The goal of this clinical trial is to learn if the intraventricular injection of Pemetrexed and Nivolumab works to treat refractory non-squamous non-small cell lung cancer with leptomeningeal metastases. The main questions to answer are:
* Is the combination of Pemetrexed and Nivolumab safe to inject?
* How effective is the combination in disease control?
Participants will：
* Intrathecal injection of 30 mg pemetrexed via Ommaya reservoir, once every 4 weeks until disease progression;
* Intrathecal injection of 40 mg nivolumab via Ommaya reservoir, once every 4 weeks until disease progression;
* Before each intrathecal administration, a preliminary intrathecal injection of dexamethasone, 5 mg/2 mL, is given.