OBJECTIVE : To assess the role of perilesional edema (PE) in non-small cell lung carcinoma (NSCLC) brain metastases (BM) undergoing radiosurgery (SRS). METHODS : This series includes 46 patients with 1-2 BM treated with SRS, selected out of all patients referred for radiotherapy (RT) for BMs over 5 years (2013 to 2017). Both the PE and gross tumor volume (GTV) were contoured on MRI images, and the PE/GTV ratio and PE + GTV value (TV, total volume) were calculated. Our clinical endpoints were brain recurrence free-survival, divided into local brain control (in field, LBC) and distant brain control (out of field, DBC) and overall survival (OS). We analyzed the role of the previously described volumetric parameters and of known clinical prognosticators (disease specific GPA, DS-GPA; chemotherapy, CHT) with Cox regression analyses. RESULTS : Only four patients (9%) developed in-field progression, whereas 10 patients (22%) showed new out-of-field BM and thirty-eight patients died in the follow up (83%). In univariate analysis, both volumetric parameters and clinical parameters were correlated with DBC and OS, whereas we did not find any correlation with LBC. In the multivariate analysis of DBC, the significant parameters were PE/GTV ratio (HR 0.302), sex (HR 0.131), and DS-GPA (HR 0.261). The OS multivariate analysis showed that the only significant parameters were DS-GPA (HR 0.478) and TV (HR: 1.038). CONCLUSIONS : Our study, although with the limitations of a monocentric retrospective study analyzing a small cohort of patients, suggests the role of PE/GTV ratio for the development of new BMs. TV also seems to be correlated with OS, together with known clinical prognosticators. These findings, if validated in a larger prospective dataset, could help in selecting patients for the most suitable RT modality (or systemic therapy approach).