Objective: The treatment of patients with ≥11 brain metastases (BMs) with stereotactic radiosurgery (SRS) alone has yet to be established. The aim of this study was to compare results of SRS alone in patients with 5–10 BMs versus 11–20 BMs. Methods: This was an institutional review board–approved, retrospective cohort study using our prospectively accumulated database including 1515 patients with 5–10 tumors and 804 patients with 11–20 tumors treated with Gamma Knife SRS by 2 experienced neurosurgeons between 1998 and 2018. The Kaplan-Meier method was applied to determine post-SRS survival times, and competing risk analyses were used to estimate cumulative incidences of the secondary end points. Results: The post-SRS median survival time was slightly longer in the group with 5–10 tumors (7.7 months) than in the group with 11–20 tumors (6.5 months) (P < 0.0001). Median survival time differences were statistically significant for patients with lung cancers but not for patients with breast, gastrointestinal tract, kidney, and other cancers. Multivariable analysis revealed female sex, better Karnofsky performance scale score, controlled primary cancer, and absence of extracerebral metastases to be statistically significant predictors of longer survival in the 2 patient cohorts. Crude and cumulative incidences of local recurrences were significantly lower in the group with 11–20 tumors than in the group with 5–10 tumors, while those of other secondary end points were similar to or lower in the group with 11–20 tumors than in the group with 5–10 tumors. Post-SRS outcomes were relatively poor in patients with 11–20 tumors from kidney or other cancers. Conclusions: Carefully selected patients with 11–20 BMs are not unfavorable candidates for SRS alone.
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