Background: Gamma knife radiosurgery (GKR) is an effective treatment modality for local control of brain metastases. The predictors of response of cystic brain metastases (CBM) to GKR is not well understood. To measure progression and determine treatment prognostic factors, we quantified the percentage cystic and solid components of brain metastases before and after GKR treatment. Methods: 71 patients with CBM treated with GKR from 2006 to 2010 were selected from our institution’s database. Volumetric analysis was performed on MRIs done on treatment date and the latest MRI. Clinical data and dosimetry parameters were reviewed to identify factors that predicted a response of cystic component and overall tumour control. Results: Metastatic lesions from the lung had significantly larger cystic components (by volume) prior to GKR than metastasis of colorectal origin (p=0.039), and also had significantly larger cystic/total ratios than metastases from the breast (p=0.023). Post-treatment, a trend of >25% improvement in both cystic and solid components of tumours was seen in lung primaries (p=0.239). Metastatic brain tumours of colorectal origin demonstrated the best treatment response of the cystic component. Conclusion: The primary cancer pathology of the CBM has an effect on the response to GKR, and can be used as a prognosticator of changes in cystic and solid volumes of lesions.