Radiosurgery is frequently used for patients with few cerebral metastases. Decisions regarding personalized treatment should include the patient's survival prognosis. Prognostic tools should be available for estimating the remaining lifetime for each primary tumor and treatment. We designed such a tool for patients receiving radiosurgery alone for cerebral metastases from lung cancer. Ten variables were analyzed in 98 patients. On multivariate analysis, extra-cranial spread was significantly associated with worse survival (p < 0.001). A trend was observed for poorer performance status (p = 0.08) and greater diameter of cerebral lesions (p = 0.07). Points for the tool were derived from 12-month survival rates of these variables and added, resulting in sum scores of 10-16 points. Three groups were built, 10-12, 14-15, and 16 points with 12-month survival rates of 22, 52, and 79% (p < 0.001). This new tool enables physicians to estimate the survival of lung cancer patients with few cerebral metastases which should impact individualized treatment choices.