Intrathecal pemetrexed

\*\*Brief Summary (English)\*\* The goal of this clinical trial is to learn if a risk-adaptive consolidation therapy, guided by cerebrospinal fluid (CSF) circulating tumor DNA (ctDNA) clearance kinetics after induction intrathecal pemetrexed, can improve intracranial progression-free survival (iPFS) compared to standard intrathecal pemetrexed consolidation in patients with leptomeningeal metastasis (LM) from EGFR-mutant non-small cell lung cancer (NSCLC) that has progressed on third-generation EGFR-TKIs. It will also learn about the safety and tolerability of proton craniospinal irradiation (pCSI) in this setting. The main questions it aims to answer are: * Does risk-adaptive consolidation therapy (pCSI with or without concurrent low-dose intrathecal pemetrexed) prolong iPFS compared to standard intrathecal pemetrexed consolidation? * What medical problems do participants have when receiving pCSI or intrathecal pemetrexed? Researchers will compare the experimental arm (risk-adaptive consolidation: pCSI for intermediate-risk, or pCSI with concurrent low-dose intrathecal pemetrexed for high-risk patients) to the control arm (standard intrathecal pemetrexed consolidation) to see if the adaptive strategy improves survival outcomes. Participants will: * Receive induction intrathecal pemetrexed (40 mg, biw, q3w for 2 cycles) via Ommaya reservoir. * Undergo CSF collection at baseline and after induction for ctDNA (maxVAF) analysis to determine risk stratification (low, intermediate, high). * If stratified as intermediate or high risk, be randomly assigned to either standard intrathecal pemetrexed consolidation or risk-adaptive consolidation (pCSI 30 GyE/10f for intermediate-risk; pCSI 30 GyE/10f with concurrent intrathecal pemetrexed 30 mg on d1, d8 for high-risk). * Receive maintenance intrathecal pemetrexed (40 mg, q4w) until intracranial progression. * Undergo regular efficacy assessments (contrast-enhanced MRI of brain and whole spine) every 8 weeks, and complete neurocognitive and quality-of-life questionnaires.