Thiotepa Administration Via Ommaya Reservoir for Leptomeningeal Metastases in Non-Small Cell Lung Cancer Resistant to EGFR-TKI.

The goal of this clinical trial is to learn if the Thiotepa administration via Ommaya Reservoir works to treat non-small cell lung cancer with leptomeningeal metastases resistant to EGFR TKI. The main questions to answer are: Is Thiotepa administration via Ommaya Reservoir safe to inject? How effective is thiotepa via Ommaya Reservoir in intracranial-disease control? Participants will： Stage 1: Intrathecal injection of 10 mg thiotepa via Ommaya reservoir, twice every week, for 4 consecutive weeks; Stage 2: Intrathecal injection of 10 mg thiotepa via Ommaya reservoir, once every week, for 4 consecutive weeks; Stage 3: Intrathecal injection of 10 mg thiotepa via Ommaya reservoir, once a month, for 4 consecutive months; Before each intrathecal administration, a preliminary intrathecal injection of dexamethasone, 2.5 mg/dose, is given.