmission rate. These findings highlight the importance of incorporating frailty-informed risk stratification and perioperative care planning to optimize patient outcomes.</p> Trial registration number <p>Not applicable.</p>