PURPOSE: Prior statistical models attempted to identify risk factors for time to 
distant brain failure (DBF) or time to salvage whole-brain radiation therapy 
(WBRT) to predict the benefit of early WBRT versus stereotactic radiosurgery 
(SRS) alone. We introduce a novel clinical metric, brain metastasis velocity 
(BMV), for predicting clinical outcomes after initial DBF following upfront SRS 
alone.
METHODS AND MATERIALS: BMV was defined as the cumulative number of new brain 
metastases that developed over time since first SRS in years. Patients were 
classified by BMV into low-, intermediate-, and high-risk groups, consisting of 
<4, 4 to 13, and >13 new metastases per year, respectively. Histology, number of 
metastases at the time of first SRS, and systemic disease status were assessed 
for effect on BMV.
RESULTS: Of 737 patients treated at our institution with upfront SRS without 
WBRT, 286 had ≥1 DBF event. A lower BMV predicted for improved overall survival 
(OS) following initial DBF (log-rank P<.0001). Median OS for the low, 
intermediate, and high BMV groups was 12.4 months (95% confidence interval [CI], 
10.4-16.9 months), 8.2 months (95% CI, 5.0-9.7 months), and 4.3 months (95% CI, 
2.6-6.7 months), respectively. Multivariate analysis showed that BMV remained 
the dominant predictor of OS, with a hazard ratio of 2.75 for the high BMV group 
(95% CI, 1.94-3.89; P<.0001) and a hazard ratio of 1.65 for the intermediate BMV 
group (95% CI, 1.18-2.30; P<.004). A lower BMV was associated with decreased 
rates of salvage WBRT (P=.02) and neurologic death (P=.008). Factors predictive 
for a higher BMV included ≥2 initial brain metastases (P=.004) and melanoma 
histology (P=.008).
CONCLUSIONS: BMV is a novel metric associated with OS, neurologic death, and 
need for salvage WBRT after initial DBF following upfront SRS alone.