Most studies evaluating factors associated with the survival of patients with brain metastases (BM) have focused on patients with newly diagnosed BM. This study aimed to identify prognostic factors associated with survival after brain re-irradiation in order to develop a new prognostic index.This 5-year retrospective study included patients treated with repeat-radiotherapy for recurrent BM at the "Instituto Nacional de Cancerología" of Mexico between 2015 and 2019. Significant variables in the multivariate Cox regression analysis were used to create the brain re-irradiation index (BRI). Survival and group comparisons were performed using the Kaplan-Meier method and the log-rank test.Fifty-seven patients receiving brain re-irradiation were identified. Most patients were women (75.4%) with a mean age at BM diagnosis of 51.4 years. Lung and breast cancer were the most prevalent neoplasms (43.9% each). Independent prognostic factors for shorter survival after re-irradiation were: Age >50 years (hazard ratio [HR]:2.5 [95% confidence interval [CI], 1.1-5.8]; p = 0.026), uncontrolled primary tumor (HR:5.5 [95% CI, 2.2-13.5]; p < 0.001), lesion size >20 mm (4.6 [95% CI, 1.7-12.2]; p = 0.002), and an interval <12 months between radiation treatments (HR:4.3 [95% CI, 1.7-10.6]; p = 0.001). Median survival (MS) after re-irradiation was 14.6 months (95% CI, 8.2-20.9).MS of patients stratified according to the BRI score was 17.38, 10.34, and 2.82 months, with significant differences between all groups.The new BRI can be easily implemented for the prognostic classification of cancer patients with progressive or recurrent BM from extracranial solid tumors.