The 5-factor modified frailty index predicts local control and overall survival following spine stereotactic body radiation therapy

PURPOSE: Frailty is an established prognostic factor in neurosurgical and oncologic populations. The association between frailty and outcomes following spine stereotactic body radiation therapy (SBRT) has not been previously evaluated using a validated clinical index. METHODS: A prospectively maintained, single-institution database of consecutive patients with spinal metastases and primary bone tumors treated with SBRT from 2007 to 2024 was analyzed. Frailty was assessed at the time of treatment using the 5-factor modified frailty index (mFI-5), with 205 patients stratified into pre-frail (mFI-5 0–1, 180 lesions) or frail (mFI-5 2–5, 131 lesions) cohorts. RESULTS: Kaplan-Meier estimates of 6-month, 1-year, 2-year, and 3-year local control (LC) were inferior in the frail cohort compared to the pre-frail cohort (87%, 77%, 70%, and 65% vs. 96%, 92%, 89%, and 87%; P < 0.001). Cumulative incidence competing risk estimates accounting for death showed higher local failure in the frail cohort (13%, 20%, 24%, and 25% vs. 4%, 7%, 9%, and 10%; P < 0.001). Kaplan-Meier estimates of 6-month, 1-year, 2-year, and 3-year overall survival (OS) were inferior in the frail cohort (70%, 51%, 34%, and 30% vs. 90%, 72%, 57%, and 47%; P < 0.001). Complete or partial pain response at 1, 3, 6, or 12 months (P = 0.345, P = 0.940, P = 0.766, P = 0.108), radiation toxicity (P = 0.832), and vertebral compression fractures (P = 0.504) following treatment did not differ between cohorts. On multivariable analysis, frailty (mFI-5 2–5; P < 0.001, HR: 2.70, 95% CI: 1.54–4.76) and Spinal Instability Neoplastic Score > 6 (P = 0.005, HR: 4.12, 95% CI: 1.55-11.0) were associated with inferior LC. Frailty (P = 0.001, HR: 1.72, 95% CI: 1.24–2.38), prior conventional external beam radiation therapy (P = 0.033, HR: 1.49, 95% CI: 1.03–2.14), and Epidural Spinal Cord Compression score > 0 (P = 0.039, HR: 1.41, 95% CI: 1.02–1.96) were associated with inferior OS. CONCLUSION: Frailty as measured by the mFI-5 was associated with inferior LC and OS following spine SBRT.