The stereotactic radiosurgery-brain prognostic index (SRS-BPI): A novel prognostic index for SRS-elligible lung cancer patients with brain metastases

Existing prognostic models for lung cancer patients with brain metastases (BM) have certain limitations and are not specifically designed for individuals eligible for BM stereotactic radiosurgery (SRS). This research seeks to assess the predictive accuracy of current indices and establish a new prognostic model tailored to this patient group. This retrospective cohort study analyzed data from 673 cancer patients with BM, ultimately including 431 individuals with lung cancer who underwent SRS at Karolinska University Hospital, Sweden, between 2009 and 2020 (representing all-comers from the Stockholm region). Demographic and clinicopathological data were extracted from electronic medical records. Predictors of overall survival (OS) were identified and incorporated into a newly developed prognostic score using a penalized Cox regression model. Internal validation was conducted through Efron-Gong optimism bootstrap analysis. The predictive performance of previously established prognostic indices was evaluated and compared to that of the newly proposed prognostic model. A novel prognostic index, named the SRS-Brain Prognostic Index (SRS-BPI), was developed, incorporating nine key variables: age, performance status, histology, genetic alterations, BM volume, neurological symptoms, BM at diagnosis, presence of extracranial metastases, and extracranial disease control. The in-sample C-index for one-year survival was 0.689, while the optimism-corrected C-index was 0.67, suggesting minimal overfitting. The SRS-BPI exhibited near-optimal calibration, with a low mean absolute error of 0.04 for one-year OS. It enabled continuous risk prediction and demonstrated superior discriminatory ability compared to previously established prognostic models. The SRS-BPI emerges as an internally validated, advanced prognostic tool, providing enhanced decision-making support for lung cancer patients with BM who are candidates for SRS. Although internally validated, external validation is essential and is currently in progress.