LMAP-11 CYSTIC BRAIN METASTASES OUTCOMES FOLLOWING GAMMA KNIFE RADIOTHERAPY

BACKGROUND: The response of cystic brain metastases (BMets) to radiotherapy is poorly understood, with conflicting results regarding local control (LC), overall survival (OS), and treatment-related toxicity. This study aims to examine the role of Gamma Knife (GK) in managing cystic BMets. METHODS: Volumetric analysis was conducted to measure tumor and edema volume at the time of GK and follow-up MRI studies. We evaluated the association of 4 variables with survival using Cox regression analysis and used the Kaplan-Meier method to estimate median survival times (MST). RESULTS: Between 2016 and 2021, 54 patients with 83 cystic BMets were treated with GK at our institution. Lung cancer was the most common pathology (51.9%), followed by breast (13.0%). The mean target volume was 2.7 cc (range, 0.1-39.0 cc) and the mean edema volume was 13.9 cc (range, 0-165.5 cc). The median prescription dose of single fraction and fractionated GK was 20 Gy (range, 14-27.5 Gy). With a median follow-up of 8.9 months, MST was 11.1 months, OS was 33.3%, and the one-year LC rate was 75.9%. GK was associated with decreased tumor and edema volumes over time, although 68.5% of patients required steroids post-GK. Patients whose tumors grew beyond baseline following GK received significantly more pre-GK whole-brain radiation therapy (WBRT) than those whose tumors declined following GK. Higher age at diagnosis of BMets and pre-GK systemic therapy were associated with worse survival, with an MST of 7.8 months in patients who received it compared to 23.3 months in those who did not. CONCLUSIONS: Pre-GK WBRT may select for BMets with increased radioresistance. This study highlights the ability of GK to control cystic BMets with the cost of high post-treatment steroid use.