Simple SummaryMetastases to the brain result in serious morbidity and mortality. Surgical resection is a treatment option; however, many patients die within a short time after surgery. It is therefore important to understand which patients may benefit from surgical resection. We have investigated prognostic factors for survival after surgery for brain metastases. We identified eight factors that significantly predicted survival in these patients and created a preoperative prognostic index to estimate survival and to guide clinical decision-making in evaluating surgery as a treatment for brain metastases. We tested our findings on patients who underwent the same treatment at two other large hospitals in Norway and Sweden, and our findings were valid. This is to our knowledge the first such index to support decision-making in this setting.AbstractBackground: Brain metastases (BM) are common in cancer patients and are associated with high morbidity and mortality. Surgery is an option, but the optimal selection of patients for surgery is challenging and controversial. Current prognostication tools are not ideal for preoperative prognostication. By using a reference population (derivation data set) and two external populations (validation data set) of patients who underwent surgery for BM, we aimed to create and validate a preoperative prognostic index. Methods: The derivation data set consists of 590 patients who underwent surgery for BM (2011–2018) at Oslo University Hospital. We identified variables associated with survival and created a preoperative prognostic index with four prognostic groups, which was validated on patients who underwent surgery for BM at Karolinska University Hospital and St. Olavs University Hospital during the same time period. To reduce over-fitting, we adjusted the index in accordance with our findings. Results: 438 patients were included in the validation data set. The preoperative prognostic index correctly divided patients into four true prognostic groups. The two prognostic groups with the poorest survival outcomes overlapped, and these were merged to create the adjusted preoperative prognostic index. Conclusion: We created a prognostic index for patients with BM that predicts overall survival preoperatively. This index might be valuable in supporting informed choice when considering surgery for BM.