Efficacy and Safety of Intraventricule Pemetrexed Disodium Administered Via Ommaya Reservoir

This study is testing whether a chemotherapy drug (pemetrexed) delivered directly into the fluid surrounding the brain and spinal cord — through a small implanted device called an Ommaya reservoir — can treat lung cancer that has spread to the lining of the brain (leptomeningeal metastasis). **You may be eligible if...** - You have non-squamous non-small cell lung cancer with a confirmed genetic mutation (EGFR, ALK, or ROS1) - Your lung cancer has spread to the lining of the brain, confirmed by tests - You have already tried multiple targeted therapy drugs (TKIs) without success - You are medically able to have an Ommaya reservoir implanted **You may NOT be eligible if...** - You are HIV positive - You have rapidly progressing cancer outside the brain that cannot be controlled - You are extremely malnourished or have end-stage disease - You have severe problems from brain tumors outside the meninges - You have another active cancer - You have very poor kidney, liver, or blood function - You are pregnant or breastfeeding Talk to your doctor to see if this trial is right for you.