Objective:To explore the efficacy and safety of whole-brain low-dose radiotherapy(LDRT)combined with PD-1 inhibitor sin-tilimab and intrathecal pemetrexed(IP)for the treatment of refractory non-small cell lung cancer(NSCLC)with leptomeningeal metastases(LM).Methods:Retrospective analysies were was performed on eight NSCLC patients with LM at the West China Hospital of Sichuan Uni-versity from December 2022 to May 2024.Among the eight patients,there were four were males and four were females,with a median age of 49 years(rangeing,between 34 to 58 years).All patients were treated with whole-brain LDRT combined with immune checkpoint inhibit-or(ICI)and intrathecal chemotherapy regimens,and the therapeutic efficacy was evaluated according to the Response Assessment in Neuro-Oncology(RANO)criteria and the Karnofsky physical status(KPS)score.Adverse reactions were assessed according to the Common Criteria for the Evaluation of Adverse Events(CTCAE version 5.0).Survival analysis was performed using the Kaplan-Meier method.The classification proportion of cerebrospinal fluid subsets before and after treatment was analyzed using by single-cell sequencing,and the differential ana-lysis of gene expression in parallel cells was performed.Results:The best clinical treatment effects in eight patients were were evaluated us-ing the RANO criteria:five patients(62.5%)were evaluated as improved and three(37.5%)as stable.The median KPS score of the eight pa-tients was 30(20-50)before treatment,which was significantly improved to 60(40-90)after treatment(P=0.000 9).The remission rate of neurological symptoms was 100%(8/8)in eight patients.The median neurological progression-free survival(NPFS)was 12 months.The res-ults of single-cell sequencing in CSF of patientss(P1)showed that the proportion of T cells in the patient samples after whole-brain LDRT treatment was significantly higher than that before treatment(6.08%vs.68.87%),and the proportion of tumor cells was significantly lower(12.92%vs.0.6%).The differential analysis of gene expression showed that CCL5 and CXCL13 were significantly upregulated in T cells of CSF after WB-LDRT treatment.Conclusions:The combination of whole-brain LDRT with ICI and IP in the treatment of NSCLC with LM can signific-antly alleviate neurological symptoms,improve quality of life and prolong the NPFS of patients,which is a safe and effective treatment.