Abstract AIMS Between 20-40% of patients with cancer develop brain metastases (BMets), with metastasis representing the most common type of brain tumour. Cystic metastasis describes a rarer type of BMet characterised by oedematous collections. Stereotactic radiosurgery (SRS) is an effective and minimally invasive method for treating non-cystic BMets but controversy exists regarding its use in cystic BMets. The traditional view that cystic lesions are less responsive to SRS however is not fully evidenced. This investigation aims to elucidate the effectiveness of Gamma Knife (GK) radiosurgery in achieving local control of cystic BMets and enhancing survival versus GK radiosurgery for non-cystic BMets. METHOD Patients who had undergone fractionated GK for either a non-cystic or cystic BMet between 2018 and 2021 were identified. BMet volume on pre-GK MRI was calculated. Information was additionally collected on BMet location, maximum cyst diameter, if present, the patient’s primary tumour and its control and their performance status at the time of GK. The follow-up of patients is ongoing with survival being calculated using the Kaplan- Meier method and local progression through volumetric analysis of repeat MRI scans. RESULTS 57 patients with 79 cystic BMets were identified and 67 patients with non-cystic BMets. The most common primary tumour in both the non-cystic and the cystic BMets group was lung, comprising 30% and 23% respectively. The second most common tumour amongst non-cystic BMets was renal (12%) however in the cystic cohort it was breast (9%). The median tumour volume before treatment was 6.2cm3 in the non-cystic group and 12.5cm3 amongst cystic BMets. Median cyst size was 17.4 mm. Karnofsky Performance Status (KFS) was similar across both groups. CONCLUSION By comparing outcomes following GK for cystic and non-cystic metastasis, with the same primary tumours, this study can uncover the efficacy of fractionated GK for cystic lesions and aid the selection of treatments for these patients.