Comparative Prognostic Value of Pretreatment Systemic Inflammatory Indices After Gamma Knife Radiosurgery for Breast Cancer Brain Metastases: A Retrospective Cohort Study

Background/Objectives: The prognostic relevance of pretreatment systemic inflammatory indices has not been well established in patients with breast cancer brain metastases (BCBM) undergoing Gamma Knife radiosurgery (GKRS). We evaluated five routinely available inflammatory indices and explored whether they provided prognostic information beyond established clinical factors and the Updated Breast Graded Prognostic Assessment (Updated Breast-GPA). Methods: We retrospectively analyzed 71 patients who underwent initial GKRS between January 2021 and December 2024. Five log2-transformed, standardized inflammatory indices were entered separately into Cox models adjusted for Updated Breast-GPA, cumulative GKRS-treated tumor volume, and prior intracranial surgery. Restricted cubic splines, 12-month time-dependent ROC curves, bootstrap internal validation, paired model comparisons, and sensitivity analyses were performed. Results: During a median follow-up of 24.0 months, 33 deaths occurred, and median overall survival was 27.6 months. Higher WLR, NLR, and SII and lower LMR were associated with shorter survival after multivariable adjustment, whereas PLR was not. The WLR model had the numerically highest apparent C-index (0.762) and 12-month AUC (0.851), with corresponding bootstrap optimism-corrected estimates of 0.737 and 0.827. Paired comparisons did not establish superiority over the other indices after adjustment for multiple comparisons. Sensitivity analyses adjusting for recent chemotherapy and including patients who subsequently received whole-brain radiotherapy yielded directionally consistent associations. Conclusions: Multiple pretreatment inflammatory indices were associated with survival after GKRS for BCBM. WLR showed the highest numerical discrimination among the indices evaluated within the GPA-based clinical framework, although paired comparisons did not demonstrate a statistically significant advantage over the other indices. These findings suggest that pretreatment inflammatory indices may complement established prognostic assessment after GKRS, warranting further validation in independent cohorts.