RADI-34. USE OF LOW-DOSE STEREOTACTIC RADIOSURGERY FOR ADVANCED BRAIN METASTASES

BACKGROUND: Gamma knife stereotactic radiosurgery (GKSRS) is commonly used to treat brain metastases. However, treatment time significantly increases as a function of increasing dose and number of lesions treated. In patients with large number of brain metastases, advanced disease, and poor performance status, low-dose GKSRS may be better tolerated and allows for safer re-treatment with radiotherapy should tumors recur. METHODS: We queried our institutional GKSRS database and identified patients treated with low-dose GKSRS for brain metastases as defined by a prescription of 12–15 Gy margin dose. Overall survival was measured from time of initial low-dose GKSRS to death or study exit. A composite endpoint of time to additional GKSRS, whole brain radiotherapy (WBRT), craniotomy, or death was used to examine disease progression. RESULTS: We identified 30 patients treated with low-dose GKSRS at a single institution between 2008 to 2018. A total of 428 brain metastases were treated, with a median of 12 (IQR=4–20) brain metastases per patient. Thirteen patients received immunotherapy concurrent with low-dose GKSRS, and 23 patients received mutation-targeted therapy or immunotherapy. Median overall survival was 238 (IQR 91–580) days, and median composite time to disease progression was 121 (IQR = 33–371) days. The two longest survivors in our cohort are alive at over three years. One had testicular cancer, and the other had melanoma. The metastatic melanoma patient had a BRAF V600E tumor and received mutation-targeted systemic therapy. He received standard-dose GKSRS and WBRT prior to low-dose GKSRS, as well as immunotherapy prior to and concurrent with low-dose GKSRS. CONCLUSIONS: A heterogenous population with large number of brain metastases was treated with low-dose GKSRS, with acceptable but varied results in terms of survival and tumor control. Further study with larger cohorts is warranted to optimize selection criteria and timing of low-dose GKSRS with other radiotherapy and systemic agent.